Ice accumulation is a dangerous sign in an aircraft operation. Many people generalize any ice accumulation as “ice.” But in reality, aviation personnel especially flight crews should know the difference between the types of ice and how it affects the aircraft.
Types of Ice that Accumulates on Aircraft
HOAR FROST
If you leave the aircraft parked outside during a cold night, expect that it will be covered by the hoar frost and/or frozen dew in the morning. Whenever icing conditions are first encountered, aircraft are protected against airframe heating by the leading edges of the airfoils and intake ducts. Ice formation on wings is due to supercooled water changing state on contact with the wing.
Appearance: White semi-crystalline
Formed by: Deposition by clear air- Where water vapor turning directly into ice crystals on the aircraft surface. When an aircraft goes under a warmer atmosphere and dampers air during descent.
Subtle to moderate Effect: A radio antenna covered with hoar frost will interfere with the signal. Radio reception is hampered and engine performance is affected due to the buildup of ice.
Severe Effect:
- Alters the aerodynamic characteristics
- Reduce lift
- Increase stall speed
What is the difference between hoar frost and frozen dew?
You can differentiate them by their appearance.
Hoar Frost: White semi-crystalline
Frozen Dew: Clear and semi-crystalline
MIXED ICING
It is the formation of both rime and clear ice. It is difficult to remove.
CLEAR ICE OR GLAZE ICE
Clear ice, also called run-back ice. It has a strong structure since it has no embedded air bubbles. Its shape is a multi-edge horn. Glaze ice deposit is formed when liquid water flows over the airframe before freezing, and which is dense, tough and sticks closely to the surface.
Appearance: Glassy ice (not translucent)
Droplets: Low supercooled water droplets
Formation:
- When flying in areas with accumulated large supercooled water droplets
- In the cumuliform clouds and/or freezing rain
Characteristic: Spreads unevenly
Effect on aircraft:
- Disrupts the airflow
- Increases drag by up to 5 time
RIME ICE
Rime ice is formed when liquid water flows over the airframe before freezing and sticks closely to the surface.
Appearance: Milky white
Droplets: Low supercooled water droplets
Formed by:
- When aircraft are flying though stratiform clouds
- Formed at below 0°C
Characteristic: Does not spread from the origin. That’s why the droplets retain their irregular shapes.
Effect on aircraft:
- Alters the aerodynamic characteristics
- Chocking of the carburetor’s orifices
Can be removed by: De-icing equipment
What is the difference between rime ice and hoar frost?
Rime ice has a bigger ice texture and sharp look than the hoar frost. The rime ice is formed by supercooled water droplets while the hoar frost is formed by low supercooled water droplets.
WHAT IS RIME-GLAZED?
It is a mixture of snow and clear ice that appears white.
Types Of Ice Accumulated in Aircraft And Their Differences
As a conclusion, here is a table to explain their differences.
| CLASSIFICATION | HOAR FROST | RIME ICE | CLEAR OR GLAZE ICE | MIXED ICING |
|---|---|---|---|---|
| Appearance | White semi-crystalline | Milky white | Glassy ice | White and Rough textured |
| Water droplets | Supercooled water droplets | Low supercooled water droplets | Low super cooled water droplet | Large and small supercooled droplets |
| Formed by | When an aircraft goes under a warmer atmosphere and damper air during descent. | When aircraft is flying though strati form clouds ; Formed at below 0°C | When flying in areas with accumulated large super cooled water droplets; In the cumuliform clouds and/or freezing rain | |
| Characteristic | Fine ice crystals | Does not spread from the origin. That’s why the droplets retain their irregular shapes. | Spreads unevenly | Formed rapidly |
Which type of ice is the most dangerous?
Considering their effect, it is still based on the type of aircraft, altitude, temperature, and the amount of ice accumulated. But to give you an idea, flight personnel should look out to the danger of clear ice due to the following reasons:
- Alters the wing camber and aerodynamic characteristics, therefore, causes the loss of lift
- Ice buildup Increases drag and reduces lift.
- Faster to accumulate, causes more weight on the aircraft
- Extensive heavy vibration on the aircraft surface. Ice buildup causes destructive vibration, and hampers true instrument readings.
CLEAR ICE can instantly ruin the aircraft structure. It forms a smooth sheet of solid ice.
Where does ice accumulate on aircraft?
- Blade/propeller
- Camber of the wings, including the leading edge
- Radio antennas
- Carburetors
- Pitot tube, alpha probes, and other probes
- Horizontal and vertical stabilizers
- Windscreen
- Engine intakes
Control surfaces become unbalanced or frozen due to the buildup of ice.
NOTE: Small parts have higher catch rate efficiency. Small amounts of ice will produce bigger ice formation.
What altitude does icing occurs?
