Aircraft Pneumatic System: Maintenance Procedures

AIRCRAFT PNEUMATIC SYSTEM

1. Definition

It operates the landing gear, flaps, brakes, cargo doors, and other forms of mechanical actuation. It is the backup source of power in the event of a failure of the hydraulic power.

2. Maintenance Task (MAINTAIN PNEUMATIC SYSTEMS, n.d.)
   1. General Method

Set up a maintenance schedule and follow it diligently.

1. Inspect filter elements that have been removed from compressors and the system for signs of failure, which may indicate that the service interval should be shortened or that there are impending system problems.
2. Use clean oil in lubricators.
3. Make certain that the oil used in lubricators is of a type recommended by the manufacturer for the system or component.
4. When adding oil to a lubricator, be sure to clean the areas around the dipstick and filler cap before removing them.
5. Before adding oil, be sure the oil already in a lubricator is clean. If it’s not, drain the oil and refill the lubricator completely with new oil that will give good performance under prevailing conditions.
6. Use clean containers, hoses and funnels when filling a lubricator.
7. Do not run the compressor or the system unless all normally provided filtration devices are in place.
8. Use common-sense precautions to prevent dirt entering components that have been temporarily removed from the circuit.
9. How to Fix Leaks (Finding and Fixing Leaks, n.d.)

1. Tighten a connection or replace faulty equipment such as couplings, fittings, pipe sections, hoses, joints, drains, and traps. 
2. In many cases leaks are caused by bad or improperly applied thread sealant.
3. Select high quality fittings, disconnects, hose, tubing, and install them properly with appropriate thread sealant.

Once leaks have been repaired, the compressor control system should be re-evaluated and adjusted, if necessary, to realize the total savings potential.

3. Testing
   1. General Method
      1. Step 1 – Know the system Study the machine’s technical specifications to obtain an understanding of how the system operates and the function of the machine’s components. Obtain a circuit drawing and check the system through. Check the machine’s maintenance records and commissioning test results, if they are available. 
      2. Step 2 – Ask the operator Determine the symptoms of the problem by asking the operator for a detailed description of the machine’s normal operating performance. 
      3. Step 3 – Inspect the machine Use your senses (touch, smell, sight and hearing) to locate problems or damage such as noisy components, air leaks, malfunctioning components and damaged air lines. 
      4. Step 4 – Operate the machine Operate the machine and check that the machine’s gauges are reading ‘normal’ and that there are no unusual noises. The operation of the machine’s controls should not be ‘sticky’ or ‘spongy’. The machine’s performance should not be slow, erratic – nor non-existent. 
      5. Step 5 – List the possible causes Once the fault has been located and recognised, list the possible causes – starting with the simplest. 
      6. Step 6 – Reach a conclusion Use a troubleshooting chart to check the list of possible causes; then decide which is the most likely. 
      7. Step 7 – Test the conclusion Before starting any repairs to the system, test the conclusion on the cause of the problem. It may be necessary to use pressure gauges, a stopwatch and rpm meters to substantiate the conclusion.

2. Testing Activities  (Pneumatic Testing Procedures, n.d.)

The procedure must:

1. Describe a graduated pressurization method to reach test pressure, including minimum durations to hold the pressure at each graduated step.
2. Require that the test pressure be reduced to design pressure before examining for leakage.
3. Require that if leaks are identified, the system shall be de-pressurized prior to proceeding with repairs or correction of the deficiency.
4. Require that depressurization must take place in a controlled manner and the dangers of a confined space and possibility of asphyxiation from the test medium (such as nitrogen) are considered.
5. Pneumatic Test Procedure
   1. Test procedures ≤ 1677 kJ stored within the pneumatic test limitations may be developed and submitted by a licensed contractor using an application-specific or standard pneumatic test procedure format.
   2. Pneumatic test procedures > 1677 kJ that are not within the pneumatic test limitations shall be developed using an application-specific procedure. The application-specific procedure shall be developed and approved by a professional engineer and submitted to Technical Safety BC for acceptance prior to conducting the test. 
6. Standard Pneumatic Test Procedure
   1. Licensed contractors who frequently perform pneumatic pressure tests may include an accepted pneumatic test procedure ≤ 1677 kJ that is within the stored energy, temperature and pneumatic test limitations in their quality control manual. The addition of the procedure will constitute a revision to the Quality Control Manual (QCM). 

The licence holder shall submit their QCM to Technical Safety BC for review and acceptance. Revisions are normally submitted to the regional boiler safety officer. The standard test procedure shall be acceptable for subsequent pneumatic tests within the limitations of the accepted procedure.

5. Pneumatic Test Limitations

Identify that the scope of equipment is within the following test limitations:

1. The stored energy value will not exceed 1677 kJ.
2. The pressure system is made of P-1, P-8 material or a material acceptable to Technical Safety BC that conforms to a specification listed in the code of construction.
3. The test medium is air or nitrogen.
4. The testing will be conducted at a temperature of at least 17°C (30°F) above the design metal’s minimum temperature.
5. Any additional requirements appropriate to the code of construction.

4. Methods Used in Inspection
   1. Pre-Test Checks and Inspections

The procedure must describe a pre-test inspection to verify the following, where appropriate:

1. The system is completed according to drawings and specifications.
2. All visual inspections and non-destructive examinations required by the code of construction shall be completed and evaluated as acceptable.
3. Parts of the system not being tested are adequately isolated.
4. A pre-test inspection shall be made to verify proper assembly and tightness of connections, positioning of valves and all supports are installed as designed.
5. Proper assembly, including tightness of connections, positioning of valves, over pressure protection and hangers and supports are in place as designed.
6. There is no damage, misalignment or anything else out of place.
7. All associated lockouts are in place.
8. Any hoses are restrained or anchored and whip checks are in place.
9. ”Walk-around”—visual check of aircraft exterior and engines for damage, leakage, and brake and tire wear.
10. Final Preparation

The procedure must:

1. Describe the test pressure value and verify that it is the minimum in accordance with the code of construction (usually 110% of the design pressure).
2. Require that gauge(s) are calibrated and that the appropriate range is used (1.5 – 4 times of test pressure).
3. Require that metal temperature is satisfactory at test time.
4. Require that the safe distance, as identified in the test procedure shall be identified by placing appropriate barriers to restrict non-essential personnel from the test area.
5. Require that overpressure protection is in place and set to the lesser of 50 psi or 10% above the test pressure.