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Aircraft Construction and Materials Reviewer


 A circumferential member that opposes hoop stress and provide shape and form to the fuselage




A __ reduces aerodynamic drag caused by air spilling off of the wing tip




In addition to two main wheels, a shock absorbing nose wheel is at the forward end of the fuselage





1. Fuselage construction that relies largely on the strength of skin or covering. 


2. The tail rotor of the helicopter, controlled by pedals operated by the pilots, 

counteracts the torque of the main rotor and provides ___________



3. Aircraft control surfaces that control the yaw axis. 


4. Aircraft control surfaces that control the roll axis. 


5. Aircraft control surface that control the pitch axis. 


6. Structural part of an aircraft that houses the engine. 


7. A type of configuration where the propeller is in a position that it pulls the aircraft 

through the air. 


8. Give two main distinguishing features of wings in an aircraft.


9. Type of landing gear used in landing on icy grounds. 


10. This type of landing gear arrangement is also known as to be “Conventional”. 


11. Main structural member of the wings. 


12. Main structural part of an aircraft that generates lift. 


13. Monocoque is from a French word that stands for 


14. Main structural part of a helicopter that generates lift.


 A fore and aft structural member of an aerofoil which has primary purpose of maintaning the correct countour of the covering but is usually also a stressed bearing component of the main structure.




Which statement is true regarding a cantiliver wing?

No external bracing is needed

It requres only one  lift strut on each side –

It has nonadjustable lift strut

The longitudunal members serves for stiffening the metal skin and prevent if from bulging or buckling under severe stresses




This construction is a modification to the monocoque type wherein the skin is stiffened by longitudinal elements




A heavy frame to contain pressures of fluids or to disperse concentrated loads




The movable surfaces which help to direct the aircraft during a flight are usually a __ located at the aft edge of the horizontal stabilizer




The principal longitudinal member of the fuselage that helps the skin support primary bending load




15. Name of combined flight control surface used in V-Tail type. 


16. Secondary control surface that slows down speed of an aircraft and creates a lifting effect during take-off/landing. 


17. Control surface that allows movement along the lateral axis. 


18. Wing shape where the wing tip is narrower than the wing root. 


19. Main structural part of an aircraft that provides electrical power needed for the technical equipment and appliances inside the aircraft. 


20. Tricycle arrangement of landing gear is composed of two main wheels and a ____


Multiple Choice 

1. The vertical flight of a helicopter is controlled by 

a. collective pitch changes. 

b. cyclic pitch changes. 

C. increasing or decreasing the RPM of the main rotor. 

d. none of the above. 

2. An airplane is controlled directionally about its vertical axis by the 

a. ailerons. 

b. elevator(s). 

C. rudder. 

d. flaps 

3. The elevators of a conventional airplane are used to provide rotation about the. 

a. vertical axis. 

b. diagonal axis. 

C. longitudinal axis. 

d. lateral axis. 

4. Movement of an airplane along its lateral axis (roll) is also movement 

a. around or about the longitudinal axis controlled by the elevator. 

b. around or about the lateral axis controlled by the ailerons. 

c. around or about the longitudinal axis controlled by the ailerons. 

d. around or about the lateral axis controlled by the elevator. 

5. In aircraft the airplane rotates about this particular point. 

a. Center of gravity 

b. Center of Pressure 

C. Moment point 

d. Axis point 

6. During flight, an aircraft is yawing to the right. The aircraft would have a tendency to fly 

a. right wing low. 

b. nose up. 

C. left wing low. 

d. nose down. 

7. If an aircraft is aerodynamically stable. 

a. aircraft becomes too sensitive.

b. aircraft returns to trimmed attitude. 

C. aircraft is easy to manoeuvre. 

d. aircraft is well-designed.

8. Ailerons control the aircraft in the. 

a. longitudinal plane. 

b. directional plane. 

C. lateral plane. 

d. vertical plane. 

9. Flight spoilers. 

a. can be used to decrease lift to allow controlled decent without reduction of 


b. can be deployed on the down going wing in a turn to increase lift on that wing. 

C. can be used with differential ailerons to reduce adverse yaw in a turn. 

d. none of the above. 

10. A balance tab. 

a. assists the pilot to move the controls. 

b. is used to trim the appropriate axis of the aircraft. 

C. effectively increases the area of the control surface. 

d. increase effects of lift on the wing. 

11. Semi-monocoque construction. 

a. utilizes the safe-life design concept. 

b. is used only for the fuselage. 

C. offers good damage resistance. 

d. none of the above. 

12. Stringers are used in which of the following types of aircraft fuselage construction? 

a. Semi-monocoque. 

b. Truss type. 

C. Monocoque. 

d. None of the above. 

13. In a monocoque structure, which component carries the majority of the loads? 

a. Longerons. 

b. Stringers. 

C. Skin. 

d. Formers. 

14. Skin panels may be strengthened by. 

a. stringers. 

b. struts. 

C. cleats. 

d. joints. 

15. A cantilever wing is a 

a. usual airliner wing. 

b. top wing of a biplane. 

C. swept-back wing. 

d. glider type. 

16. What is a cantilever wing? 

a. One that folds for access to limited space. 

b. One that has external supporting struts. 

C. One that has no external supporting struts. 

d. One that unfolds when needed. 

17. The term ’empennage’ incorporates. 

a. rudder, ailerons, spoilers. 

b. elevators, stabiliser, ailerons. 

C. elevators, stabiliser, rudder. 

d. elevators, rudder, spoiler. 

18. Which of the following are primary control surfaces? 

a. Roll spoilers, elevators, tabs. 

b. Elevators, roll spoilers, tabs. 

C. Elevators, ailerons, rudder. 

d. Elevators, stabiliser, rudder. 

19. Non-shock absorbing landing gear types commonly uses this component to 

moderated impact loads during landing. 

a. bungee cord 

b. hydraulic struts 

C. pneumatic tire aids 

d. impact reducers 

20. This type of aircraft is designed without a tail unit nor a definite fuselage. 

a. Blended body 

b. Flying wing 

C. Lifting body 

d. Glider wing 



An aircraft is a device that is used for, or is intended to be used for, flight in the air. 


  • Lighter-than-air vehicles            • Rotacraft
  • Airplane                    • Glider
  • Rotacraft


  • Airships
  • Balloons, etc

Both are lighter-than-air aircraft but have differentiating features and  are operated differently. 


The wings on this type  of flying machine are attached to the fuselage and are not intended to move independently  in a fashion that results in the creation of lift. One, two, or three sets of wings have all been  successfully utilized. [Figure 1-12] 

Rotary-wing aircraft such as helicopters are also widespread.  

This handbook discusses features and maintenance aspects common to both fixed-wing and  rotary-wing categories of aircraft. Also, in certain cases, explanations focus on information  specific to only one or the other. 

Glider airframes are very similar to fixed-wing aircraft. Unless  otherwise noted, maintenance practices described for fixed-wing aircraft also apply to gliders.  T

Lighter-than-air aircraft – The same is true for this, although thorough coverage of the unique  airframe structures and maintenance practices for lighter-than-air flying machines is not  included in this handbook. 

The airframe of a fixed-wing aircraft consists of five major structural components: the fuselagewings, powerplant, empennage, and landing gear. [Figure 1-13] 


Helicopter airframes consist  of the:

  • Fuselage
  • Main rotor and related gearbox
  • Tail rotor (on helicopters with a single main  rotor)
  • Landing gear


Airframe structural components are constructed from a wide variety of materials. The earliest  aircraft were constructed primarily of wood. 


Steel tubing and the most common material,  aluminum, followed. 

Many newly certified aircraft are built from molded composite materials,ex. carbon fiber. 


Wing spar – main structural member in a wing    • Stringers    
Bulkheads                        • Longerons
Stringers                            • Ribs, etc


The skin of aircraft can also be made from a variety of materials:

Impregnated fabric        • Aluminum
Plywood            • Composites

Under the skin and attached to the structural  fuselage are the many components that support airframe function. 


Rivets           • Screws         
Bolts            • Other fasteners

TECHNIQUES: Welding, adhesives, and  special bonding techniques


  • Wings are airfoils that, when moved rapidly through the air, create lift. 
  • They are built in many  shapes and sizes. Wing design can vary to provide certain desirable flight characteristics.  
  • Control at various operating speeds, the amount of lift generated, balance, and stability all  change as the shape of the wing is altered. 
  • Both the leading edge and the trailing edge of  the wing may be straight or curved, or one edge may be straight and the other curved. One  or both edges may be tapered so that the wing is narrower at the tip than at the root where it  joins the fuselage. 
  • The wing tip may be square, rounded, or even pointed. 
  • The wings of an aircraft can be attached to the fuselage at the top, mid-fuselage, or at the  bottom. They may extend perpendicular to the horizontal plain of the fuselage or can angle  up or down slightly. This angle is known as the wing dihedral. The dihedral angle affects the  lateral stability of the aircraft. 
Machine generated alternative text:


The fuselage is the central body of an airplane and is designed to accommodate the crew,  passengers, and cargo. It also provides the structural connection for the wings and tail  assembly. Older types of aircraft design utilized an open truss structure constructed of wood,  steel, or aluminum tubing. The most popular types of fuselage structures used in today’s aircraft  are the monocoque (French for “single shell”) and semi-monocoque. 


Truss Type A truss is a rigid framework made up of members,such as beams, struts, and bars to resist deformationby applied loads. The truss-framed fuselage is generally coveredwith fabric.Machine generated alternative text:


                                         MONOCOQUE TYPE

The monocoque (single shell) fuselage relies largely on the strength of the skin or covering to carry the primary loads. 

MonocoqueThe true monocoque construction uses formers, frame assemblies, and bulkheads to give shape to the fuselage.  The heaviest of these structural members arelocated at intervals to carry concentrated loads and at points where fittings are used to attach other units such as wings, powerplants, and stabilizers.Machine generated alternative text:
Semi-monocoque TypeTo overcome the strength/weight problem of monocoque construction. It also consists of frame assemblies, bulkheads, and formers as used in the monocoque design but, theskin is reinforced by longitudinal members calledlongerons.  Longerons usually extend across several frame members and help the skin support primarybending loads. They are typically made of aluminum alloy either of a single piece or a built-up construction.Machine generated alternative text:



  • Includes the entire tail group & consists of fixed surfaces: vertical stabilizer &  horizontal stabilizer
  • The movable surfaces include the rudder, the  elevator, and one or more trim tabs  


  • Attached to the back of the vertical stabilizer. 
  • During flight, it is used to move the  airplane’s nose left and right. 


  • Attached to the back of the horizontal stabilizer, is used to move the nose of the airplane up and down during flight. 


  • Small, movable portions of the trailing edge of the control surface. These movable trim tabs,  which are controlled from the flight deck, reduce control pressures. 
  • Trim tabs may be installed  on the ailerons, the rudder, and/or the elevator. 


  • Aircraft tail types


The powerplant usually includes both the engine and the propeller. 


  • PRIMARY FUNCTION: to provide the power to turn the propeller.
  • It also generates electrical power, provides  a vacuum source for some flight instruments.
  • In most single-engine airplanes, provides a  source of heat for the pilot and passengers. 
  • The engine is covered by a cowling, or a nacelle,  which are both types of covered housing. 
    • The purpose of the cowling or nacelle is to streamline  the flow of air around the engine and to help cool the engine by ducting air around the  cylinders.  


 Single, twin, triple, four (multiple) engine



The principal support of the airplane when parked, taxiing, taking off, or  landing. 

The most common type of landing gear consists of wheels, but airplanes can also be  equipped with floats for water operations or skis for landing on snow.  

Wheeled landing gear consists of three wheels—two main wheels and a third wheel positioned  either at the front or rear of the airplane. Landing gear with a rear mounted wheel is called  conventional landing gear.  

Tailwheel – Airplanes with conventional landing gear are sometimes referred to as tailwheel airplanes. 

Nosewheel – When the third wheel is located on the nose, it is called a nosewheel, and the design is referred  to as a tricycle gear. 

A steerable nosewheel or tailwheel permits the airplane to be controlled  throughout all operations while on the ground. Most aircraft are steered by moving the rudder  pedals, whether nosewheel or tailwheel. Additionally, some aircraft are steered by differential  braking.  



Leaf Type Spring Gear 

Non-shock absorbing struts made from steel, aluminum, or composite material  transfer the impact forces of landing to the airframe at a non-damaging rate. Machine generated alternative text:
pring Steel Strut  


Shock load transfer to the airframe is direct  with this design. Use of pneumatic tires aids  in softening the impact loads. Machine generated alternative text:


Bungee Cord 

Shock load transfer to the airframe is direct with this design. Use of pneumatic  tires aids in softening the impact loads. Bungee cords are positioned between  the rigid airframe structure
and the flexing gear assembly to take up the loads  and return
them to the airframe at a non-damaging rate. 
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True shock absorption occurs when the  shock energy
of landing impact is converted  into heat energy, as in a shock strut landing  gear.  This is the most common method of  landing shock dissipation in aviation. It is used  on aircraft of all sizes. Shock struts are self-contained hydraulic units that support an  aircraft while on the ground and protect the structure during landing. An oleo-pneumatic (air-oil) shock strut  consists of two
telescoping tubes with sealed  ends. The resulting
variable-displacement chamber is partially
filled with hydraulic fluid  and partially with
compressed air or nitrogen. 
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Rough Field and soft field


Primary Flight Control Surfaces 

The primary flight control surfaces on a fixed-wing aircraft include: 


Attached to the trailing edge of both wings and when moved, rotate  the aircraft around the longitudinal axis. 


Attached to the trailing edge of the  horizontal stabilizer. When it is moved, it alters aircraft pitch, which is the attitude about the  horizontal or lateral axis. 


Hinged to the trailing edge of the vertical stabilizer. When  the rudder changes position, the aircraft rotates about the vertical axis (yaw). 

There are several secondary or auxiliary flight control surfaces. 

Classification of Aircraft

  • Classified by the lift they produce
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  • Heavier than air
  • Dynamics
  • Propels themselves into the air to generate lift


  • Lighter than air
  • Statics
  • Uses buoyancy to generate lift (Airship, Free Balloon [air balloon])

Differences of Gyrocopter and Helicopter

  • Helicopter has blades only on the top and controls the whole helicopter
  • Gyrocopter derives its support from the propeller on the back and the propeller on the top is a free-wheel.

Why is there a Broad Classification of Aircrafts?

  • Functions
  • Flying Characteristics
  • Flying Operations
  • Innovations
  • Flight Principles
  • Used with respect to the Certification, Ratings, Privileges, and Limitations of Airmen

Aircraft Categories

  • Acrobatic
  • Certified design for acrobatic maneuvers a normal aircraft cannot perform
  • Commuter
    • Certified. Anything in the transport category meaning used for transport for passengers or cargo
  • Experimental
  • Light Sport
    • Non-certified but may be included in the acrobatic category.
  • Limited
    • Certified to a specific profile such as the Cessna 152 which is normally used for flying (also included in Normal category). It may be certified as limited if they have a specific function such as using a plane solely for agriculture.
  • Normal
  • Primary
  • Transport
    • Difference between transport and commuter is the weight requirements.
  • Utility

Pilot Certificate Categories

  • Airplane
  • Lighter-than-Air
  • Powered Parachute
  • Rotorcraft
  • Weight-Shift-Control

Airplane Classifications

  • Airplane
  • Lighter-than-Air
  • Powered Parachute
  • Rotorcraft
  • Weight-Shift-Control
    • Gliders

Aircraft Types

  • Makes a specific make and basic model aircraft, including modifications thereto that do not change its handling or flight characteristics.
  • Example: The Engine of the A321 CEO and the A321 NEO has different engines with the CEO having a streamline engine and the wing tip has a triangular shape, while the NEO has a trailing edge turbine and sharklet wing tip.