Icing does not occur above 40,000 feet (12,000 meters). The droplets there are all frozen and they will not stick in the aircraft’s surface since it is already in ice crystal formation.
Supercooled droplets are formed below freezing temperature. It can exist between 0 to 40 degrees Celsius and if there is enough moisture in the atmosphere.
Does icing occurs on land? Yes.
Does icing occur every time the aircraft is flying at any altitude? No.
What is the difference between de-icing and anti-acing?
DE-ICING
Ice is accumulated then removed. Think of it as how you remove the ice from your freezer, “DEfrost.”
Includes engine ice protection from hot air and electrical systems. Note that the leading edge fluid de-icing unit does not disturb the aerodynamic shape.
ANTI-ACING
A preventative measure to avoid ice formation. It is usually done by the continuous flow of heat or fluid.
Includes engine ice protection from hot air only. Air for anti-icing of the wings is obtained from engine compressors.
Whenever icing conditions are first encountered, the anti-icing system is operated during flight to protect against airframe icing by heating the airfoil edges and intake ducts. Air used for the anti-icing of the wings is from the combustion heater. The anti-ice system protects the leading edges slats and flaps.
Another difference between the two is their procedure. The de-icing fluid is applied without heating and the anti-icing fluid is applied hot.
How does modern aircraft detect ice accumulation on aircraft?
There ice numerous ice detection systems:
ACCRETION TYPE DETECTOR
- They sense the buildup of ice through visual detection systems
- Examples:
- Beta particle ice detection
- Napier and Rosemount ice detector
- Smith ice detector
- Teddington ice detector – A hot rod type of ice detector that is switched on when selected by the flight crew.
INFERENTIAL TYPE DETECTOR
- Sense the ice by measuring the humidity and temperature
Example: - Element ice sensing unit
Other are other types of ice detector just like the Rotary Knife Edge Ice Detector that provides warning of ice when the torque is decreased caused by ice formation slowing the rotating wheel and illuminating a warning light in the cockpit.
How does ice on aircraft is remove?
There are three main methods used for ice protection.
- LIQUID – By the freezing-point depressant fluids
- MECHANICAL – By the expanding rubber boots on the wings. In general aviation, pneumatic de-icing system or deicer boots remove ice accumulations by breaking up the ice formations. Deicer boots on modern aircraft are bonded with adhesive to the leading edge of a wing. Tubes in deicer boots alternate inflation of deicer boot tubes keeps disturbance of the airflow to a minimum.
- THERMAL – Thermal anti-icing system are from the compressor bleed air and electrical system. Thermal heat also came from the engine oil’s heat or hot air. Some modern large aircraft are using electrical thermal anti‐icing systems. Thermal wing anti‐ice systems for business jet and large transport category aircraft will typically use bleed air from turbine engine compressor.
Some aircraft has anti-icing systems installed to prevent the bled from the engine compressor. Most modern turboprop airplanes have a thermal anti-icing system for the wings.
Frost deposits can be removed by placing the aircraft in a warm hangar or by using a deicing fluid. General aviation aircraft equipped to fly in icing conditions use liquid, thermal, or electric anti‐icing systems that might be controlled automatically or manually to prevent the formation of ice.
A mixture of Ethylene glycol and isopropyl alcohol is a solution as a de-icing fluid to remove frost outside in the aircraft outer skin.
How do flight crews protect the windscreen from ice?
They have windscreen ice protection systems where fluid spray and electrical heating equipment is equipped in the windscreen.
De-misting of passenger windows is provided by the air from the cabin. Cabin air is used for de-misting purposes and wipers can clear rain and another rain repellent can be used to clear heavy rain. Windshield rain repellent is only applied when the rain is on windows and spread by the windshield’s wipers. The windshield pneumatic rain removal system is where an air blast forms a barrier that prevents raindrops from striking the windshield surface.
Another way to prevent ice buildup to the windscreen is by heating them for impact resistance enhancement. Heat sensors located on most airplanes fitted with electrically heated windshields embedded in the glass.
Can a pilot use the wipers on a dry windscreen?
Usually, pilots can’t utilize the wipers on dry windscreen because heavy undiluted repellent will restrict window visibility. Also, rain repellant cannot be used on dry windshields because it will restrict visibility. Note that the dry air by volume in the troposphere has 21% oxygen, 78% nitrogen, and other gases.
Anti-icing system for the aircraft’s windshields can be any of the following methods:
- Windshield wipers
- Anti-icing fluid
- Heated air system
- Electric heating element
NOTE
The air inside the aircraft is pressurized for the safety of everyone on board. Note that the percentage of oxygen when air is compressed may be unaffected.
When planning a flight with icing conditions, the aircraft must be equipped with the approved ice-protection system. All ice must be removed on the wings, empennage (tail), and control surfaces before take-off.