OVERSPEED LIGHT (Cont.) If the 'overspeed' light comes ON after an automatic shut down the APU should be checked for vibration or unusual noise after the subsequent restart. It is not satisfactory to simply reset the M280 module to extinguish the 'overspeed' light as further operation of the APU could cause severe damage if the APU overspeed was caused by mechanical damage to the APU.
When reports of APU automatic shut downs occur, the following action should be carried out:
(a)
Select APU master switch to "OFF."
(b)
Press the 'overspeed' reset switch on the M280 module (situated in the E & E compartment) to extinguish the overspeed light.
(c)
Start the APU. If the APU starts and over-speeds, replace the fuel control unit or three speed switch. If, however, the APU vibrates excessively or has unusual noise shut down the APU immediately and replace the APU. If there is no indication of vibration or overspeed on subsequent starts the APU may continue in service without further investigation.
Self Initiated APU Shutdowns:
If a self initiated shut down of an APU occurs in service inspect the APU and its auxiliary equipment for any obvious cause of malfunction, viz. broken electrical lead, ruptured air line etc. Despite the fact that this initial inspection may detect a seemingly obvious reason for the self initiated shutdown of the APU; it should be noted that these defects could be caused by vibration due to a bearing failure. If this is suspected the following checks should be carried out to ensure it is not an internal engine failure which has caused the problem:
(a)
Inspect the oil for discoloration and metal particles.
(b)
Inspect for presence of excessive metal particles within the oil filter or housing.
501 APU Controls and Indicators
Feb 20/85 Figure 101 (Sheet 4) BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 49-00 Page 106A
OVERSPEED LIGHT (Cont.) (c) Check for high oil consumption. An APU requiring the addition of a quart of oil at a time other than at its normal oil level inspection period is an indication of internal failure.
(d)
Check for signs of oil in the exhaust section due to damaged turbine oil seal.
(e)
If any of the above defects are found the APU should be removed for further investigation.
(f)
If no defects are found start the APU. If the APU vibrated excessively or has unusual noise shutdown the APU immediately and replace the APU. If the APU starts and over-speeds it may be caused by a defective speed control system; the most likely component to cause this problem is a defective fuel control unit or a three-speed switch.
If a self initiated shutdown of an APU occurs in service and there is insufficient time available to check the APU oil system for possible bearing failure the APU must be placarded to the effect that the APU may be used on the ground only provided that there is compliance with the following requirements:
(a)
Start the APU.