Major Structural Stresses

  • Tension
  • Compression
  • Torsion (Twisting)
  • Shear (Resits a material to slide over its adjacent layer)
  • Bending (compression + tension)

Airplane Basic Construction

Five Major Components

  1. Fuselage
    • The main structure or body of the aircraft that provides space for cargo, controls, accessories, passengers, and other equipment
    • Helps withstand Hoop Stress
    • Types
      1. Truss Type
        • It is a rigid framework made up of members such as beams, struts, and bars to resist deformation by applied loads.
        • It is constructed of steel tubing welded that can carry both tension and compression loads
        • Bulkheads are used to maintain the shape of the fuselage
      2. Monocoque Type
        • Monocoque
          1. Small diameter
          2. No other weight support
            1. If we attempt to make it stronger, its weight will rise.
        • Semi-Monocoque
          1. Monocoque type with stringers and longerons
          2. The loads imposed on the skin are shared by a series of frames, stringers and formers that are attached to it.
      3. Other Structural Terms
        • Longeron
          1. Main longitudinal member of a fuselage or nacelle
        • Tie Rod (Tension Rod)
          1. Member taking a tensile load
        • Strut
          1. Member taking a compression load
        • Stressed Skin
          1. Structure where loads are shared between skin and framework
        • Bulkhead
          1. A partition within the structure. Usually lateral but can be longitudinal. If it forms the boundary of a pressurized structure it is called a pressure bulkhead.
        • Crack Stopper
          1. A reinforcing member normally placed at right angles to the path of an anticipated crack which will reduce the rate of further propagation.
        • Gussets
          1. A flat sheet triangular in shape used to reinforce the corners of structure
        • Keelson (Keel Beam)
          1. Structural element frequently used to carry the fuselage bending loads through the portion of the lower fuselage which is cut up by the wheel wells.
  2. Wings
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  • These are airfoils, when moved rapidly through the air, create Lift.
  • Parts
  • Wing Flaps
  • Spar
  • Resists bending and axial loads.
  • Is the main member of the wings (like the longerons to the fuselage).
  • It is designed to be subjected to shear, bending, and torsion.
  • Form the wingbox for stable torsion resistance.
  • It may be classified as tension-field  beam or shear resistant beam.
  • Parts
  1. Spar cap/ Flange
  1. Carries the Bending Moment generated by the wing in flight
  2. The Upper Spar Cap will be loaded in compression. The Lower Spar Cap will be loaded in Tension for a positive load factor (wing bending upward).
  3. Spar Web
  4. It is responsible for carrying the shear loads which arises from aerodynamic loading of the wing.
  1. Aileron
  2. Stringers
    • Also considered as stiffeners
    • It experiences both tension and compression loads.
  3. Ribs
    • Provides shape to the wings.
    • Maintains the aerodynamic profile
    • Supports the skins and stiffeners
    • Transmits the pressure on the wing to the spanwise members
  4. Wing Tip
  5. Skin
    • If there is load felt on the skin, it will distribute it on the spar, rubs, and stringers to help withstand the load.
  6. Fuel Tank
  7. Torsion Box
  8. Wing Planforms
  1. Rectangular Wing
    • Commonly used for smaller aircrafts
    • Simplest to manufacture
  2. Elliptical Wing
    • Most aerodynamic efficient wing
    • Hard to manufacture
    • Thinnest possible wings
  3. Tapered Wing
    • Compromise of efficiency and manufacturing
    • To meet halfway between the rectangular wing and elliptical wing
  4. Delta Wing
    • Common in supersonic aircrafts
  5. Efficient in any flight regime
  • Large fuel tank storage
  • Large Torsion Box
  1. Trapezoidal Wing
    • Commonly used for combat or military flights.
    • The problem is with its maneuverability
    • Also known as diamond wing
    • Has stealth characteristics
  2. Swept-back Wings
    • Common in commercial aircrafts
    • Has reduced drag especially on transonic speed
  3. Forward-swept Wings
    • Has controllability issues (with rolling and other maneuvers)
    • Greater wing root stress
  4. Variable sweep wings
    • Only for optimization of the wings
    • Has complexity and prone to repairs
    • Can move (?)
  5. Wing Positions
  1. Low Wing
    • Landing gear needs no stiffening.
    • Shorter landing gear (with enough clearance).
    • Sometimes has dihedral to avoid collision with the ground.
  2. Mid Wing
    • Needs more stiffening which means greater weight.
    • In terms of construction the passenger/cargo space is limited.
  3. High Wing
    • Fuselage is closer to the ground.
    • Has easier loading/unloading.
    • Has sufficient ground clearance.
    • Less landing gear height.
  4. Inverted Gull Wing
  5. Gull Wing
  6. Dihedral Wing
  7. Anhedral Wing (Negative Dihedral)
  8. Wing Bracing
Aircraft Construction and Materials Reviewer
  1. Cantilever Wing
  • No bracing
  1. Semi-Cantilever
    • Has wing struts
  2. Empennage
  3. It includes the entire tail group and consists of fixed surfaces such as the vertical stabilizer and horizontal stabilizer.
  4. Vertical Stabilizer
  1. Its function is to provide directional stability and control in flight through the rudder (yaw)
  2. It is important that these tail surfaces be located that they are not blanketed by the fuselage
  • Horizontal Stabilizer
  1. This should be located so that any blanketing by the wing/fuselage is avoided.
  2. Elevator (pitch)
  • Dorsal Fins and Ventral Fins add additional directional stability
  • Tail Configurations
  1. Conventional
    • Simplest to manufacture
    • Commonly used with small aircrafts
    • Low structural weight
  2. T-Tail
    • Shaped like a T
    • Horizontal Stabilizer is more efficient (compared to the conventional)
    • Has a thin stabilizer which means we need to strengthen the vertical stabilizer which adds more weight.
  3. Cruciform Tail
    • Looks like a cross
    • Compromise between conventional and T-tail
  4. Dual Tail
    • Two (2) Vertical Stabilizers which means two (2) Rudders which is good for directional control and stability.
  5. Triple Tail
    • Commonly used during the 1940s
  6. V-Tail
    • Called butterfly tail
    • Horizontal and Vertical Stabilizer is combined called ruddervator
    • Has less weight which means less drag
    • Complexity contrasts with the conventional tail
  7. Inverted V-Tail
    • Same as V-Tail… just…. Inverted
  8. Y-Tail
    • Effective, especially the horizontal stabilizer
    • Efficient ruddervator
    • Has additional directional control
    • When there is a higher angle of attack (AOA), it still has good directional control
  9. Boom Tail
    • Just like the dual tail with a boom pipe
    • Avoids the propwash/engine
  10. High Boom Tail
    • Just like boom tail with higher horizontal stabilizer
    • Has two rudders which means good directional stability
  11. Multiple-Plane Tail
    • Wright Flyer
    • Commonly used during the World War I
  • Landing Gear
  • The principal support of the airplane when parked, taxiing, take-off, or landing.
  • Wheels Arrangement
  1. Conventional (Tail Dragger)
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  • Advantages
  1. Allows Adequate ground clearance for a large propeller
  2. More desirable operations on unimproved fields
  • Disadvantages
  1. Directional control becomes more difficult while on the ground.
  2. Lack of good forward visibility when the tail wheel is on the rear.
  3. Brake application must be monitored to prevent nosing over.
  • Tricycle
  • Advantages
  1. Allows more forceful application of brakes during landing at high speeds without causing the aircraft to nose over.
  2. Great forward visibility for the pilot during take-off, landing and taxiing.
  3. It prevents ground looping by providing more directional stability
  4. Single Main
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  • Advantages
  1. The design is simple, lightweight and low drag. (it may include skids rather than wheels).
  2. Impractical for large and heavy aircrafts.
  • Bicycle
Aircraft Construction and Materials Reviewer
  • Advantages
  1. It has lower weight and drag than either the tail dragger or tricycle arrangements
  • Disadvantages:
  1. Very demanding on the pilot who must maintain a very level attitude during take-off and landing while carefully managing airspeed.
  2. The pilot must compensate for any rolling motion that could cause the plane to land unevenly.
  3. Crosswinds are particularly difficult to deal with.
  • Quadricycle
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  • Advantages
  1. The planes floor can be very close to the ground for easier loading and/or unloading of cargo
  • Disadvantages
  1. It is very demanding for the pilot since it requires a very flat attitude during take-off and landing.
  2. It is sensitive to roll, crosswinds and proper alignment with the runway. 
  • Multi-bogey
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  • Advantages
  1. Applicable for large commercial airplanes
  2. Wheel arrangement promotes fail safe.
  • Disadvantages
  1. This type of arrangement has greater weight
  • Skid
  • Disadvantages
  1. The skids are not as efficient as the oleo shock absorbers
  1. Floats
    • Used for seaplanes
    • Similar with skids
  2. Releasable Rail
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  • It is designed for the aircraft to take-off while airborne and is not expected to land on the ground or sea.
  • The main function of the attachment is to hold the vehicle when launched
  1. Landing Gear Operations
  • Rigid/Fixed Landing Gear
  • Greater Drag than Retractable
  1. Retractable Landing Gear
    • Less drag than Rigid/Fixed
    • Has a complex system
  2. Power Plant
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  1. Usually includes both Engine and Propeller
  2. Its primary function is to provide power to turn the propeller.
  3. Parts (?)
  4. Nacelles (Engine Pods)
    • They are streamline enclosures used to primarily house the engine and its components
    • Usually present a round and elliptical profile to the wind thus reducing aerodynamic drag
    • Contains the engine and accessories, engine mounts, structural members, a firewall, and skin and cowling on the exterior to fare the nacelle to the wind.
    • Cowling – provides access to the engine (second picture ^) and protects the engine from damage.
  5. Has cowl flaps that are fully opened to regulate engine temperature.
  • Engine Mount – it is the assembly where the engines are fastened or attached
  1. Power plant Types
  1. Reciprocating Engine
    • In-Line
    • Radial
    • Horizontally Opposed
    • V-Type
    • W-Type
  2. Jet Engines
    • Turbojet
    • Turbofan
    • Turboprop
    • Turboshaft
    • Ramjet
  3. Propeller Installation (location)
  1. Tractor Type
    • In front of the fuselage
  2. Pusher Type (boom mounted)
    • Propeller is behind the aircraft
  3. Push-pull configuration
    • One in front and one additional behind so that the propwash is streamlined.
  4. Engine Placement
  1. Wing mounted
    • Example: Pylons hanging
  2. Fuselage Mounted
  3. Nose Mounted
  4. Number of Engines
  1. Single Engine
  2. Twin Engine
  3. Triple Engine
  4. Four Engine

Airplane Flight Controls

Primary Flight Controls

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  1. An aircraft turns with both rudder and aileron controls
  2. Ailerons
  • When the control wheel is moved to the right, the aircraft will roll right.
  • Aileron deflection is opposite
  • In the case of the situation, the left aileron is downward, and the right aileron is upward.
  1. Elevator
    • Control column (pulling or pushing the control wheel)
    • When pulled, the elevator will deflect upwards meaning that it will go nose up. 
  2. Rudder
    • Uses Rudder Pedals as its control
      • Two Rudder Pedals
      • When the left pedal is pushed, the rudder will deflect left and yaw left.

Secondary Flight Controls

  1. Flight controls attached to the primary flight controls


  1. Most common high lift devices meaning they increase lift.
  2. Located on the trailing edge which is at the back of the wings
  3. Plain Flap
    • Simplest flap
    • When flap is deflected down, there is greater lift and induced drag (which is drag due to lift).
  4. Split Flap
    • Greater drag
  5. Slotted Flap
    • Popular with commercial aircrafts
    • Meaning it has a pathway or space for wing to pass
    • Lift produced is greater than the plain or split flap
  6. Fowler Flap
    • This is a type of slotted flap
    • Instead of deflecting it farther, the flap instead extends.

Leading Edge Devices

  1. Fixed Slot
    • Delays the airflow separation at a higher angle of attack
    • Minimal deflection
    • Daanan ng hangin – Ma’am
  2. Slats
    • The slats retract meaning the space under them is greater
    • To direct the air there.
  3. Leading Edge Flaps
    • Minimize/Reduce the pitch down of an aircraft
  4. Stall Strip
    • To delay the airflow separation
    • Improve controllability at high angle of attack


  1. Intended for the trim system.
    • Relieves pressure from the control surface.
      • Example: When crosswinds push the nose of the fuselage, the trim system relieves the pressure that the pilot feels.
    • Attached on the trailing edge of the control surfaces.
    • Trim Tab
      • Common for small aircrafts
    • Balance Tab
      • Automatically deflects when the pilot moves the control surface
      • Deflects opposite of the movement of the control surface.
        1. Example: The elevators are deflected downwards, the balance tab goes upwards.
    • Anti-Servo Tab
      • Automatically deploys when the pilot moves the controls surface
      • Deflects the same directions as the control surface.
      • Servo Tab – destabilizes the aircraft
    • Spring Tab
      • Like a servo tab
      • Difference is their linkages
    • Ground Adjustable Tab
      • Trial and error
      • Prevents skidding for small aircrafts
    • Adjustable Stabilizer
      • Can be adjusted mid-flight (?)

Dual Purpose Flight Controls

  1. Stabilator
  2. Stabilizer + elevator
  3. Elevons
  4. Elevator + Ailerons
  5. Ruddervator
  6. Rudder + Elevator
  7. Flaperons
  8. Flaps + Ailerons

Helicopter Basic Construction

  1. Airframe
  2. Fuselage
  3. Landing gear
  4. Main Rotor System
  5. Classified based on 
  1. blade attachment
  2. Movement relative to hub
  • Parts
  1. Mast
  2. Hub
  3. Rotor Blades
  • Types of Main Rotor System
  1. Rigid Rotor System
  • The simplest is the rigid rotor system. In this system, the rotor blades are rigidly attached to the main rotor hub and are not free to slide  back and forth (drag) or move up and down (flap).
  1. Semirigid Rotor System
  • The semirigid rotor system uses a teetering hinge at the blade attach point. While held in check from sliding back and forth, the teetering hinge DOES ALLOW the BLADES TO FLAP UP AND DOWN. With this hinge, when on eblade flaps up, the others flap down.
  1. Fully Articulated System
  • It provides hinges that allow the rotors to move forward and backward, as well as up and down
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  • Tail Rotor System
  • Main Function is to counter the torque effect of the main rotor [Helicopter will spin without the tail rotor]
  • NoTaR- No Tail Rotor
  • Flight Controls
  1. Collective Lever
  1. Increases or decreases height
  2. Affects angle of attack
  3. Affects the engine RPM
  4. Cyclic Stick
  5. Affects longitudinal and lateral [forward, backward, left, and right] movement
  6. Anti-Torque Pedals
  1. For stability
  2. Affects the Yaw
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  • Powerplant
  1. Commonly a turboshaft
  • Transmission
  • Group: Saynes, Doria, Ilagan



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  • Stiff. They have  relatively high elastic moduli.  
  • Most, when pure, are soft and easily deformed.  
  • They can be made strong by  

alloying and by mechanical and heat  

treatment, but they remain  ductile, allowing them to be formed by deformation  processes. 

  • Certain high 

strength alloys (spring 

steel, for instance) have  

ductility as low as 1%, but  

even this is enough to ensure that the material yields before it fractures and that fracture, when it occurs, is of a tough, ductile type. 

  • Partly because of  their ductility, metals are prey to fatigue and of all the classes of material, they are the least resistant to  corrosion. 


  • Have high moduli, they are brittle. 
  • Their “strength” in tension means the  brittle fracture strength; in compression it is the brittle crushing strength, which is about 15 times  greater. 
  • And because ceramics have no ductility, they have a low tolerance for stress concentrations (like  holes or cracks) or for high-contact stresses (at clamping points, for instance).
  • Ductile materials  accommodate stress concentrations by deforming in a way that redistributes the load more evenly, and  because of this, they can be used under static loads within a small margin of their yield strength. 
  • Ceramics cannot. Brittle materials always have a wide scatter in strength, and the strength itself depends on the  volume of material under load and the time over which it is applied. 
  • So, ceramics are not as easy to design  with as metals. 
  • Despite this, they have attractive features. They are stiff, hard, and abrasion-resistant  (hence their use for bearings and cutting tools);
  • they retain their strength to high temperatures; and they  resist corrosion well. 


  • Non-crystalline (“amorphous”) solids. 
  • The most common: soda-lime and borosilicate  glasses familiar as bottles and ovenware, etc.
  • Metals, too, can be made non crystalline by cooling them sufficiently quickly. 
  • The lack of crystal structure suppresses plasticity, so, like  ceramics, glasses are hard, brittle, and vulnerable to stress concentrations.  


  • They have moduli that are low, roughly 50 times lower  than those of metals, but they can be strong— nearly as strong as metals. 
  • Elastic deflections can be large. 
  • They creep, even at room temperature, meaning that a polymer  component under load may, with time, acquire a permanent set. 
  • Their properties depend on  temperature so that a polymer that is tough and flexible at 20°C may be brittle at the 4°C of a household  refrigerator, yet may creep rapidly at the 100°C of boiling water. Few have useful strength above 200°C. 
  • Some polymers are mainly crystalline, some are amorphous (non-crystalline), some are a mix of  crystalline and amorphous—transparency goes with the amorphous structure. If these aspects are  allowed for in the design, the advantages of polymers can be exploited. 
  • When  combinations of properties, such as strength per unit weight, are important, polymers can compete with  metals. They are easy to shape. Complicated parts performing several functions can be molded from a  polymer in a single operation. 
  • The large elastic deflections allow the design of polymer components that  snap together, making assembly fast and cheap. And by accurately sizing the mold and precoloring the  polymer, no finishing operations are needed. 
  • Polymers resist corrosion (paints, for instance, are  polymers) and have low coefficients of friction.


  • Long-chain polymers above their glass-transition temperature, Tg. 
  • The covalent bonds that  link the units of the polymer chain remain intact, but the weaker Van der Waals and hydrogen bonds that,  below Tg, bind the chains to each other, have melted. 
  • This gives elastomers unique properties: Young’s  moduli as low as 10−3 GPa (105 times less than that typical of metals) increase with temperature (all other  solids show a decrease), and have enormous elastic extension. 
  • Their properties differ so much from those  of other solids that special tests have evolved to characterize them. 
  • This creates a problem: If we wish to  select materials by prescribing a desired attribute profile, as we do later in this book, then a prerequisite  is a set of attributes common to all materials. 
  • To overcome this, we use a common set of properties in the early stages of design, estimating  approximate values for anomalies like elastomers.  
  • Specialized attributes, representative of one family  only, are stored separately; they are for use in the later stages. 


  • Are combinations of two or more materials in a predetermined configuration and scale. They  combine the attractive properties of the other families of materials while avoiding some of their  drawbacks.
  • The family of hybrids includes fiber and  particulate composites, sandwich structures, lattice structures, foams, cables, and laminates; almost all  the materials of nature—wood, bone, skin, and leaf—are hybrids. 
  • MOST FAMILIAR: Fiber-reinforced composites
  • Most of those at present available to the engineer have a polymer matrix  reinforced by fibers of glass, carbon, or Kevlar (an aramid). They are light, stiff, and strong, and they can  be tough.
  • They, and other hybrids using a polymer as one component, cannot be used above 250°C  because the polymer softens, but at room temperature their performance can be outstanding. 
  • Hybrid  components are expensive, and they are relatively difficult to form and join. So, despite their attractive  properties, the designer will use them only when the added performance justifies the added cost. 
  • Today’s  growing emphasis on high performance and fuel efficiency provides increasing drivers for their use.


There are six important families of materials for mechanical design: metals, ceramics, glasses, polymers,  elastomers, and hybrids (which combine the properties of two or more of the others). 

