ATA 36 Pneumatics

ATA 36 Pneumatics

2 - Test procedure 490°F switch (Shop procedure different from AMM!) 737-ALL

Test procedure 490°F switch:

1. Preheat to 510°F.

2. Decrease temperature to 455°F.

3. Slowly increase temperature to 500°F.

4. Stabilize temperature for ~ 7 minutes.

In this time or during the temperature
increase action the switch has to close.

5. Slowly decrease temperature to 470°F.

6. Stabilize temperature for ~ 7 minutes.

In this time or during the temperature
decrease action the switch has to open

-Switch close limits 490 –5 +10°F.

-Switch open limits 470°F minimum.

3 - Precooler control valve: control line damaged against the bracket. 737-6/7/8/900

When you do inspections under the engine thrust reversers, take a look at the precooler control valve control line, found on the LH side above the engine. The control line can be positioned against the bracket, causing a chafed/damaged line, which results in a precooler valve not closing. So make sure the line is free from the bracket and not damaged. See pictures of a line which is cut half through.

6 - Engine bleed system flexible lines, an easy overview for troubleshooting. 737-6/7/8/900

The engine bleed system on the NG has 7 flexible lines incorporated in the various systems. They are the cause of many engine bleed problems because they are very sensitive and fragile. The flexible lines are made of very thin wounded material inside, which cracks and breaks very quick when not installed tension free or due to engine vibrations. The picture below shows a cross-section of such a line.

When troubleshooting on the engine bleed system, you can use the overview in the picture below, where the flexible lines are highlighted. The numbered pictures refer to these lines, so you can easily find them on the engine.

Click on the picture to enlarge in PDF format.

10 - Example of ACMS data that shows a failing precooler control valve sensor 390F.

Below you find 2 pictures of ACMS data from the same aircraft, in which the 390 F sensor of the precooler control valve failed. You can see how the PRSOV is closing and making the bleed duct pressure lower and lower. After replacement of the sensor, the ACMS data shows a correct duct pressure again, proving that the 390 F sensor is working correctly and opening the PCV to cool the bleed air. The 390 F sensor was replaced after the bleed air health check was performed iaw amm, and all other parts where found working ok. The 390 F and 450 F sensors are not tested with this test. They have a special shop-test which cannot be performed during normal aircraft maintenance and troubleshooting. Experience learns that this sensor is often defective. Thanks to Ferry B. for his input !

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