Within a family  there is a certain commonality. Ceramics and glasses as a family are hard, brittle, and corrosion resistant.  Metals are ductile, tough and good thermal and electrical conductors. Polymers are light, easily shaped,  and electrical insulators. Elastomers have the ability to deform elastically to large strains. 

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  • The ability of a material to stand up to forces being applied without it breaking, bending, shattering, or deforming in any way. 
  • ability of a material to resist abrasion, penetration, cutting action, or permanent distortion
  • Can be increased by: Hardening and cold working; Ex. trip or plate in rollers
  • Can be measured by:
M ineral 
Gold' Silver 
Copsx•r vrnny 
Knife blade 
Iron pyrite 
Hardened steel file


  • A characteristic of a material that does not break or shatter when receiving a blow or under a sudden shock.
  • Ability of a material to withstand cracks, i.e. to prevent the transfer or propagation of cracks across its section, hence causing failure.
  • A material that absorbs impact (sudden forces or shocks such as hammer blows)well is tough
  • The opposite to brittleness ability to absorb energy, and may be stretched or deformed without breaking.
Resistance to scratching,cutting or abrasionResistance to fracture
Materials are scratchproofMaterials are not easily breakable and can withstand high pressures
Affected by the strength and plasticity of the materialAffected by the rate of loading, temperature, and notch effect
Measured by:Rockwell hardness testBrinell HTVickers HTKnoop HTMeasured by:Impact toughnessNotch TFracture T


  • The ability of a material to absorb force and flex in different directions, returning to its original position.
  • The property of returning to the original shape when the force causing the change of shape is removed.

Aircraft Construction: Designed that the maximum applied loads to which the airplane may be subjected will never stress them above their elastic limit.

Force - Extension Graph 
Elastic Limit 
This is reached when the 
graph line starts to curve 


  • The ability of a material to be change in shape permanently
  • The capability of an object or material to be stretched and to recover its size and shape after its deformation.
  • Behavior that takes place beyond the yield point in a material (plastic region). When a material goes into the plastic region, the strain cannot be recovered.
Ability of an object or material to resume its normal shape after being stretched or compressedDeformation is irreversible The capability of an object or material to be stretched and to recover its size and shape after its deformation.
Materials showing elasticity have elastic propertiesMaterials showing plasticity do not have elastic properties
Materials do not break quickly apart when stretchedBreaks apart quickly when stretched
Materials that can reversibly deform to a high extentMaterials that are either ductile or brittle when comparatively a small stress is applied


  • The ability of a material to be reshaped in all directions without cracking
  • The ability plastically deforms, and shape a material by forging, rolling or by any other method of applying pressure. Being easy to beat into a thin sheet is the literal meaning
  • Property of metals which allows them to be bent or permanently distorted without rupture


  • The ability of a material to change shape (deform) usually by stretching along its length
  • The property of metals which allows them to be drawn out without breaking
  • It is a measure of how easily a material can be worked
Ability of a material to stretch under tensile stress allows to be drawn, bent or twisted into various shapes without breaking.Ability to deform and change shape under compressive stress Allows to be hammered, rolled, or pressed into various shapes without cracking, breaking, orleaving some other detrimental effect
Materials can be rolled into wiresMaterials can be rolled into sheets
Measured by bend testMeasured by the ability to withstand pressure
Affected by the grain sizeAffected by the crystal structure
ability of a material to resist applied stress before failure without deformationresistance to elastic deformation – ability to resist bending
Capacity to withstand great force or pressure without breakage or plastic deformationRigidity of an object
Ability of an object to withstand stress without breaking or plastic deformationAbility of an obj to resist the deformation when a stress is applied
Physical failure of a subsFunctional failure of a sub
Associated w/ elastic and brittle subsApplicable to elastic substances
TYPES:Impact strengthTensile SFatigue SYield STYPES:Rotational stiffnessAxis stiffness

FUSIBILITY – the ability of a metal to become liquid by the application of heat.

DENSITY – the weight of a unit volume of a material. -Important consideration for weight and balance.

CONDUCTIVITY – the property which enables a metal to carry heat or electricity. 

– Important for Fusion and to control expansion and contraction.

BRITTLENESS – the property of a metal which allows little bending or deformation without shattering. -structural metals are often subjected to shock loads, brittleness is not a very desirable property.

THERMAL EXPANSION – compression and expansion when changes the temperature

DURABILITY – Withstand force over a long period of time

Contraction and Expansion – reactions produced in metals as the result of heating or cooling.

STRESS AND STRAIN – loads in computing young’s modulus



  • Structural parts are often formed from metals in their soft state and are then heat treated to harden them so that the finished shape will be retained.


  • Structural metals are often subjected to shock loads, brittleness is not a very desirable property. 


  • Necessary in sheet metal that is worked into curved shapes – such as cowlings, fairings, or wingtips. 


  • Greatly preferred for aircraft use because of their case of forming and resistance to failure under shock loads. 
  • For this reason, aluminum alloys are used for cowl rings.
  • Fuselage and wing skin. And formed or extruded parts, such as ribs, spars, and bulkheads.
  • Chrome molybdenum steel is also easily formed into desired shapes.


  • Members and parts are so designed that the maximum loads to which they are subjected will not stress them beyond their elastic limits  


  • Desirable property in aircraft metals. 


  • Important consideration when choosing a material to be used in the design of a part in order to maintain the proper weight and balance of the aircraft. 


  • Welded parts of the aircraft.


  • Must be considered in conjunction with bonding, to eliminate radio interference. 


  • Cooling and heating affect the design of welding jigs,castings and tolerances necessary for hot-rolled material.


Properties of Materials

Stress – Strain Curve

  • To get the Young’s Modulus
  • Tensile Test = FA
  • Tells us the load an aircraft can handle
  • Strain is the possible deformation of the materials 
  • Proportional Limit
  • Linear relation between elongation and the axial force
  1. Elastic Limit
    • The limit beyond which the material will no longer go back to its original shape when the load is removed.
    • The maximum stress that may be developed such that there is zno permanent or residual deformation when the load is entirely removed.
  2. Yield Point
    • The point at which the material will have an appreciable elongation without increase in load
    • Once the yield point is reached there will be a deformation or an elongation.
  3. Ultimate Strength
    • The maximum ordinate in the stress-strain diagram, also known as tensile strength.
  4. Rupture Strength
    • Also known as the breaking strength
    • The strength of the material at rupture
  5. Hooke’s Law
    • It states that the stress is directly proportional to strain
  6. Modulus of Elasticity
    • The proportionality constant (k) on Hooke’s Law. It is equal to the slope of the stress-strain diagram of 0 to P.
  7. Modulus of Resilience
    • It is the work done on a unit volume of a material as the force is gradually increased from 0 to P.
  8. Modulus of Toughness
    • It is the work done on a unit volume of a material as the force is gradually increased for 0 to R
  9. Working Stress
    • The actual stress of a material under a given loading
  10. Allowable Stress
    • The maximum safe stress that a material can carry
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Factor of Safety

Aviation Factor of Safety =Ultimate StressLimit Stress= 1.5

*The higher the factor of safety means that the weight is higher, so if the factor of safety is higher than 1.5, the aircraft becomes heavier and less efficient. Another thing is that aircrafts are dynamic which means that the aircrafts need to be reinforced rather than the static buildings.

Materials Science vs. Materials Engineering

Materials Science

  1. Study between the structure and its properties
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How do we classify materials?

  1. Chemical Composition
  2. Occurrence in Nature
    • Abundance
  3. Manufacturing Process
    • Ultimate Strength and Allowable Strength
  4. Crystalline or Non-Crystalline Structure
  5. Technical Application Use

Classification of Materials

  1. Metals
  1. Ferrous Metals
    1. Has Iron
  2. Non-ferrous Metals
    1. No iron or small percentage of iron
  3. Usually good Electrical and Thermal Conductors
  4. At ordinary temperatures metals are usually solid
  5. To some extent metals are malleable and ductile.
  6. The freshly cut surfaces of metals are Lustrous (shiny)
  7. When struck metal produce typical sound
  8. Most of metals form alloys
  • Ceramics
  1. Inorganic materials consisting of both metallic and non-metallic elements bonded together chemically.
  2. Can be crystalline, non-crystalline or a mixture of both.
  3. Have high melting points and high chemical stability
    1. High temp = good corrosion resistance
  4. Usually Poor Electrical Conductors
  • Glasses
  1. Are considered to be Non-Crystalline Solids
  2. Can be crystalline, non-crystalline or a mixture of both
  3. Considered to be corrosion resistant
  • Polymers
  1. It consists of long molecular chains containing carbon compounds
  2. Most are non-crystalline, but some consists of both crystalline and non-crystalline regions
  3. Typically have low densities and are mechanically flexible
  4. Most are Poor Electrical Conductors
  • Elastomers
  1. A type of polymer
  2. More elastic than polymers
  3. Are based on polymers which have proper elasticity
  4. The most versatile of engineering materials
  5. Complex materials that have unique combinations of useful properties especially being elastic and its Resilience. Without permanent distortion.
  6. Ability to adhere to various fibers, metals and rigid plastics.
  • Hybrid
  1. Composite or advanced metals
  2. A combination of the previous 5 metals
  3. Combination of Two or More Materials in predetermined configuration and scale.
  4. Has remarkable benefits:
    1. Lightweight
    2. High strength
    3. Corrosion resistance
    4. High strength-to-weight ratio
    5. High electric strength
    6. High impact strength
    7. Non-magnetic
    8. Durable
    9. Design geometry
    10. Low maintenance

Properties of materials

  • Mechanical
  1. Hardness
    1. Ability of a material to resist penetration, wear or cutting action.
  2. Toughness
    1. Ability of a material to be deformed without breaking.
  3. Strength
    1. Ability of a material to resist deformation
  4. Stiffness
    1. Ability of a material to resist elastic deformation
  5. Brittleness
    1. Ability of a material to break when deformed or hammered
  6. Durability
    1. Ability of a material to withstand force over a period of time
  7. Ductility
    1. Ability of a material to be drawn into thinner sections without breaking
  8. Malleability
    1. Ability of a material to be stretched or shaped when drawn into thin sheets without breaking
  9. Conductivity
    1. Ability of a material to transmit heat or electricity
  10. Density
    1. The weight of a unit volume of a material
  11. Elasticity
    1. Ability of a material to be stretched and recover back to its original shape after deformation
  12. Plasticity
    1. Ability of a material to be stretched undergoing non-reversible changes when applied by force.
  13. Fusibility
    1. Ability of a material to become liquid when heated
  14. Thermal Expansion
    1. Ability of a material to expand/contract when there’s a change in temperature
  15. Corrosion Resistance
    1. Ability of a material to resist or withstand damage caused by oxidation or other chemical reactions
  16. Machinability
    1. A property of a material which governs the ease or difficulty that material can be machined/welded
  • Thermal Properties
  1. Thermal expansion
  • Magnetic Properties
  1. Magnetic or non magnetic
  • Electrical Properties
  1. Conductive or Insulator
  • Optical Properties
  1. Transparent or Translucent
  2. Reflectivity
  • Chemical Properties

Materials Engineering

  • Study of the process of materials (raw material ⇒ steel)



Deterioration and tension are two of the common problem areas of aircraft fabric


Probing is to determine whether a bonded joint shows signs of separation.


Repair or replace wood if any amount or form of decay is found.


A faint line running across the grain of a wood spar generally indicates decay.


Cellulose acetate and acrylic are thermoplastics.


Plywood is made up of two or three pieces of thin wood glued together with the same direction.


Finishing tapes should have the same properties as the fabric used to cover the aircraft.


Time is an important consideration in the bonding process of wood. Close assembly time is the period from the moment that the structure parts are placed together until clamping pressure is applied.



It is the maximum tensile laod per unit area which a material can withstand

Tensile Strength

The amount of expansion or contraction is predictable at specific temperatures is called__

Thermal expansion

Coefficient of Expansion/contraction?

Load per unit area acting on a material


Strain – extension/ unit length or original length

Number that measures an object or substance’s resistance to being deformed elastically when a force is applied to it.

Modulus of elasticity

Ratio of maximum energy absorbed per unit volume

Modulus of Resistance

It is defined as a material’s ability to resist deformation under load. The flexural strength represents the highest stress experienced within the material at its moment of rupture.

Flexural Strength

It is the difference in gage length before being subjected to any strength and after rupture.

Tensile Stress/Elongation??

The slope of tangent to the stress-strain curve

Tangent Modulus

The ability to carry heat

Thermal conductivity

The slope of secant to the curve that passes through the origin and a point on the curve

Secant modulus

It is this property that permits the manufacture of sheets, bar stocks, forgings, and fabrication by bending and hammering.


It is the ability of metal to resist deformation. Once the yield point is reached the metal deforms without an increase in the applied stress.

Yield strength

Bearing strength is the ability of a joint to withstand any form of crushing or excessive compressive distortion. Material under a compression load usually falls by buckling or bending. The force at which something buckles while being compressed varies with an object’s length, cross-sectional area, and shape.

Bearing strength

Refers to its ability to resist cutting, penetration, or abrasion


Material’s tendency to break or shatter when exposed to stress. Opposite of ductility and malleability.


Coefficient of elastic for a shearing force

Shear Modulus


Machine generated alternative text:
Federal Aviation 
Date: 9/8/98 
AC No: 43.13-1B 
Change: 1 
1. PURPOSE. This advisory circular (AC) contains methods. techniques. and practices acceptable to the 
Administrator for the inspection and repair of nonpressurized areas of civil aircraft. only when there are no 
manufacturer repair or maintenance instructions. This data generally pertains to Ininor repairs. The repairs 
identified in this AC may only be used as a basis for FAA approval for major repairs. The repair data may 
also be used as approved data, and the AC chapter. page, and paragraph listed in block S of FAA form 337


Machine generated alternative text:


  • Cross grain. Spiral grain, diagonal grain, or a combination of the two is acceptable providing the grain does not diverge from the longitudinal axis. 
  • Wavy, curly, and interlocked grain. Acceptable if local irregularities do not exceed limitations specified for spiral and diagonal grain.
  • Hard knots. Sound, hard knots up to 3/8 inch in diameter are acceptable providing: 
  • (1) they are not projecting portions of I-beams, along the edges of rectangular or beveled un-routed beams, or along the edges of flanges of box beams except in lowly stressed portions; 
  • (2) they do not cause grain divergence at the edges of the board or in the flanges of beams more than specified;
  • (3) they are in the center third of the beam and are not closer than 20 inches to another knot or other defect. 
  1. Pin knot clusters. Small clusters are acceptable providing they produce only a small deviation of grain direction. 
  2. Pitch pockets. Acceptable in the center portion of a beam providing they are at least 14 inches apart when they lie in the same growth ring and do not exceed 1-1/2 inches in length by 1/8 inch in depth.
  3. Mineral streaks. Acceptable providing that there is no decay indicated anywhere on the wood.


  1. Cross grain. Not acceptable if the grain exceeds longitudinal axis. 
  2. Wavy, curly, and interlocked grain. Not acceptable unless they are within the limitations specified in the description of acceptable defects listed previously. 
  3. Hard knots. Not acceptable unless they are within the limitations specified in the description of acceptable defects listed previously. 
  4. Pin knot clusters. Not acceptable if they produce a large effect on the direction of the grain. 
  5. Spike knots. Reject wood that contains this type of defect. 
  6. Pitch pockets. Not acceptable unless they are within the limitations specified in the description of acceptable defects listed previously. 
  7. Mineral streaks. Not acceptable if any decay is found. 
  8. Checks, shakes, and splits. Reject wood containing these defects. 
  9. Compression wood. Reject ‘wood that indicates compression wood. 
  10. Compression failures. Reject wood that contains an obvious compression failure. If there is a question as to whether wood indicates compression failure, perform a microscopic inspection or toughness test. 
  11. Decay. Reject wood that indicates any form of decay or rot including indications of red heart or purple heart.


The adhesive used in aircraft structural repair plays a critical role in the overall finished strength of the structure.


  1. Casein adhesive performance which was a powdered glue made from milk. Casein glue deteriorates over the years after it is exposed to moisture in the air and to wide variations in temperature. 
  1. Synthetic-resin adhesives comprise a broad family which includes plastic resin glue, resorcinol, hot-pressed Phenol, and epoxy
  1. Resorcinol glue is a two-part synthetic resin glue consisting of a resin and a hardener and is the most water-resistant of the glues used. The glue is ready for use as soon as the appropriate amount of hardener and resin has been thoroughly mixed. Resorcinol adhesive meets the strength and durability requirements of the FAA, making it one of the most common types of glue used in aircraft wood-structure repair.
  1. Phenol-formaldehyde glue is most commonly used in the manufacturing of aircraft-grade plywood. Phenol-formaldehyde glue requires high curing temperatures and pressures making it impractical for use in the field.
  1. Epoxy resins are two-part synthetic resins that generally consist of a resin and a hardener mixed together in specific quantities. Epoxies have excellent working properties and usually require less attention to joint quality or clamping pressures as compared to other aircraft adhesives. They penetrate evenly and completely into wood and plywood structures. However, varying degrees of humidity and temperature affects the joint durability in different epoxies.
  1. Plastic resin glue is a urea-formaldehyde resin that is water-, insect-, and mold-proof. This type of glue usually comes in a powdered form. It has been used in wood aircraft for many years. Caution should be used due to possible rapid deterioration (more rapidly than wood) of plastic resin glue in hot, moist environments and under cyclic swell-shrink stress. Plastic resin glue rapidly deteriorates in hot, moist environments, and under cyclic stresses, making it obsolete for all aircraft structural repairs. 



  1. Use moisture meters to determine the moisture content of the wood structure. The moisture content of any wooden member is an important factor in its structural integrity. Wood that is too wet or too dry may compromise the strength and integrity of the structure. A moisture meter reads the moisture content through a probe that is inserted into a wooden member. Use a correction card to correct for temperature and the type of wood being tested. 


  1. The wood structure may be inspected for structural integrity by tapping the suspect area with a light plastic hammer or screwdriver handle. Tapping should produce a sharp, solid noise from a solid piece of wood. If the wood area sounds hollow or feels soft, inspect further.


  1. Use prying to determine whether a bonded joint shows signs of separation. When prying a joint, be cautious not to use too much force, otherwise you may forcibly separate it. Light prying is sufficient to check the integrity of a joint. If there is any movement between the wood members of the joint, a failure of the bond is confirmed. Repair or replace the bonded structure if a failure has occurred. 


  1. Smell is a good indicator of musty or moldy areas. When removing the inspection panels, be aware of any odors that may indicate damage to the wood structure. Odor is an essential indicator of possible wood deterioration. Musty and moldy odors reveal the existence of moisture and possible wood rot.


  1. Visual inspection techniques are used to determine any visible signs of damage. Both internal and external visual examinations are imperative to a complete inspection of the wood structure. 
    • External Visual InspectionSplit or tear on the fabric covering, bulging on the surface of the wood caused by de-laminating. 
    • Internal Visual Inspection – Open and examine the internal structure if there is any reason to suspect glue failure or wood rot. This may entail creating inspection openings or even removing part of the skin.


The bonding process is critical to the structural strength of an aircraft wooden structure. To ensure the structural integrity of a wood joint, the bonding process must be carefully controlled

1. PREPARATION of the wood surface prior to applying the adhesive. 

  1. Wood Preparation Wood surfaces ready for bonding must be free from oil, wax, varnish, shellac, lacquer, enamel, dope, sealers, paint, dust, dirt, adhesive, crayon marks, and other extraneous materials. 

2. UTILIZATION of a good quality aircraft-standard adhesive that is properly prepared. Time is an important consideration in the bonding process. considering the ff (four type periods): 

  1. Pot life is the usable life of the adhesive from the time that it is mixed until it must be spread onto the wood surface. Once pot life has expired, the remaining adhesive must be discarded. Do not add thinning agents to the adhesive to extend the life of the batch. 
  2. Open assembly time is the period from the moment the adhesive is spread until the parts are clamped together. Where surfaces are coated and exposed freely to the air, some adhesives experience a much more rapid change in consistency than when the parts are laid together as soon as the spreading has been completed.
  3. Closed assembly time is the period from the moment that the structure parts are placed together until clamping pressure is applied. The consistency of the adhesive does not change as rapidly when the parts are laid together. 
  4. Pressing (or clamping) time is the period during which the parts are pressed tightly together and the adhesive cures. The pressing time must be sufficient to ensure that joint strength is adequate before handling or machining the bonded structure. 

Clamping Pressure is used to squeeze adhesive out into a thin, continuous film between the wood layers. This forces air from the joint and brings the wood surfaces into intimate contact. Pressure should be applied to the joint before the adhesive becomes too thick to flow and is accomplished by means of clamps, presses, or other mechanical devices. 

3. PERFORMING a good bonding technique consistent with the manufacturer’s instructions.



    • Low density polyethylene is made in thin, flexible sheet or film and is used for plastic bags, protective sheeting, and electrical insulation.
    • High-density polyethylene is used for containers such as fuel tanks, large drums and bottles. 
  3. VINYLS – manufactured in a variety of types and has a wide range of application. Their used in aircraft includes seat covering, electrical insulation, moldings, and tubing. They are flexible and resistant to most chemical and moisture
  4. ACRYLIC RESIN – a water clear plastic that has a light transmission of 92%. This property, together with its weather and moisture resistance, makes it an excellent product for aircraft windows and windshields. 
  5. POLYTETRAFLOUROETHYLENE (Teflon) – is encountered in non-lubricated bearings, tubing, electrical devices, and other applications.


Aircraft fabric covering is a term used for both the material used and the process of covering aircraft open structures. It is also used for reinforcing closed plywood structures.



  1. Fabric deteriorates only by exposure to ultraviolet radiation as used in an aircraft covering environment 


  1. Most fabrics obtains maximum tension on an airframe at 350 degrees Fahrenheit and will not be excessive on aircraft originally covered and doped

Aircraft Dope 

  1. A plasticized lacquer that is applied to fabric-coated aircraft. It tautens and stiffens fabric stretched over airframes and adheres and protects fabric applied to other skin material. 
  2. Typical doping agents include nitrocellulose, cellulose acetate and cellulose acetate butyrate. Liquid dopes are highly flammable; nitrocellulose – explosive propellant “guncotton”. Dopes will often include colouring pigments to facilitate even application.

Aircraft Fabric Synthetic 

  1. STC approved covering material – Difference in fabric may be denier, tenacity, thread count, weight, shrink, tension and weave style 
    • Tenacity– customary measure of strength of a fiber or yarn. 
    • Denier – measure of the linear density, the tenacity works out to be not a measure of force per unit area, but rather a quasidimensionless measure analogous to specific strength 
  1. Polyester Filaments 
    • Manufactured by polymerization of various select acids and alcohols, then extruding the resulting molten polymers through spinnerets to form filaments
  1. Covering Procedures 
    • Coating types, covering accessories, and covering procedures also may vary; therefore, the covering procedures given in the pertinent manuals must be followed to comply with the STC. 
  1. Installation 
    • Initial installation of polyester fabric is similar to natural fabric. The fabric is installed with as little slack as possible, considering fittings and other protrusions. 
    • slack-not using due diligence, care, or dispatch

Aircraft Fabric-Natural 

  1. Physical Specifications and minimum strength requirements for natural fabric fiber, cotton, and linen, used to recover or repair components of an aircraft.

Recovering Aircraft

  1. Recover or repair aircraft with a fabric of equal quality and strength to that used by the original aircraft manufacturer 
  2. note: recovering or repairing aircraft with any type fabric and/or coating other than the type used by the original aircraft manufacturer is considered a major alteration. Obtain approval form from then FAA on fabric and installation data. Cotton and linen rib lacing cord, machine and hand sewing thread, and finishing tapes should not be used with polyester and glass fabric covering

Reinforcing Tape

  1. Reinforcing tape should have a minimum 40 lbs. resistance without failure when static tested in shear against a single rib lace, or a pull through resistance when tested against a single wire clip, rivet screw, or any other type of fabric to rib attachment

Finishing Tape

  1. Sometimes referred as surface tape, should have the same properties as the fabric used to cover the aircraft

Lacing Cord

  1. Should have a minimum breaking strength of 40 lbs. Rib lace cord should have a micro-crystalline fungicidal wax, paraffin free wax, or beeswax coating, or other approved treatment to prevent wearing and fraying when pulling through the structure

Machine Thread and Hand sewing Thread 

  1. Machine Thread – Shall have a minimum breaking strength of 5 lbs 
  2. Hand sewing Thread – Shall have a minimum breaking strength of 14 lbs

Preparation of the structure for covering 

Battery Box Treatment 

  1. An Asphaltic, rubber-based acid-proof coating should be applied to the structure in the area of a battery by box, by brush, for additional protection from battery acid

Worn holes 

  1. Oversized screw holes or worn size 4 self-tapping screw holes through ribs and other structures used to attach fabric may be redrilled a minimum 1-1/2 hole diameter distance from the original hole location with a # 44 (0.086) drill bit.

Fairing Precaution 

  1. Aluminum leading edge replacement fairings installed in short sections may telescope during normal spar bending loads or from thermal expansion and contraction. 
  2. This action may cause a wrinkle to form in the fabric, at the edge of the lap joint. Trailing edges should be adequately secured to prevent movement and wrinkles.

Dope Protection

  1. Solvents found in nitrate and butyrate dope will penetrate, wrinkle, lift, or dissolve most-one part wood varnishes and one-part metal primers. All wood surfaces that come in contact with doped fabric should be treated with a protective coating such as aluminum foil, cellulose tape, or dope proof paint to protect them against the action of the solvents in the dope.


SEALANTS – used to contain fuel, maintain cabin pressure, reduce fire hazards, exclude moisture, prevent corrosion, and fill gaps and smooth discontinuities on the aircraft exterior. 

SEALING – is a process that confines liquids and gases within a given area or prevents them from entering areas from which they must be excluded.

Categories of Compounds

  1. Silicone compounds – are usually white, red, or grey in colour and are used in general where heat resistance is required. 
  2. Nonsilicone compounds – can be any colour and are used where heat resistance is not required

Specification / Classification 

  1. Class ABrushcoat Sealant. (Thinned with solvent to provide viscosity suitable for brushing). 
    • Class BFilleting Sealant. (Relatively heavy consistency with good thixotropic (lowslump) properties). 
    • Class CFaying Surface Sealant. (Medium consistency for good spreadability). 
    • Class DHole-Filling Sealant. (Similar to Class B but with very low slump.
    • Classes E and FSprayable sealant



  1. Time in hours after thawing during which the sealant can be readily extruded from the sealant gun and applied to the structure.
  2. Included in the BMS classification system as a dash number ff the classification letter (except for Class C).
    • Ex. Class B-2 indicates a fillet sealing  material with a minimum application time of 2 hours.
  3. Not applicable to one-part sealants.


  1. Time in hours after thawing during which a faying surface sealant can be squeezed out of a joint when fasteners are installed.
  2. Included in the BMS classification system as a dash number ff the Class C designation.
    • Ex. Class C-20 indicates a faying surface sealant with a minimum squeeze-out life of 20 hours.


  1. Time in hours after thawing (application for one-part silicones) that is required for the sealant to cure sufficiently so that it will not transfer to the finger or to a plastic film.


  1. For manufacturing purposes; time in hours after thawing (after application for one-part silicones) that is required for the sealant to cure firmly enough to be handled without damage or deformation.
  2. After cure time has elapsed, manufacturing operations such as drilling, and fastening can be performed without damage to the sealant. Maximum allowable cure time are specified in the applicable BMS, and typical cure time for most sealants are given in process specification BAC 5000.
  3. Curing of two-part materials is greatly retarded by temperature below 60-degree F and/or relative humidities below 40%.
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The curing may be accelerated by application of heat to accelerate the cure of sealant already applied. Heat may be furnished by the use of hot air blower, heat lamps, etc., or by prewarming the structure.




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o Grain Deviation 

▪     Whatever species of wood used for aircraft construction; its grain orientation should  always be straight. 

  1. A limit of variation 1:15 is permitted, indicating that the grain  must not have an angle of more than an inch per 15 inches. 

o Knots 

▪      Knots are the part where the tree branch sprouted from the trunk.

  1. There are different  types of shapes of knots depending on the cut from the wood e.g. round, oval, or  spiked. 
  2. Hard knots are allowed up to 3/8 inch of its dimension but with some restrictions. 
  3. Small pin-knots will only be allowed if it does not cause any grain imperfections to  the wood.

o Pitch Pockets 

▪     It is a type of wood imperfection caused by small holes in the annual rings of a tree. 

  1. Woods with pitch pockets are only allowed to be used if they are 14 inches apart and  with a volume of 11/2-inches x 1/8-inch x 1/8-inch deep. 

o Compression Wood 

  1. NEVER a good wood for aircraft construction and repair. 

▪     Wood taken from a tilted tree as it grows that lead to its wood having a denser and  weaker wood structure than a normal grown tree. 

o Compression Failure 

▪ Should not be mistaken for compression wood 

▪ Usually can be identified by its irregular and thread-like line on the grains 

▪ Also, NEVER a good wood for aircraft construction and repair. 

o Checks, Shakes, and Splits 

▪ A check is a crack running through or across the annual grain 

* A shake is a crack or separation in which it can be seen from a detached two annual  rings along its boundaries.

o Stains and Decay 

▪ Stains are caused by a decay on the wood usually appearing as streaks in the grains. 

▪ An evidence of a decay is the stain that uniformly discolored the annual rings. 

▪ Decay on woods varies in color from red to white stains. 

▪ Decay on woods no matter what its stage will lessen the toughness of the wood until  it gets brittle with little to no strength at all. 

▪ Decay on wood is caused by fungi growing in damp woods that eats its fiber. 

▪ Decay can be minimized by properly drying the wood up to 20% and the application  of wood varnishes for the wood to be protected from the elements. 


Its application to aircraft construction ranges from a  thermoset plastic reinforced fiberglass to thermoplastic material for windows. 





 Transparent Plastic 

Plastics or resins can be classified to 2 different classification according to their reaction to heat: 


o Thermoplastics when applied with can be mold to its desired shape and by cooling it down  will help it maintain its shape.  

o Thermoplastics can also be reheated and reshaped multiple times without changing its  chemical composition. 


Cellulose acetate • Tansparent • Light weight • Tendency to shrink • Tendency to turn its transparent  appearance to slightly yellow shade  Acrylic • Also known as Lucite, Plexiglass, or  Perspex • Stiffer that cellulose •  Clear transparent appearance 


o Thermosetting plastic when applied with heat can also be molded and shaped but when  cooled down, it cannot be reheated to be reshaped as it is fully cured (by heat or catalyst). o Can also be used as an adhesive and bonding agent 

o Can be combined and poured into different kind of materials 

Polyester Resin • Low-cost • Fast treating • Common handling techniques for  Fiber-reinforced polyester: o Autoclaving o Pultrusion o Filament winding o Press (vacuum bag) molding o Wet layup o Injection molding o Metal molding  Vinyl Ester Resin • All properties and characteristics are the  same as Polyester resin • Higher corrosion resistance than polyester  resin • Higher mechanical properties than  polyester resin  Phenolic Resin • Also known as Phenol-formaldehyde  resin • Shows low smoke and flammability  characteristics • Used in interior components  
Epoxy  • Have different variety of viscosity from liquid to solid • Used as structural adhesives and for  prepreg materials • High strength and modulus • Low volatility • Exceptional adhesion • Low shrinkage • Exceptional chemical resistance • Ease of use • Brittle • Reduced mechanical properties when  subjected to moisture • Usually longer to process than polyester resin • Common handling techniques: o Autoclaving o Pultrusion o Filament winding o Press (vacuum bag) molding o Resin transfer molding  Polyimides • Mostly used in high-temperature  settings e.g.: o Airframe structure o Hot engine o Circuit boards • Excellent high thermal resistance • Oxidative stability • Low amount of thermal expansion • Have high-temperature curing ▪ Polybenzimidazoles (PBI) • Best used in environments where  dangerously high heat-resistant resin is  needed. • Available in the form of fiber and adhesive  Bismaleimides (BMI) • Higher temperature and toughness characteristic  than epoxy resin • Used in airplane engines and high-temperature  components • Common handling techniques is similar to epoxy  resin: o Autoclave o Injection molding o Resin transfer molding  o Sheet molded compound (SMC)  


One example of a famous fabric covered aircraft is the Wright Flyer, due to its lightweight characteristics have been used for decades in aircraft  design and building.  

In the early times, finely woven organic fabrics like cotton and linen have been the initial choice for covering  airframes. Nowadays, the standard and mostly used for aircraft covering is Polyester fabric while cotton and  linen have been ceased in production due to its low resistance to environmental effects. 

Warp — the direction along the length of fabric 

Fill or weave — the direction across the width of the fabric. 

Count — the number of threads per inch in warp or  filling. 

Ply — the number of yarns making up a thread. 

Bias — a cut, fold, or seam made diagonally to the warp or fill threads. 

Pinked edge — an edge which has been cut by machine or special pinking shears in a continuous series  of Vs to prevent raveling. 

Selvage edge — the edge of cloth, tape, or webbing woven to prevent raveling. 

Greige — condition of polyester fabric upon completion of the production process before being heat  shrunk. 

Cross coat — brushing or spraying where the second coat is applied 90° to the direction the first coat  was applied. The two coats together make a single cross coat. 

Fabric Covering Processes 

Blanket Method 

o In this method, fabrics are attached to the airframe by trimming multiple flat sections and  using adhesives to stick to the airframe.  

Envelope Method 

o This method uses precut and pre-sewn envelope fabrics made-to-fit the airframe where it is  needed to be placed. Through the help of patterns, fabrics can be cut and sewn to the exact  size where it needed to be slid into position and will be fastened to the airframe by adhesives. 

Reinforcing Materials : Fibers used for reinforcing thermoset resin to produce high strength materials. 


Glass Fiber 

  1. Can be in the form of woven cloth (higher cost) or loosen mat(cheaper). Provide the resin  material matrix with enhanced strength and durability. 

Ceramic Fiber 

  1. Designed to be used in high-temperature components. It is  however more expensive and heavier in weight of glass fiber. 

Kevlar Fiber 

  1. Frequently used type of fiber where high impact resistance is needed. Identifiable by its  soft yellow color in the form of woven cloth. 

Graphite Fibers

obtained from Rayon fibers where the cellulose is heated and stretched to change the  molecular structure of the fiber into an extremely lightweight, strong, and tough material.


  1. Rubber is used to stop the entry of foreign materials like dirt, water, or air, and to stop a seepage of fluids,  gasses, or air. 
  2. Rubber is also a vibration absorbent, noise deadening, and impact load safeguard. 


Natural Rubber 

  1. has better ease of processing and properties including tensile strength, tear strength,  elasticity, flexibility, and accumulate heat buildup than of synthetic rubber. 
  2. The problem of  the general-purpose rubber is that the decline in its physical properties when subjected to fluids especially solvents in an aircraft causes faster deterioration than of synthetic rubber. 

Synthetic Rubber


Available in different composition that gives it different properties depending on  where it will be used. 

a. Butyl 

i. Superior resistance to gas saturation 

ii. High resistance to deterioration 

iii. Low water absorption 

iv. Good temperature resistance 

v. Lower physical properties than Natural  


vi. Best used for phosphate ester hydraulic  

fluid (also known as Skydrol), silicone  

fluids, ketones, acetones, and gases. 

b. Buna-S 

i. Have the same physical and processing  

characteristic to natural rubber 

ii. Water resistant 

iii. Good resistance to heat in the absence  

of harsh flexing  

iv. Poor resistance to oil, gasoline, solvents,  

and concentric acids 

v. Better substitute to natural rubber 

c. Buna-N 

i. Also known as nitrile rubber 

ii. Exceptional resistance to hydrocarbons & solvents 

iii. Low resistance to solvents at low  temperature 

iv. Good resistance to abrasion 

v. Used in automotive and aviation  industry to handle

oil and gasoline hoses, seals, tank lining, and for gaskets.

d. Neoprene 

i. Superior oil resistance 

ii. Good to use with nonaromatic gasoline  but bad with aromatic gasoline 

iii. Have similarity to natural rubber in the  appearance and texture 

iv. Tougher than natural rubber 

v. Tear and abrasive resistance are less than  of natural rubber 

vi. Exceptional resistance to the elements e. Thiokol 

i. Also known as polysulfide rubber 

ii. Best resistance to deterioration 

iii. Worst physical characteristics 

iv. Have similarity in usage as Buna-N 

f. Silicone 

i. Best to use where flexibility is needed in high and low temperatures 

ii. Good resistance to oils but adversely reactive to aromatic and nonaromatic gasoline 

g. Silastic 

i. One of the best kind of silicones 

ii. Good for insulating electrical and electronic components 

1. A thermoplastic cannot be reheated and reformed more than once. 

2. All wood and plywood used in the repair of aircraft structures should be of aircraft


3. A category of plastic material that is capable of softening or flowing when reheated is described as a thermoplastic. 

4. The strength classification of fabric used in aircraft covering is based upon tensile

strength. 5. Mineral streaks can lead to wood decay. 

6. Repair or replace wood if any amount or form of decay is found. 

7. Probing is to determine whether a bonded joint shows signs of separation. 

8. Deterioration and tension are two of the common problem areas of aircraft fabric. 

9. Britain Perspex is a tradename for Cellulose acetate.

 10. Repair, rather than replace extensively damaged transparent plastic. 

11. Plastics should not be rubbed with a dry cloth since this is likely to cause scratches, and to build up an electrostatic charge that attracts dust particles to the surface. 

12. Plastics have many advantages over glass for aircraft use, but they lack the

toughness property. 

13. Finishing Tapes should have the same properties as the fabric used to cover the aircraft. 

14. Cellulose acetate and acrylic are thermoplastics. 

15.Acrylic was used in the past aircraft but since it is dimensionally unstable and turns yellow after it has been installed for a time. 

16. Time is an important consideration in the bonding process of wood. Close assembly time is the period from the moment that the structure parts are placed together until clamping pressure is applied. 

17. Fabric must be protected from deterioration so that coatings can be applied and give its infinite service life. 

18. The strength classification of fabric used in aircraft covering is based upon compressive strength. 

19. Laminated wood spars may be substituted for solid rectangular wood spars only in

certain instances where the primary load is shared by one or more other original structural member. 

20. Wood with checks, shakes or splits can be used providing the damage is repaired by gluing and clamping. 

21. A faint line running across the grain of a wood spar generally indicates decay. 

22. Plywood is made up of two or three pieces of thin wood glued together with the

same direction.


1. The property of a metal which allows little bending or deformation without shattering. 


2. Units used in measuring the hardness of a material. Pascal 

3. The ease with which a material can be forged, rolled, and extruded without fracture is an indication of a material’s. 


4. The ability of a metal to melt. 


5. Loads that are used in computing the Young’s modulus. 

Stress and Strain

6. The property that defines the mass per unit volume of a material. 


7. The ability of a materials to deform and change shape under compressive stress.


8. The ability of a materials to deform and change shape under tensile stress.


9. Property that is necessary in sheet metal that is worked into curved shapes such as cowlings, fairings, or wingtips. 


10. The lack of crystal structure suppresses what property? 



MODULE 4 Non-Ferrous Metals and its Alloys 


  1. The term “nonferrous” refers to all metals which have elements other than iron  as their base or principal constituent. 
  2. This group includes such metals as  aluminum, titanium, copper, and magnesium, as well as such alloyed metals  as Monel and Babbitt. 

Aluminum and Aluminum Alloys 

Figure 1. 


Pure aluminum is a white lustrous metal which stands second in  the scale of malleability, sixth in ductility, and ranks high in its resistance to  corrosion. 

Aluminum combined with various percentages of other metals forms  alloys which are used in aircraft construction. 

Aluminum alloys in which the principal alloying ingredients are manganese,  chromium, or magnesium and silicon show little attack in corrosive  environments. 

Alloys in which substantial percentages of copper are used are  more susceptible to corrosive action. The total percentage of alloying  elements is seldom more than 6 or 7 percent in the wrought alloys. 


It is vital to the aviation industry because of its high strength to  weight ratio and its comparative ease of fabrication. The outstanding  characteristic of aluminum is its light weight. Aluminum melts at the  comparatively low temperature of 1,250 °F. It is nonmagnetic and is an  excellent conductor.  

Commercially pure aluminum has a tensile strength of about 13,000 psi, but its  strength may be approximately doubled by rolling or other cold working  processes. By alloying with other metals, or by using heat-treating processes,  the tensile strength may be raised to as high as 65,000 psi or to within the  strength range of structural steel. 

Aluminum alloys, although strong, are easily worked because they are  malleable and ductile. They may be rolled into sheets as thin as 0.0017 inch or  drawn into wire 0.004 inch in diameter. Most aluminum alloy sheet stock used  in aircraft construction range from 0.016 to 0.096 inch in thickness; however,  some of the larger aircraft use sheet stock which may be as thick as 0.356 inch. 


  1. Casting Alloy – those suitable for casting in sand, permanent mold, or die  castings
  2. Wrought alloys – those which may be shaped by rolling,  drawing, or forging. Most widely used  in aircraft construction, being used for stringers, bulkheads, skin, rivets, and  extruded sections. 


  1. Properties of the alloys are determined by the alloying elements and cannot  be changed after the metal is cast.
  2. Alloying elements make it  possible to heat treat the casting to produce the desired physical properties. 

The casting alloys are identified by a letter preceding the alloy number. When  a letter precedes a number, it indicates a slight variation in the composition of  the original alloy. This variation in composition is simply to impart some desirable  quality. 

In casting alloy 214, for example, the addition of zinc to improve its  pouring qualities is indicated by the letter A in front of the number, thus  creating the designation A214. 

When castings have been heat treated, the heat treatment and the  composition of the casting is indicated by the letter T, followed by an alloying  number. 

An example of this is the sand-casting alloy 355, which has several  different compositions and tempers and is designated by 355-T6, 355-T51, or  C355-T51. 


  1. Permanent mold,    Die cast,    Sand mold

In casting aluminum, in most cases different types of alloys must be used for  different types of castings. Sand casting san & die castings require different  types of alloys than those used in permanent molds.


  1. Parts produced by pouring molten  metal into a previously prepared mold, allowing the metal to solidify or freeze,  and then removing the part
  2. Produced by pouring liquid metal  into the mold, the metal flowing under the force of gravity alone. 


The two principal types of sand-casting alloys are 112 and 212. Little difference  exists between the two metals from a mechanical properties’ standpoint, since  both are adaptable to a wide range of products. 


Metallic mold    Ex. Cast Iron

Major difference being in the material from which the molds are  made: MOLD PROCESS

The permanent mold process is a later development of the sand casting  process. The advantage of this process is that there are fewer openings (called  porosity) than in sand castings. The sand and the binder, which is mixed with  the sand to hold it together, give off a certain amount of gas which causes porosity in a sand casting. 

Permanent mold castings are used to obtain higher mechanical properties,  better surfaces, or more accurate dimensions. 


  1. Permanent metal mold with metal cores
  2. Semi permanent types containing sand cores

Because finer grain structure is  produced in alloys subjected to the rapid cooling of metal molds, they are far  superior to the sand type castings. Alloys 122, A132, and 142 are commonly  used in permanent mold castings, the principal uses of which are in internal  combustion engines. 


Die castings used in aircraft are usually aluminum or magnesium alloy. If weight  is of primary importance, magnesium alloy is used because it is lighter than  aluminum alloy. 

Aluminum alloy is frequently used because it is  stronger than most magnesium alloys. 

A die casting is produced by forcing molten metal under pressure into a  metallic die and allowing it to solidify; then the die is opened, and the part  removed. 


  1. Permanent mold processes the metal flows into the die  under gravity.
  2. In the die casting operation, the metal is forced under great  pressure. 

Die castings are used where relatively large production of a given part is  involved. Remember, any shape which can be forged can be cast. 



Those in which the mechanical properties are  determined by the amount of cold work introduced after the final annealing  operation. The mechanical properties obtained by cold working are destroyed  by any subsequent heating and cannot be restored except by additional cold  working, which is not always possible. 

The “full hard” temper is produced by  the maximum amount of cold work that is commercially practicable. 

Metal in  the “as fabricated” condition is produced from the ingot without any  subsequent controlled amount of cold working or thermal treatment. There is,  consequently, a variable amount of strain hardening, depending upon the  thickness of the section. 


The mechanical properties are obtained  by heat treating to a suitable temperature, holding at that temperature long  enough to allow the alloying constituent to enter into solid solution, and then  quenching to hold the constituent in solution. The metal is left in a  supersaturated, unstable state and is then age hardened either by natural  aging at room temperature or by artificial aging at some elevated  temperature. 

WROUGHT ALUMINUM                            Wrought vs. Cast Aluminum 

Wrought aluminum and wrought aluminum alloys are designated by a four-digit index system. 

The system is broken into three distinct groups: 

  1. 1xxx group     ● 2xxx through 8xxx group    ● 9xxx group (which is currently unused)
  1. The first digit of a designation identifies the alloy type. 
  2. The second digit  indicates specific alloy modifications. 

IF 2ND NUMBER IS ZERO: it would indicate no special control over individual impurities.

Digits 1 through 9,  however, when assigned consecutively as needed for the second number in  this group, indicate the number of controls over individual impurities in the  metal. 

The last two digits of the 1xxx group are used to indicate the hundredths of 1  percent above the original 99 percent designated by the first digit. Thus, if the  last two digits were 30, the alloy would contain 99 percent plus 0.30 percent of  pure aluminum, or a total of 99.30 percent pure aluminum. Examples of alloys  in this group are: 

• 1100—99.00 percent pure aluminum with one control over individual  impurities. 

• 1130—99.30 percent pure aluminum with one control over individual  impurities. 

• 1275—99.75 percent pure aluminum with two controls over individual  impurities. 

In the 2xxx through 8xxx groups,

 First digit indicates the major alloying  element used in the formation of the alloy as follows: 

• 2xxx—copper 

• 3xxx—manganese 

• 4xxx—silicon 

• 5xxx—magnesium 

•6xxx—magnesium and silicon 

• 7xxx—zinc 

• 8xxx—other elements 

Second digit in the alloy designation  indicates alloy modifications. 

IF THE 2ND DIGIT IS ZERO: It indicates the original  alloy.

Digits 1 through 9 indicate alloy modifications. 

The last two of the  four digits in the designation identify the different alloys in the group. [Figure 5- 4]


1000 series

  1. 99 percent aluminum or higher, 
  2. excellent corrosion resistance, 
  3. high  thermal and electrical conductivity,
  4. low mechanical properties, 
  5. excellent  workability. 
  6. Iron and silicon are major impurities. 

2000 series

  1. Copper is the principal alloying element. 
  2. Solution heat treatment,  
  3. optimum properties equal to mild steel, 
  4. poor corrosion resistance unclad. 
  5. It is  usually clad with 6000 or high purity alloy. 
  6. Its best-known alloy is 2024. 

3000 series

  1. Manganese is the principal alloying element

of this group which is  generally non-heat treatable. 

  1. The percentage of manganese which will be  

alloy effective is 1.5 percent. 

  1. The most popular is 3003, 

which is of moderate  strength 

and has good working characteristics. 

4000 series

  1. Silicon is the principal alloying element of this

group, lowers  melting temperature. 

  1. Its primary use is in welding and brazing.
  2. When used in  welding heat-treatable alloys,

this group will respond to a limited amount 

of  heat treatment. 

5000 series

  1. Magnesium is the principal alloying element
  2. It has good welding
  3. corrosion resistant characteristics
  4. High temperatures (over 150 °F) or  excessive

cold working will increase susceptibility to corrosion. 

6000 series

  1. Silicon and magnesium form magnesium silicide

which makes  alloys heat treatable.

  1. It is of medium strength, good forming qualities,
  2. Has  corrosion resistant characteristics. 

7000 series

  1. Zinc is the principal alloying element. 
  2. The most popular alloy of the  series is 6061. 
  3. When coupled with magnesium, it results in 

heat-treatable alloys  of very high strength.

  1. It usually has copper and chromium added.
  2. The principal  alloy of this group is 7075.


Where used, the temper designation follows the alloy designation and is  separated from it by a dash: i.e., 7075-T6, 2024-T4, and so forth. 

The temper  designation consists of a letter indicating the basic temper which may be more  specifically defined by the addition of one or more digits. These designations  are as follows: 

• F — as fabricated 

• O — annealed, recrystallized (wrought products only) 

• H — strain hardened 

• H1 (plus one or more digits) — strain hardened only 

• H2 (plus one or more digits) — strain hardened and partially annealed

• H3 (plus one or more digits) — strain hardened and stabilized 

The digit following the designations H1, H2, and H3 indicates the degree of  strain hardening,

number 8 representing the ultimate tensile strength equal to  that achieved by a cold reduction of approximately 75 percent following a full  anneal, 0 representing the annealed state. 


Text Box: Northrop XP-56 Black Bullet
• Experimental Flying Wing Fighter

Mg Components on Aircraft – Historical 1943 – 1944 (prototypes)

The All Magnesium Aircraft

  1. Mg Alloy Airframe & Skin
  2. Heliarc welded structure


Magnesium, the world’s lightest structural metal, is a silvery white material  weighing only two-thirds as much as aluminum.

Magnesium does not possess  sufficient strength in its pure state for structural uses, but when alloyed with zinc,  aluminum, and manganese it produces an alloy having the highest strength to  weight ratio of any of the commonly used metals.

Magnesium is probably more  widely distributed in nature than any other metal. It can be obtained from such  ores as dolomite and magnesite, and from sea water, underground brines, and  waste solutions of potash. With about 10 million pounds of magnesium in 1  cubic mile of sea water, there is no danger of a dwindling supply. 

Some of today’s aircraft require in excess of one-half ton of this metal for use  in hundreds of vital spots. Some wing panels are fabricated entirely from  magnesium alloys, weigh 18 percent less than standard aluminum panels, and  have flown hundreds of satisfactory hours.

Among the aircraft parts that have  been made from magnesium with a substantial savings in weight are  nosewheel doors, flap cover skin, aileron cover skin, oil tanks, floorings, fuselage  parts, wingtips, engine nacelles, instrument panels, radio masts, hydraulic fluid  tanks, oxygen bottle cases, ducts, and seats. 

Magnesium alloys possess good casting characteristics.

Their properties  compare favorably with those of cast aluminum. In forging, hydraulic presses  are ordinarily used, although, under certain conditions, forging can be  accomplished in mechanical presses or with drop hammers. 

Magnesium alloys are subject to such treatments as annealing, quenching,  solution heat treatment, aging, and stabilizing.

Sheet and plate magnesium  are annealed at the rolling mill. The solution heat treatment is used to put as  much of the alloying ingredients as possible into solid solution, which results in  high tensile strength and maximum ductility. Aging is applied to castings  following heat treatment where maximum hardness and yield strength are  desired. 

Magnesium embodies fire hazards of an unpredictable nature.

When in large  sections, its high thermal conductivity makes it difficult to ignite and prevents it  from burning. It will not burn until the melting point of 1,204 °F is reached.  However, magnesium dust and fine chips are ignited easily. Should a fire occur, it can be extinguished  with an extinguishing powder, such as soapstone or graphite. Water or any  standard liquid or foam fire extinguisher cause magnesium to burn more rapidly and can cause explosions. 

Magnesium alloys produced in the United States consist of magnesium alloyed  with varying proportions of aluminum, manganese, and zinc. These alloys are  designated by a letter of the alphabet, with the number 1 indicating high purity  and maximum corrosion resistance. 

Many of the magnesium alloys manufactured in the United States are  produced by the Dow Chemical Company and have been given the trade name of Dowmetal ™ alloys. To distinguish between these alloys, each is  assigned a letter. Thus, we have Dowmetal J, Dowmetal M, and so forth.  

Another manufacturer of magnesium alloy is the American Magnesium  Corporation, a subsidiary of the Aluminum Company of America. This  company uses an identification system like that used for aluminum alloys, with  the exception that magnesium alloy numbers are preceded with the letters  AM. Thus, AM240C is a cast alloy, and AM240C4 is the same alloy in the heat treated state. AM3S0 is an annealed wrought alloy, and AM3SRT is the same  alloy rolled after heat treatment. 



  1. Used mainly in aerospace, marine, chemical, biomedical application and sports (ex. Motorcycle)
  2. Turbine blades, hip-joint component, shape memory alloy


Titanium was discovered by an English priest named Gregot. A crude  separation of titanium ore was accomplished in 1825.

In 1906 enough pure  titanium was isolated in metallic form to permit a study. Following this study, in  1932, an extraction process was developed which became the first  commercial method for producing titanium. The United States Bureau of Mines  began making titanium sponge in 1946, and 4 years later the melting process  began. It is used in many commercial enterprises  and is in constant demand for such items as pumps, screens, and other tools  and fixtures where corrosion attack is prevalent. 

In aircraft construction and repair, titanium is used for fuselage skins, engine shrouds, firewalls, longerons,  frames, fittings, air ducts, and fasteners. 

Titanium is used for making compressor disks, spacer rings, compressor blades  and vanes, through bolts, turbine housings and liners, and miscellaneous  hardware for turbine engines. 

Titanium is like stainless steel. One quick method used to  identify titanium is the spark test. Titanium gives off a brilliant white trace ending  in a brilliant white burst. 

Identification can be accomplished by  moistening the titanium and using it to draw a line on a piece of glass. This will  leave a dark line similar in appearance to a pencil mark. 

  1. Titanium falls between aluminum and stainless steel in terms of elasticity,  density, and elevated temperature strength.
  2. It has a melting point of from 2,730  °F to 3,155 °F,
  3. low thermal conductivity, and a low coefficient of expansion. 
  4. It  is light, strong, and resistant to stress corrosion cracking. 
  5. Titanium is  approximately 60 percent heavier than aluminum and about 50 percent  lighter than stainless steel. 
  6. Because of the high melting point of titanium, high temperature properties are  disappointing. 

The ultimate yield strength of titanium drops rapidly above 800  °F. The absorption of oxygen and nitrogen from the air at temperatures above  1,000 °F makes the metal so brittle on long exposure that it soon becomes  worthless. However, titanium does have some merit for short time exposure up  to 3,000 °F where strength is not important. Aircraft firewalls demand this  requirement. 

Titanium is nonmagnetic and has an electrical resistance comparable to that  of stainless steel. 

Heat  treating and alloying do not develop the hardness of titanium to the high levels  of some of the heat-treated alloys of steel. It was only recently that a heat treatable titanium alloy was developed. Prior to the development of this alloy,  heating and rolling was the only method of forming that could be  accomplished. However, it is possible to form the new alloy in the soft condition  and heat treat it for hardness. 

Iron, molybdenum, and chromium are used to stabilize titanium and produce  alloys that will quench harden and age harden. The addition of these metals  also adds ductility. The fatigue resistance of titanium is greater than that of  aluminum or steel. 

Titanium becomes softer as the degree of purity is increased. It is not practical  to distinguish between the various grades of commercially pure or unalloyed  titanium by chemical analysis; therefore, the grades are determined by  mechanical properties. 


The A-B-C classification of titanium alloys was established to provide a  convenient and simple means of describing all titanium alloys. 


A (alpha) 

  1. all-around performance;             ● tough and strong both cold and hot;
  2. good weldability;                 ● resistant to oxidation.

B (beta)

  1. bendability;                     ● strong both cold and hot,
  2. excellent bend ductility;             ● but vulnerable to contamination.

C (combined alpha and beta for compromise performances) 

  1. strong when  cold and warm, but weak when hot; 
  2. good bendability;         ● moderate  contamination resistance; excellent forgeability. 


Commercially Pure Titatium

  1. A-55 is an example of a  commercially pure titanium.
  2. It has a yield strength of 55,000 to 80,000 psi and is  a general-purpose grade for moderate to severe forming. It is sometimes used  for nonstructural aircraft parts and for all types of corrosion resistant  applications, such as tubing. 
  3. Type A-70 titanium is closely related to type A-55  but has a yield strength of 70,000 to 95,000 psi. It is used where higher strength  is required, and it is specified for many moderately stressed aircraft parts. For  many corrosion applications, it is used interchangeably with type A-55. Both  type A-55 and type A-70 are weldable. 

Alloyed Titanium

  1. One of the widely used titanium base alloys is designated as C-110M.
  2. It is used  for primary structural members and aircraft skin, has 110,000 psi minimum yield  strength, and contains 8 percent manganese. 
  3. Type A-110AT is a titanium alloy which contains 5 percent aluminum and 2.5  percent tin. 
  4. It also has a high minimum yield strength at elevated temperatures  with the excellent welding characteristics inherent in alpha-type titanium  alloys. 


  1. The resistance  of the metal to corrosion is caused by the formation of a protective surface  film of stable oxide or chemi-absorbed oxygen. 
  2. Film is often produced by the  presence of oxygen and oxidizing agents. 
  3. Corrosion of titanium is uniform.
  4. There is little evidence of pitting or other serious  forms of localized attack. Normally, it is not subject to stress corrosion, corrosion  fatigue, intergranular corrosion, or galvanic corrosion. Its corrosion resistance is  equal or superior to 18-8 stainless steel. 
  5. Copper and copper alloys, suited to a wide range of ...

Laboratory tests with acid and saline solutions show titanium polarizes readily.  

  1. The net effect, in general, is to decrease current flow in galvanic and corrosion  cells. 
  2. Corrosion currents on the surface of titanium and metallic couples are  naturally restricted. This partly accounts for good resistance to many  chemicals; also, the material may be used with some dissimilar metals with no  harmful galvanic effect on either.



  1. Copper is one of the most widely distributed metals. 
  2. It is the only reddish  colored metal and is second only to silver in electrical conductivity. 
  3. Its use as  a structural material is limited because of its great weight.
  4. Its high electrical and heat conductivity,  in many cases overbalance the weight factor.
  5. COPPER: IDEAL FOR MAKING WIRE: malleable and ductile.
  6. It is  corroded by salt water but is not affected by fresh water. The ultimate tensile  strength of copper varies greatly. For cast copper, the tensile strength is about  25,000 psi, and when cold rolled or cold drawn its tensile strength increases to  a range of 40,000 to 67,000 psi. 
  7. In aircraft, copper is used primarily in the electrical system for bus bars,  bonding, and as lockwire. 


  1. Beryllium copper is one of the most successful of all the copper base alloys. 
  2. It  is a recently developed alloy containing about 97 percent copper, 2 percent  beryllium, and sufficient nickel to increase the percentage of elongation. 
  3. The  most valuable feature of this metal is that the physical properties can be  greatly stepped up by heat treatment, the tensile strength rising from 70,000 psi  in the annealed state to 200,000 psi in the heat-treated state.
  4. The resistance of  beryllium copper to fatigue and wear makes it suitable for diaphragms,  precision bearings and bushings, ball cages, and spring washers. 


  1. Brass is a copper alloy containing zinc and small amounts of aluminum, iron,  lead, manganese, magnesium, nickel, phosphorous, and tin. Brass with a zinc  content of 30 to 35 percent is very ductile, but that containing 45 percent has  relatively high strength. 


  1. Muntz metal is a brass composed of 60 percent copper and 40 percent zinc. 
  2. It has excellent corrosion resistant qualities in salt water. 
  3. Its strength can be  increased by heat treatment. 
  4. As cast, this metal has an ultimate tensile  strength of 50,000 psi, and it can be elongated 18 percent. 
  5. It is used in  making bolts and nuts, as well as parts that encounter salt water. 


  1. Red brass, sometimes termed “bronze” because of its tin content, is used in fuel  and oil line fittings. This metal has good casting and finishing properties and  machines freely. 


  1. Bronzes are copper alloys containing tin.
  2. The true bronzes have up to 25  percent tin, but those with less than 11 percent are most useful, especially for  such items as tube fittings in aircraft. 
  3. Aluminum bronzes rank very high in aircraft usage. 
  4. They would find greater  usefulness in structures if it were not for their strength to weight ratio as  compared with alloy steels. 
  5. Wrought aluminum bronzes are almost as strong  and ductile as medium carbon steel, and they possess a high degree of  resistance to corrosion by air, salt water, and chemicals. They are readily  forged, hot or cold rolled, and many react to heat treatment. 
  6. These copper base alloys contain up to 16 percent of aluminum (usually 5 to  11 percent), to which other metals, such as iron, nickel, or manganese, may  be added.
  7. Aluminum bronzes have good tearing qualities, great strength,  hardness, and resistance to both shock and fatigue.
  8. Because of these properties, they are used for diaphragms, gears, and pumps. Aluminum  bronzes are available in rods, bars, plates, sheets, strips, and forgings. 


  1. Using about 89 percent copper, 9 percent aluminum,  and 2 percent of other elements,
  2. have high strength combined with ductility,  and are resistant to corrosion, shock, and fatigue.
  3. Because of these properties,  cast aluminum bronze is used in bearings and pump parts.
  4. These alloys are  useful in areas exposed to salt water and corrosive gases. 


  1. Exceptionally high strength, tough, corrosion resistant  copper zinc alloy containing aluminum, manganese, iron and, occasionally,  nickel or tin.
  2. This metal can be formed, extruded, drawn, or rolled to any  desired shape.
  3. In rod form, it is generally used for machined parts, for aircraft  landing gears and brackets. 


  1. A more recent development composed of about 95 percent  copper, 3 percent silicon, and 2 percent manganese, zinc, iron, tin, and  aluminum. 
  2. Although not a bronze in the true sense because of its small tin  content, silicon bronze has high strength and great corrosion resistance. 


  1. The leading high nickel alloy,
  2. combines the properties of high strength  and excellent corrosion resistance.
  3. This metal consists of 68 percent nickel, 29  percent copper, 0.2 percent iron, 1 percent manganese, and 1.8 percent of  other elements. It cannot be hardened by heat treatment. 
  4. Adaptable to casting and hot or cold working, can be successfully  welded. It has working properties similar to those of steel.
  5. When forged and  annealed, it has a tensile strength of 80,000 psi. This can be increased by cold  working to 125,000 psi, sufficient for classification among the tough alloys. 
  6. Monel has been successfully used for gears and chains to operate retractable  landing gears, and for structural parts subject to corrosion.
  7. In aircraft, Monel is  used for parts demanding both strength and high resistance to corrosion, such  as exhaust manifolds and carburetor needle valves and sleeves. 


  1. A nonferrous alloy containing mainly nickel, copper, and aluminum. 
  2. It is produced by adding a small amount of aluminum to the Monel formula.
  3. It  is corrosion resistant and capable of being hardened by heat treatment.  
  4. K-Monel has been successfully used for gears, and structural members in  aircraft which are subjected to corrosive attacks. 
  5. This alloy is nonmagnetic at  all temperatures. K-Monel sheet has been successfully welded by both  oxyacetylene and electric arc welding.




  1. Monel contains about 68 percent nickel and 29 percent copper, plus  small amounts of iron and manganese.
  2. Nickel alloys can be welded or easily  machined. Some of the nickel Monel, especially the nickel Monels containing  small amounts of aluminum, are heat-treatable to similar tensile strengths of  steel. 
  3. Nickel Monel is used in gears and parts that require high strength and  toughness, such as exhaust systems that require high strength and corrosion  resistance at elevated temperatures.


  1. Inconel alloys of nickel produce a high strength, high temperature alloy  containing approximately 80 percent nickel, 14 percent chromium, and small  amounts of iron and other elements.
  2. The nickel Inconel alloys are frequently  used in turbine engines because of their ability to maintain their strength and  corrosion resistance under extremely high temperature conditions. 
  3. Inconel and stainless steel are similar in appearance and are frequently found  in the same areas of the engine. Sometimes it is important to identify the  difference between the metal samples.
  4. A common test is to apply one drop  of cupric chloride and hydrochloric acid solution to the unknown metal and  allow it to remain for 2 minutes. At the end of the soak period, a shiny spot  indicates the material is nickel Inconel, and a copper colored spot indicates  stainless steel.



MODULE 05: Ferrous Metals and  its Alloys 


  1. The term ferrous applies to the group of metals having  iron as their principal constituent. 
  2. If carbon is added to iron, in percentages ranging up to approximately 1  percent, the product is vastly superior to iron alone and is classified as carbon  steel.
  3. Carbon steel forms the base of those alloy steels produced by combining  carbon steel with other elements known to improve the properties of steel.
  4. base metal (such as iron) to which small quantities of other metals have been  added is called an alloy
  5. The addition of other metals changes or improves the  chemical or physical properties of the base metal for a particular use. 

The most common ferrous metal in aircraft structures is steel, an alloy of iron with a controlled amount of carbon added.

Alloy – metals created by mixing two other metals together (a catalyst and an ore)

Iron – soft, malleable and ductile in its pure form.

Carbon – makes the steel heat treatable and hard

Society of Automotive Engineers (SAE) – Due to the large number of elements

that will combine with iron, an infinite number of steels is obtainable.


1ST – Basic alloying element

2nd – Percentage of the basic element in the alloy

3rd – Percentage of carbon in the alloy in hundredth of a percent

Carbon Steel1xxx
Nickel Steel2xxx
Nickel Chromium Steel3xxx
Chromium Molybdenum Steel4xxx
Chromium Steel5xxx
Chromium Vanadium Steel6xxx
Tungsten Steel7xxx
National Emergency Steel8xxx
Silicon Manganese Steel9xxx

TUNGSTEN STEEL – Red hardness property. Typically used for breaker contacts in magnetos and for high speed cutting tools.


  1. Silicon – deoxidizer; improves ductility.
  2. Manganese –desulphurizer, hardener, improves penetration hardness and forging qualities
  3. Silicon Manganese – good impact resistance



  1. AUESTENITIC STEEL – 200 and 300 series; chromium nickel alloy, and some manganese (200 ser.); non heat treatable
  2. FERRITIC STEEL – Chromium alloy but may also contain little aluminum; no carbon content; non heat treatable
  3. MARTENSITIC STEEL – 400 series; chromium alloy only; magnetic


Numerical Index

  1. Used to identify the chemical compositions of the structural steels.
  2. The list of  standard steels is altered from time to time to accommodate steels of proven  merit and to provide for changes in the metallurgical and engineering  requirements of industry. 


  1. A four numeral series is used to designate the plain carbon and alloy steels;
  2. Five numerals are used to designate certain types of alloy steels.
  3. The first two  digits indicate the type of steel,
  4. the second digit also generally (but not  always) gives the approximate amount of the major alloying element,
  5. the last two (or three) digits are intended to indicate the approximate  amount of carbon. However, a deviation from the rule of indicating the  carbon range is sometimes necessary.


  1. Small quantities of certain elements are present in alloy steels that are not  specified as required.
  2. These elements are considered as incidental and may  be present to the maximum amounts as follows: copper, 0.35 percent; nickel,  0.25 percent; chromium, 0.20 percent; molybdenum, 0.06 percent. 


  1. Metal stock manufactured in several forms and shapes, including sheets,  bars, rods, tubing’s, extrusions, forgings, and castings. 
  2. Sheet metal is made in  several sizes and thicknesses. Specifications designate thicknesses in  thousandths of an inch.
  3. Bars and rods are supplied in a variety of shapes, such  as round, square, rectangular, hexagonal, and octagonal.
  4. Tubing can be  obtained in round, oval, rectangular, or streamlined shapes. The size of tubing  is generally specified by outside diameter and wall thickness. 
  5. The sheet metal is usually formed cold in such machines as presses, bending  brakes, draw benches, or rolls.
  6. Forgings are shaped or formed by pressing or  hammering heated metal in dies.
  7. Castings are produced by pouring molten  metal into molds. The casting is finished by machining. 
  8. Spark testing is a common means of identifying various ferrous metals. In this  test the piece of iron or steel is held against a revolving grinding stone and the  metal is identified by the sparks thrown off.

Each ferrous metal has its own  peculiar spark characteristics. The spark streams vary from a few tiny shafts to a shower of sparks several feet in length. (Few nonferrous metals give off sparks  when touched to a grinding stone. Therefore, these metals cannot be  successfully identified by the spark test.) 

Identification by spark testing is often  inexact unless performed by an experienced person, or the test pieces differ  greatly in their carbon content and alloying constituents.

  1. Wrought iron  produces long shafts that are straw colored as they leave the stone and white  at the end.
  2. Cast iron sparks are red as they leave the stone and turn to a straw  color. Low-carbon steels give off long, straight shafts having a few white sprigs.  As the carbon content of the steel increases, the number of sprigs along each  shaft increases, and the stream becomes whiter in color. 
  3. Nickel steel causes the spark stream to contain small white blocks of light within the main  burst. 



Carbon steels 

  1. The first type of steel derives its physical properties from the presence of varying  quantities of carbon.
  2. The most common type of steel produced  worldwide.

Low-carbon steel 

  1. Steel containing carbon in percentages ranging from 0.10 to 0.30 percent is  classed as low-carbon steel.
  2. The equivalent SAE numbers range from 1010 to 1030. Steels of this grade are used for making such items as safety wire, certain  nuts, cable bushings, or threaded rod ends.
  3. This steel in sheet form is used for  secondary structural parts and clamps and in tubular form for moderately  stressed structural parts. 

Medium-carbon steel 

  1. Steel containing carbon in percentages ranging from 0.30 to 0.50 percent.
  2. This steel is especially adaptable for  machining or forging, and where surface hardness is desirable.
  3. Certain rod  ends and light forgings are made from SAE 1035 steel. 

High-carbon steel 

  1. Steel containing carbon in percentages ranging from 0.50 to 1.05 percent
  2. The addition of other elements in varying  quantities adds to the hardness of this steel.
  3. In the fully heat-treated condition,  it is very hard, will withstand high shear and wear, and will have little deformation. 

Nickel steel 

  1. The various nickel steels are produced by combining nickel with carbon steel. 
  2. Steels containing from 3 to 3.75 percent nickel are commonly used. 
  3. Nickel  increases the hardness, tensile strength, and elastic limit of steel without  appreciably decreasing the ductility.
  4. It intensifies the hardening effect of  heat treatment. SAE 2330 steel is used extensively for spacecraft parts, such as  bolts, terminals, keys, clevises, and pins. 

Chromium steel 

  1. High in hardness, strength, and corrosion-resistant  properties and are particularly adaptable for heat-treated forgings that  require greater toughness and strength than may be obtained in plain-carbon  steel.
  2. It can be used for such articles as the balls and rollers of antifriction  bearings. 

Chrome-nickel (stainless) steel 

  1. Chrome-nickel or stainless steels are the corrosion-resistant metals.
  2. The  anticorrosive degree of this steel is determined by the surface condition of the  metal as well as by the composition, temperature, and concentration of the  corrosive agent.
  3. The principal alloy of stainless steel is chromium.
  4. The corrosion resistant steel most often used in construction is known as 18–8 steel because  of its content of 18 percent chromium and 8 percent nickel. One of the  distinctive features of 18–8 steel is that its strength may be increased by cold  working.
  5. Stainless steel may be rolled, drawn, bent, or formed to any shape. Because  these steels expand about 50 percent more than mild steel and conduct heat only about 40 percent as rapidly, they are more difficult to weld.
  6. Stainless steel  can be used for almost any part of a spacecraft. Some of its common applications are in the fabrication of exhaust collectors, stacks and manifolds,  structural and machined parts, springs, castings, tie rods, and control cables. 

Chrome-vanadium steel 

  1. Made of approximately 18 percent  vanadium and about 1 percent chromium.
  2. When heat-treated, they have  strength, toughness, and resistance to wear and fatigue.
  3. A special grade of  this steel in sheet form can be cold formed into intricate shapes. It can be  folded and flattened without signs of breaking or failure.
  4. SAE 6150 is used for  making springs; and chrome-vanadium with high-carbon content
  5. SAE 6195, is  used for ball and roller bearings. 

Chrome-molybdenum steel 

  1. Molybdenum in small percentages is used in combination with chromium to  form chrome-molybdenum steel, which has various uses.
  2. Molybdenum is a  strong alloying element. It raises the ultimate strength of steel without affecting  ductility or workability. 
  3. Molybdenum steels are tough and wear resistant, and  they harden throughout when heat-treated.
  4. They are especially adaptable for welding and, for this reason, are used principally for welded structural parts  and assemblies.
  5. This type steel has practically replaced carbon steel in the  fabrication of fuselage tubing, engine mounts, and other structural parts. For example, a heat-treated SAE X4130 tube is approximately four times as strong  as an SAE 1025 tube of the same weight and size.
  6. A series of chrome-molybdenum steel most used in spacecraft construction is  that series containing 0.25 to 0.55 percent carbon, 0.15 to 0.25 percent  molybdenum, and 0.50 to 1.10 percent chromium. These steels, when suitably  heat-treated, are deep hardening, easily machined, readily welded by either  gas or electric methods, and are especially adapted to high-temperature  service. 


  1. A nickel-chromium-iron alloy closely resembling stainless steel  (corrosion-resistant steel, CRES) in appearance.
  2. Exhaust systems use both  alloys interchangeably. Because the two alloys look very much alike, a  distinguishing test is often necessary.
  3. One method of identification is to use an electrochemical technique to identify the nickel (Ni) content of the alloy.  Inconel has nickel content greater than 50 percent, and the electrochemical  test detects Ni.
  4. The tensile strength of Inconel is 100,000 psi annealed and  125,000 psi, when hard rolled. It is highly resistant to salt water and is able to  withstand temperatures as high as 1,600° F.
  5. Inconel welds readily and has  working qualities quite similar to those of corrosion-resistant steels.


Heat Treatment

Any process involving controlled heating and cooling to develop certain desirable characteristics.


Heat – soak – cool

HEAT – Heat Treatment any process involving controlled heating and cooling to develop certain desirable characteristics.

SOAK – maintain high temperature for a time period appropriate to the mass and thickness of metal which permits the rearrangement of the internal structure.

COOL – fast cooling (quenching) makes the steel hard while slow cooling(air furnace, sand pack) makes steel soft


  1. HARDENING – Controlled by rate of cooling
  2. ANNEALING – reduces brittleness and some hardness but improves ductility and toughness. Usually done after hardening.
  3. NORMALIZING – reduces brittleness and some hardness but improves ductility and toughness. Usually done after hardening.
  4. CASE HARDENING – relieves stress due to mechanical working of the metal (forging, welding, machining) that can cause cracking.
    • CARBURIZING – heating steels in contact with a carbonaceous material
  1. Pack – oldest method; bone, wood charcoal, coke
  2. Liquid – shallow penetration; salt bath; fast
  3. Gas – exposure to carbon atmosphere; electric furnace
  1. CYANIDING – heating steels in contact with a cyanide salt, followed by

quenching; speed and cheapness; core is hard and brittle; seldom used in aircraft work.

  1. NITRIDING – heating special alloy steels (nitra alloys) in contact with ammonia gas or other nitrogenous material; a harder case is obtainable


  • defined as a natural phenomenon which attacks metal by chemical or electrochemical action and converts it into a metallic compound, such as an oxide, hydroxide, or sulfate. 
  • The corrosion occurs because metals tend to return to their natural state. It is required for an aircraft to be regularly inspected and cleaned to control corrosion that may leave damage. 
  • Dirt and grease are visually unpleasant and can also accelerate corrosion formation by trapping moisture and corrosive materials between aircraft structures. 
  • Awareness of proper cleaning procedures and detection is needed to remove the build-up contamination in structural components without damaging its materials. If the corrosion effects have become established, you must also select appropriate methods to minimize and control these. 


1) Type of metal 

2) Heat treatment and grain direction 

3) Presence of a dissimilar, less corrodible metal (galvanic corrosion) 

4) Anode and cathode surface areas (in galvanic corrosion) 

5) Temperature 

6) Presence of electrolytes (such as hard water, salt water, or battery fluids) 

7) Availability of oxygen 

8) Presence of different concentrations of the same electrolyte 

9) Presence of biological organisms

10) Mechanical stress on the corroding metal 

11)Time of exposure to a corrosive environment

12)Lavatory fluids with hard water, salt water, or battery 


  • Pure metals are combined with other metals to form alloy because it is not suitable alone for aircraft construction. Corrosion can occur on the grain boundaries of alloy materials. However, metals have a wide range of corrosion resistance. 
  • The most noble metals, does not lose electrons easily thus do not corrode easily, are gold and silver. While magnesium and aluminum are the most active metals, they lose electrons easily thus corrode easily. 


  • The environmental conditions of where an aircraft is operated and maintained can significantly affect the corrosion characteristics. It is harmful to an aircraft to be operated in a hot and moist climate because an electrochemical attack would likely increase rather than in a. dry climate


  • These include flight routes and bases of operation that expose aircraft to more corrosive conditions than others. The corrosion severity of any area may be increased by many factors, including airborne industrial pollutants, chemicals used on runways and taxiways to prevent ice formation, humidity, temperatures, prevailing winds from a corrosive environment, etc.


  • The following are controllable factors that can affect the spread of corrosive attack caused by foreign materials to metal surfaces. 

• Soil and atmospheric dust 

• Oil, grease, and engine exhaust residue 

• Salt water and salt moisture condensation 

• Spilled battery acids and caustic cleaning solutions 

• Welding and brazing flux residues


  • Slimes, molds, fungi and other living organisms (some microscopic) can grow on damp surfaces. If the damp surfaces were not cleaned and treated, there is a possibility that corrosion in the area will occur. 


  • Manufacturing processes, such as machining, forming, welding, or heat treatment, can leave stresses in aircraft parts. 
  • The residual stress can cause cracking in a corrosive environment when the threshold for stress corrosion is exceeded. 


  • Corrosive agents – Substances that cause corrosion.

Corrosion is a natural phenomenon which attacks metal by chemical or electrochemical action and converts it into a metallic compound, such as an oxide, hydroxide, or sulfate. 

Water or water vapor containing salt combine with oxygen in the atmosphere to produce the most prominent corrosive agents. Additional corrosive agents include acids, alkalis, and salts. 

The corrosion process involves two simultaneous changes: 

  • the metal that is attacked or oxidized suffers what is called anodic change.
  • The corrosive agent is reduced and is considered as undergoing cathodic change
  • Direct Chemical Corrosion 

Direct chemical attack or pure chemical corrosion results from direct exposure of a bare surface to caustic liquid or gaseous agents. The most common agents causing direct chemical corrosion include: 

1. Spilled battery acid or fumes from batteries. 

2. Residual flux deposits resulting from inadequately cleaned, welded, brazed, or soldered joints 

3. Entrapped caustic cleaning solutions.

  • Electrochemical Corrosion

An ion is unstable, always seeking to lose or gain electrons so it can change back into a balanced, or neutral, atom. The earlier a metal appears in the series, the more easily it gives up electrons. 

ANODAL METAL – A metal that gives up electrons and corrodes easily. 

CATHODIC METAL – Metals that appear later in the series do not give up electrons easily.

My Share Learning Content: 6.6 The Electrochemical Series

Many metals become ionized due to galvanic action when brought into contact with dilute acids, salts, or alkalis, such as those found in industrially contaminated air. 

Corrosion is an electrochemical action in which one metal is changed into a chemical salt. When two dissimilar metals are in contact with each other in the presence of some electrolyte such as hydrochloric acid or plain water, the less active metal acts as the cathode and attracts electrons from the anode. As the electrons are pulled away from the anode the metal corrodes. 


  • It is when two dissimilar metals are placed in an electrolyte, an electrical potential exists. This potential forces electrons in the more negative material, the anode, to flow to the less negative material, the cathode, when a conductive path is provided.

The following four conditions must exist before electrochemical corrosion can occur 

1. Presence of a metal that will corrode (anode).

 2. Presence of a dissimilar conductive material (cathode) which has less tendency to corrode. 

3. Presence of a conductive liquid (electrolyte). 

4. Electrical contact between the anode and cathode (usually metal-to-metal contact, or a fastener). 

Corrosion control – consists of preventing the chemical action by eliminating one or more of these basic requirements.


  • Surface Corrosion
    • Damage Rating: 3
    • Cause:  Direct chemical attack
    • Electro-chemical attack 
    • Effect: General roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products.
    • Detection: Metal surfaces
    • Other Information: It is called as uniform etch or uniform attack corrosion.
    • It is the most common form of corrosion.
  • Filiform Corrosion
  • Damage Rating: 4
  • Cause: Polyurethane finishes
  • Between 78–90 percent relative humidity of the air.
  • Surface is slightly acidic.
  • Effect: Worm-like trace of corrosion products beneath the paint film.
  • Can lead to intergranular corrosion Detection o Beneath paint film o Steel and aluminum surfaces. 
  • Removal: Glass bead blasting material with portable abrasive blasting equipment or sanding. 
  • Prevention/Control: 
    • Storing aircraft in an environment with a relative humidity below 70 percent 
    • Using coating systems having a low rate of diffusion for oxygen and water vapors
    • Washing the aircraft to remove acidic contaminants from the surface, such as those created by pollutants in the air.
  • Pitting Corrosion
  • Damage Rating: 5 
  • Cause: Metals that form protective oxide films. 
  • Effect: White or gray powdery deposit, similar to dust, which blotches the surface. 
  • Tiny holes or pits can be seen in the surface. 
  • Detection: Aluminum and magnesium alloys. 
  • Other Information: Most destructive and intense form of corrosion.
  • Dissimilar Metal Corrosion
    • Damage Rating: 5
    • Cause
      • Contact between dissimilar metal parts in the presence of a conductor.
      • Improper use of steel cleaning products, such as steel wool or a steel wire brush on aluminum or magnesium, can force small pieces of steel into the metal being cleaned. 
    • Effect: Extensive pitting damage. 
    • Detection: 
      • Points or areas of contact where the insulation between the surfaces has broken down or been omitted. 
      • Electrochemical attack is taking place out of sight.
    • Prevention/Control
      • Disassembly and Inspection.
      • Carefully monitor the use of nonwoven abrasive pads, so that pads used on one type of metal are not used again on a different metal surface

Concentration Cell Corrosion / Crevice Corrosion

  • Corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though the joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material 

I. Metal Ion Concentration Cell 

  • Damage Rating: 3
  • Cause
    • Solution may consist of water and ions of the metal that are in contact with water. 
  • Effect
    • An electrical potential exists between the two points: the area of the metal in contact with the low concentration of metal ions is anodic and corrodes; the area in contact with the high metal ion concentration is cathodic and does not show signs of corrosion. 
  • Detection
    • High concentration of metal ions normally exists under faying surfaces where the solution is stagnant and a low concentration of metal ions exist adjacent to the crevice, created by the faying surface.

II. Oxygen Concentration Cell 

  • Damage Rating: 3
  • Cause
    • Solution in contact with the metal surface normally contains dissolved oxygen.
  • Effect
    • Difference in oxygen concentration between two points. 
    • Corrosion around low oxygen concentration areas (anode). 
  • Detection:
    • Develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points.
    • Under gaskets, wood, rubber, and other materials in contact with the metal surface commonly alloys such as stainless steel. 

III. Active-Passive Cell 

  • Damage Rating: 3
  • Cause:
    • Metals that depend on a tightly adhering passive film, usually an oxide for corrosion protection. o Starts as an oxygen concentration cell. 
    • Passive film is broken beneath the dirt particle exposing the active metal to corrosive attack.
  • Effect
    • Electrical potential will develop between the large area of the passive film and the small area of the active metal. 
  • Rapid Pitting Detection: Beneath the dirt particle
  • Intergranular Corrosion 
    • Damage Rating: 5
    • Cause
      • Changes that occur in the alloy during the heating and cooling process of the material’s manufacturing. 
      • Corrosive Environment 
    • Effect
      • Attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure.
    • Detection: May exist without visible surface evidence 
    • Other Information: High strength aluminum alloys and some stainless steels are particularly susceptible to this form of electrochemical attack.
  • Exfoliation Corrosion
    • Damage Rating: 5
    • Cause: Extruded sections such as spars where grain thickness is usually less than in rolled forms. 
    • Effect: Lifting up the surface grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface. 
    • Detection
      • Difficult to detect in its initial stage. 
      • Through ultrasonic and eddy current inspection. 
      • Other Information: o Advanced form of Intergranular Corrosion
  • Stress-Corrosion/Cracking 
  • Damage Rating: 5
  • Cause
    • Constant or cyclic stress acting in conjunction with a damaging chemical environment. o Internal or external loading. 
    • Internal stress may be trapped in a part of structure during manufacturing processes, such as cold working or by unequal cooling from high temperatures. 
    • Externally introduced in part structure by riveting, welding, bolting, clamping, press fit, etc. 
    • Specific environments that promotes stress to certain alloys. 
    • High strength heat treated aluminum alloys: Salt solution and seawater. 
    • Titanium Alloys: Methyl alcoholhydrochloric acid solutions. 
    • Magnesium Alloys: Moist air o Level of stress that varies from point to point within the metal. 
  • Effect
    • Cracking
    • Structural Failure 
  • Detection
    • Difficult to recognize/detect
    • Prevention/Control: o Applying protective coatings 
    • Stress relief heat treatments 
    • Using corrosion inhibitors o Controlling the environment 
    • Shot peening – ametal surface increases resistance to stress corrosion cracking by creating compressive stresses on the surface which should be overcome by applied tensile stress before the surface sees any tension load. Therefore, the threshold stress level is increased.
  • Fretting Corrosion 
    • Damage Rating: 4
    • Cause
      • Two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion. 
      • Presence of water vapor. 
    • Effect
      • Pitting of the surfaces and the generation of considerable quantities of finely divided debris. 
      • Localized abrasion occurs. 
      • If the contact areas are small and sharp, deep grooves resembling Brinell markings or pressure indentations may be worn in the rubbing surface. This type of corrosion on bearing surfaces has also been called false brinelling
    • Detection: Smoking rivet black (ring around the rivet) found on engine cowling and wing skins. 
    • Other Information: Reaction that is not driven by an electrolyte, and in fact, moisture may inhibit the reaction.
  • Fatigue Corrosion 
  • Damage Rating: 5
  • Cause
    • Involves cyclic stress and a corrosive environment. 
    • Metals may withstand cyclic stress for an infinite number of cycles so long as the stress is below the endurance limit of the metal. Once the limit has been exceeded, the metal eventually cracks and fails from metal fatigue. 
  • Effect
    • First stage – pitting and crack formations to such a degree that fracture by cyclic stress occurs.
    • Second stage – essentially a fatigue stage where failure proceeds by propagation of the crack (often from a corrosion pit or pits). 
  • Detection: Metal crack and fractures 
  • Other Information
    • Fracture of a metal part due to fatigue corrosion generally occurs at a stress level far below the fatigue limit of an uncorroded part, even though the amount of corrosion is relatively small. 
    • Principal difference between these two types of environment enhanced cracking is in the character of loading, which is static in stress corrosion cracking and alternating/repeated/cycling/periodically fluctuating in corrosion fatigue.
  • Galvanic Corrosion
    • Damage Rating: 4
    • Cause: Two dissimilar metals make electrical contact in the presence of an electrolyte. 
    • Effect: The rate which corrosion occurs depends on the difference in the activities. The greater the difference in activity, the faster corrosion occurs.
    • Detection: If the surface area of the corroding metal is smaller than the surface area of the less active metal, corrosion is rapid and severe. 
    • When the corroding metal is larger than the less active metal, corrosion is slow and superficial.


Most corrosive agents fall into one of two categories, acids, and alkalis. However, care must be taken not to overlook other less obvious corrosive agents such as the atmosphere which contains moisture, salts, or corrosive industrial agents.

Corrosive AgentsCorrosion Prone Materials
Acids o Sulfuric acid (battery acid) o Halogen acids (hydrochloric, hydrofluoric, and hydrobromic) o Nitrous oxide compounds o Organic acids found in the wastes of humans and animals.most of the alloys used in airframes
Alkaliso washing soda o potash (wood ashes) o lime (cement dust)Aluminum and magnesium alloys ( unless the solution contains corrosion inhibitor ) 
Salt solutionsAluminum alloy, magnesium alloys, and other steels (except from stainless steel alloys)
Atmosphere o Oxygen o Airborne moisture o Corrosive gases and contaminant particularly industrial and marine salt spray.Ferrous alloys
Watero Type and quantity of dissolved mineral o Organic impurities o Dissolved gasses (particularly oxygen) in the water. o Physical factors, such as water temperature and velocity 


Detection Methods using:

  • Magnifying glasses, mirrors, borescopes, fiber optics, and other optical inspection tools imperative for a good visual inspection. 
  • Ultrasonic equipment – for stress corrosion cracks
    • Pulseecho 
    • Resonance method
  • Radiological inspection – used to determine if there is any corrosion on the inside of a structure. 
    • X-ray inspection – requires extensive training and experience for proper interpretation of the results. Furthermore, the use of x-ray involves some danger because exposure to the radiation energy used in this process can cause burns, damage to the blood, and possibly death.

Aircraft Corrosion Prone Areas

  • Exhaust Trail Areas
    • Engine exhaust is corrosive 
  • Trouble Areas 
  • Gaps, seams, hinges, fairings 
  • Areas not reached by cleaning 
  • Rivet heads, skin lap joint, crevices 
  • Inspect by removing and inspecting: 
  • Fairings 
  • Access plates 
  • Empennage (tail assembly) surfaces
  • Battery Compartments and Vent Openings
    • Electrolyte fumes are hard to contain 
    • Inspect battery compartment and vent opening 
    • Minimize corrosion: 
      • Regular cleaning
      • Neutralization of acid deposits
  •  Bilge Areas
    • Sump for collecting waste liquids 
      • Hydraulic fluids 
      • Water 
      • Residual Oil 
      • Dirt and debris 
  • Protection from Corrosion 
  • Chemical conversion coatings 
  • Lap joint sealants 
  • Paint application interior
  • Inspection focus
    • Areas under galleys 
    • Areas under lavatories 
  • Maintenance
    • Clean and paint affected areas
  • Wheel Well and Landing Gear
    • Vulnerable to corrosion because water, gravel, debris 
    • Corrosion prevention challenge 
      • Paint film coverage difficult to attain 
      • Partial preservative masks corrosion 
  • Heat from braking precludes some preservatives 
  • Common trouble spots 
    • Magnesium wheels 
    • Exposed rigid tubing 
    • Exposed indicator switches 
    • Crevices
  • Water Entrapment Areas
  • Aircraft designed with drains 
  • Daily inspection standard requirement 
  • Neglected drains may accumulate 
  • Debris 
  • Grease 
  • Sealant
  • Engine Frontal Areas and Cooling Vents
  • Runways and rain erode protective finish 
    • Dirt
    • Dust
    • Gravel
  • Inspection 
    • Cooling air path
    • Areas of salt deposits 
  • Maintenance to prevent corrosion 
    • Paint touchups
    • Hard film preservative coatings
  • Wing Flap and Spoiler Access 
  • Potential for corrosion
  • Dirt and water collect often undetected 
  • Spoiler and flaps retracted 
  • Inspect with spoilers and flaps deployed.
  • External Skin Areas 
    • Consistent maintenance prevents corrosion 
    • Trimming, drilling, riveting destroys finish 
    • Inspection should include edges, fasteners, and missing paint 
    • Piano hinges corrosion prone
      • Dissimilar metal
      • Trap dirt, salt, and water
      • Include lubrication when inspecting 
    • Spot welded areas 
      • Corrosive agents trapped between metal layers.
      • Corrosion appears at entry point. 
      • Skin buckles with advanced corrosive attack. 
      • Detect by sighting along welding seam or using a straightedge 
  • Preventive Maintenance
    • Moisture entry points caused by broken spot welds
    • Identify and fill with sealant or preservative compound
  • Miscellaneous Trouble Areas 
    • Helicopter rotor heads
      • Exposed to elements
      • Bare steel surface
      • External working parts
      • Dissimilar metals contacts 
    • Steel control cables
      • Check for external corrosion
      • Check for internal corrosion 
    • Remove external corrosion 
    • Non-woven abrasive pad soaked in oil 
    • Steel wire brush 
    • Recoat with Preservative


1. Remove as much of the corrosion as possible. 

2. Neutralize any residual material. 

3. Restore the protective surface film. 

Metal/Corrosion ProductREMOVALTREATMENT
Iron RustMechanical Removal o Except on highly stressed steel surfaces, the  following can be use  ✓ abrasive papers and compounds ✓ small power buffers and buffing  compounds, ✓ hand wire brushing, or steel wool o Residual rust must be considered   Chemical Removalo A variety of commercial products that  actively remove the iron oxide without  chemically etching the base metal are  available and can be considered for use. o The steel part is removed from the airframe for treatment, as it can be nearly impossible to  remove all residue.  
SteelHighly Stressed Steel Partso For Highly stresses steel parts, careful  removal of corrosion products is required because overheating can cause sudden  failire. The following can be use: ✓ mild abrasive papers, such as  rouge or fine grit aluminum oxide  or fine buffing compounds on cloth  buffing wheels. ✓ Nonwoven abrasive pads. It is  essential that steel surfaces are  not overheated during buffing.   o After careful removal of surface corrosion,  reapply protective paint finishes  immediately.  Use of chemical corrosion removers is prohibited  with engineering authorization, because high strength steel parts are subject to hydrogen  embrittlement.  Chemical Surface Treatment of Steel Approved methods for converting activerust to phosphates and other protectivecoatings.o Rinsing and neutralizing of residual acid.o Use of chemical inhibitors on installedsteel parts is not only undesirable, butalso extremely dangerous.Must be always reminded the danger ofentrapment of corrosive solutions.
Aluminum and Aluminum AlloysUnpainted Aluminum Surfaceso Care must be taken to prevent staining and  marring of the exposed aluminum.  o Avoid unnecessary mechanical removal of  the protective Alclad layer and the exposure  of the more susceptible aluminum alloy base  material.   ANODIZED SURFACESo Nonwoven abrasive pads are used for  cleaning corroded anodized surfaces. Do  not use steel wool or steel wire brushes. Do  not use severe abrasive materials.  Intergranular Corrosion in Heat-Treated Aluminum Alloy Surfaces The mechanical removal of all corrosion products and visible delaminated metal layers must be accomplished to determine the extent of the destruction and to evaluate the remaining structural strength of the component. Unpainted Aluminum SurfacesA thin coating of relatively purealuminum is applied over the basealuminum alloy. The protection obtainedis good and the pure-aluminum cladsurface, commonly called “Alclad,” canbe maintained in a polished condition.The thickness of the outer cladding layertypically varies between 1% and 15% ofthe total thickness. ANODIZED SURFACESAnodizing is a common surfacetreatment of aluminum alloys. Chromic acid and other inhibitive treatments canbe used to restore the oxide film.Intergranular Corrosion in Heat-Treated Aluminum Alloy SurfacesAny loss of structural strength must be evaluated prior to repair or replacementof the part. If the manufacturer’s limits do not adequately address the damage, a designated engineering representative (DER) can be brought into assess the damage.
Magnesium AlloysWrought Magnesium Sheet and Forgingso Limit such mechanical cleaning to the use of stiff, hog bristle brushes and similar non-metallic cleaning tools (including nonwoven abrasive pads), particularly if treatment is to be performed under field conditions. Installed Magnesium Castings If extensive removal of corrosion products from a structural casting is involved, a decision from the manufacturer may be necessary to evaluate the adequacy ofstructural strength remaining. Specific structural repair manuals usually include dimensional tolerance limits for critical structural members and must be referred toif any question of safety is involvedInstalled Magnesium Castings Earliest practicable treatment is required if dangerous corrosive penetration is to be avoided. Separate parting surfaces to effectively treat the existing attack and prevent its further progress.
Titanium and its alloysUse of steel wool, iron scrapers, or steel  brushes for cleaning or for the removal of  corrosion from titanium parts is prohibited. The surface is treated following cleaning  with a suitable solution of sodium  dichromate. Wipe the treated surface  with dry cloths to remove excess  solution, but do not use a water rinse. 


Aircraft cleaners and cleaning materials


Cleaner and Cleaning MaterialsPurposeRequirement/Precautions
SolventUsed in aircraft cleaningFlashpoint of not less than 105 °F if explosion proofing of equipment and other special precautions are to be avoided.Safety precautions must be observed in their use. Particular attention must be paid to recommended protective measures including gloves, respirators, and face shields.Use of carbon tetrachloride is to be avoided.
DRY CLEANING SOLVENT Stoddard solvent is the most common petroleum base solvent used in aircraft cleaning.Dry cleaning solvent is preferable to kerosene for all cleaning purposes, but like kerosene, it leaves a slight residueupon evaporation that may interfere with the application of some final paint films.Flashpoint is slightly above 105 °F and can be used to remove grease, oils, or light soils.
ALIPHATIC AND AROMATIC NAPTHAAliphatic naphtha used for cleaning acrylics and rubber. Recommended for wipe down of cleaned surfaces just before painting.Aliphatic naphtha flashes at approximately 80 °F and must be usedwith care.Aromatic naphtha is toxic, attacksacrylics and rubber products, and mustbe used with adequate controls.
SAFETY SOLVENTTrichloroethane (methyl chloroform) is used for general cleaning and grease removal.NonflammableReplacement for carbon tetrachlorideThe use and safety precautionsnecessary when using chlorinatedsolvents must be observed. Prolongeduse can cause dermatitis on somepersons.
METHYL ETHYL KOTEONE (MEK)Solvent cleaner for metal surfaces and paint stripper for small areas. A highly active solvent and metalcleaner with a flashpoint of about 24°F.    Toxic when inhaled, and safetyprecautions must be observed duringits use.
KEROSENESoftening heavy preservative coatingsGeneral solvent cleaning, but its usemust be followed by a coating or rinsewith some other type of protectiveagent.Generally leaves an appreciable filmon cleaned surfaces that may actuallybe corrosive. Kerosene films may beremoved with safety solvent, wateremulsion cleaners, or detergentmixtures
CLEANING COMPOUND FOR OXYGEN SYSTEMSClean accessible components of theoxygen system, such as crew masksand lines.Fluids must not be put into tanks orregulators.Instructions of the manufacturer of theoxygen equipment and cleaningcompounds must always be followed.


Cleaner and Cleaning MaterialsPurposeRequirement/Precautions
EMULSION CLEANERS Used in general aircraft cleaningRemoval of heavy deposits, such ascarbon, grease, oil, or tar.When used in accordance withinstructions, these solvent emulsionsdo not affect good paint coatings ororganic finishes. 
WATER EMULSION CLEANERCleaning compound intended for useon both painted and unpainted aircraftsurfaces.o For cleaning fluorescent paintedsurfaces and is safe for use onacrylics.MIL-C-22543A✓ A sample application must be checkedcarefully before general uncontrolleduse.
SOLVENT EMULSION CLEANERNonphenolic– safely used on painted surfaces without softening the base paint. Phenolic based – more effective for heavy-duty application, but it alsotends to soften paint coatings. It must be used with care around rubber,plastics, or other nonmetallic materials.Nonphenolic – Persistent materials areto be given a second or third treatmentas necessary. Phenolic based – Wear rubber glovesand goggles for protection whenworking with phenolic base cleaners.  


    Cleaner & Cleaning MaterialsPurposeRequirement/Precautions
SOAPS AND DETERGENTS CLEANERS Available for mild cleaning use—-
CLEANING COMPOUND, AIRCRAFT SURFACESGeneral cleaning of painted and unpainted aircraft surfaces for theremoval of light to medium soils,operational films, oils, or greases.MIL-C-5410 Type I and II materials✓ safe to use on all surfaces, includingfabrics, leather, and transparentplastics. 
NONIONIC DETERGENT CLEANERSFor softening and removing heavypreservative coatings.Similar to the emulsion cleanersMay be either water-soluble or oilsoluble. The oil-soluble detergentcleaner is in a 3 to 5 percent solutionwith dry cleaning solvent 


Cleaner and Cleaning MaterialsPurposeRequirement/Precautions
MECHANICAL CLEANING MATERIALS ✓ must be used with care and inaccordance with directions given ifdamage to finishes and surfaces is tobe avoided.
MILD ABRASIVE MATERIALSCan be used on most metalso Powdered pumice – used for cleaningcorroded aluminum surfaces. o Impregnated cotton wadding material -used for removal of exhaust gas stains and polishing corroded aluminum surfaces. It may also be used on other metal surfaces to produce a high reflectance. o Aluminum metal polish – used to produce a high luster, long lasting polish on unpainted aluminum clad surfaces. It must not be used on anodized surfaces, because it removes the oxide coat. o Three grades of aluminum wool(coarse, medium, and fine) – used for general cleaning of aluminum surfaces.Impregnated nylon webbing material is preferred over aluminum wool for the removal of corrosion products andstubborn paint films and for the scuffing of existing paint finishes prior to touchup. o Lacquer rubbing compound material -used to remove engine exhaust residues and minor oxidation. Avoidheavy rubbing over rivet heads or edges where protective coatings maybe worn thin. o Abrasive Papers – used on aircraft surfaces but must not contain sharp orneedle-like abrasives that can imbed themselves in the base metal beingcleaned or in the protective coating being maintained.same pad should not be used ondifferent metals


Cleaner and Cleaning MaterialsPurposeRequirement/Precautions
CHEMICAL CLEANERS Used with great care in cleaning assembled aircraft.✓ The danger of entrapping corrosivematerials in faying surfaces andcrevices counteracts any advantagesin their speed and effectiveness.✓ All residues must be removed.
PHOSPHORIC-CITRIC ACIDCleaning aluminum surfaceso Phosphoric-citric acid mixture (Type I)is ready to use while Type II is a concentrate that must be diluted with mineral spirits and water.Wear rubber gloves and goggles toavoid skin contact.✓ Any acid burns may be neutralized bycopious water washing, followed bytreatment with a diluted solution ofbaking soda (sodium bicarbonate).
BAKING SODAused to neutralize acid deposits in lead- acid battery compartments and to treat acid burns from chemical cleaners andinhibitors. 


The type and extent of soiling and the final desired appearance determine the cleaning method to be used.

Wet wash removes oil,grease, carbon deposits, and most soils, except forcorrosion and oxide films.The cleaning compoundsused are generally applied by spray or mop.High-pressure running water is used as a rinse. Eitheralkaline or emulsion cleaners can be used in the wetwash method.
Dry wash is used to remove airport film, dust, and small accumulations of dirt and soil when the use of liquids is neither desirable nor practical.This method is not suitable for removing heavy deposits of carbon, grease, or oil, especially in theengine exhaust areas. Dry wash materials are applied with spray, mops, or cloths and removed by dry mopping or wiping with clean, dry cloths.
Polishing restores the luster to painted and unpainted surfaces of the aircraft and is usually performed after thesurfaces have been cleaned. Polishing is also used toremove oxidation andcorrosion.Polishing materials are available in various forms anddegrees of abrasiveness. It is important that theaircraft manufacturer’s instructions be used in specificapplications.

Interior Cleaning

  • Corrosion can establish itself on the inside structure to a greater degree, because it is difficult to reach some areas for cleaning. Nuts, bolts, bits of wire, or other metal objects carelessly dropped and neglected, combined with moisture and dissimilar metal contact, can cause electrolytic corrosion.
  • When performing structural work inside the aircraft, clean up all metal particles and other debris as soon as possible. To make cleaning easier and prevent the metal particles and debris from getting into inaccessible areas, use a drop cloth in the work area to catch this debris. A vacuum cleaner can be used to pick up dust and dirt from the interior of the flight deck and cabin.
  • Aircraft interior present certain problems during cleaning operations due to the fact that aircraft cabin compartments are relatively small enclosures. The possibility of restricted ventilation and quick buildup of flammable vapor/air mixtures can occur when there is any indiscriminate use of flammable cleaning agents or solvents. Additionally, there may also exist the possibility of an ignition source from concurrent maintenance work in the form of an electrical fault, friction or static spark, an open flame device, etc.

Powerplant Cleaning

  • Grease and dirt accumulations on an air – cooled engine provide an effective insulation against the cooling effect of air flowing over it. Such an accumulation can also cover up cracks or other defects.
  • When cleaning an engine, open or remove the cowling as much as possible. Beginning with the top, wash down the engine and accessories with a fine spray of kerosene or solvent. A bristle brush may be used to help clean some of the surfaces. 
  • Fresh water, soap, and approved cleaning solvents may be used for cleaning propeller and rotor blades. Except in the process of etching, caustic material must not be used on a propeller.
  • Scrapers, power buffers, steel brushes, or any tool or substances that mar or scratch the surface must not be used on propeller blades, except as recommended for etching and repair. 
  • Water spray, rain, or other airborne abrasive material strikes a whirling propeller blade with such force that small pits are formed in the blade’s leading edge.
  • If preventive measures are not taken, corrosion causes these pits to rapidly grow larger. The pits may become so large that it is necessary to file the blade’s leading edge until it is smooth.
  • Steel propeller blades have more resistance to abrasion and corrosion than aluminum alloy blades. Steel blades, if rubbed down with oil after each flight, retain a smooth surface for a long time. 
  • Examine the propellers regularly, because cracks in steel or aluminum alloy blades can become filled with oil that tends to oxidize. This can readily be seen when the blade is inspected. Keeping the surface wiped with oil serves as a safety feature by helping to make cracks more obvious.
  • Propeller hubs must be inspected regularly for cracks and other defects. Unless the hubs are kept clean, defects may not be found. 
  • Clean steel hubs with soap and fresh water or with an approved cleaning solvent. These cleaning solvents may be applied by cloths or brushes. Avoid tools and abrasives that scratch or otherwise damage the plating.
  • In special cases where a high polish is desired, the use of a good grade of metal polish is recommended. Upon completion of the polishing, all traces of polish must be removed immediately, the blades cleaned, and then coated with clean engine oil. 
  • All cleaning substances must be removed immediately after completion of the cleaning of any propeller part. Soap in any form can be removed by rinsing repeatedly with fresh water. 
  • After rinsing, all surfaces must be dried and coated with clean engine oil. After cleaning the powerplant, all control arms, bell cranks, and moving parts must be lubricated according to instructions in the applicable maintenance manual.

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