(4)
Examine the rubber cushion of each vibration isolator on the shroud mounting brackets for cracks, separations and displacements.
(5)
APU shroud for loose or broken fasteners.
(6)
All external electrical or tubing connections for looseness.
(7)
Open and close door in side of shroud. Door should open easily and close completely.
(8)
APU shroud drains (AMM 49-12-0/701).
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4. APU Condition Monitoring Check
A. This is a preventive maintenance check, which if performed at regular intervals, will identify a potential problem. Appropriate corrective action can then be taken before the problem results in flight delays, expensive part replacement, and/or APU failure.
B. The following explanation is provided for each item appearing on the data sheet (Fig. 602). The reference values in parentheses are suggested based upon experience with this monitoring procedure:
Item 1. RECORD OUTSIDE AIR TEMPERATURE
(a) This reading will be used for heat exchanger effectiveness, for a comparison of prior
no-load EGT recordings, and for a check of acceleration time during starting. Item 2. CHECK BATTERY VOLTAGE (22 VOLTS MINIMUM)
(a)
Low batteries cause slow or hung starts.
(b)
Battery voltage should drop to approximately 16-18 volts and recover to 20 volts during starter motor in-rush. If voltage does not drop, then clutch slippage or corroded terminals may be the cause.
Item 3. CHECK THAT PACK VALVES ARE OFF, APU AND MAIN ENGINE BLEED VALVES ARE CLOSED AND THE ISOLATION VALVE IS CLOSED
(a) No recording required.
Item 4. START APU - CHECK ACCELERATION - RECORD TIME FROM STARTER ENGAGED AS NOTED BY DC VOLTAGE DROP, TO APU GENERATOR "OFF" BUS LIGHT ON (20-40 SECONDS)
(a) This is to evaluate condition of battery circuit, starter motor and clutch, fuel control, centrifugal switch and EGT control system.
CAUTION: TO AVOID DAMAGING APU, OBSERVE OPERATING LIMITS (REF 49-11-0, MAINTENANCE PRACTICES).
Slow acceleration time may be due to: 1) Aircraft battery - low state. 2) APU starter electrical cables faulty. 3) Starter clutch slipping.
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APU Condition Monitoring Check 501
49-11-0 Figure 602 Feb 01/95
Page 606
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4) Fuel control accelerator limiter diaphragm leaking.
5) Fuel control accelerator limiter valve leaking (half ball piston out of position or
piston limiter O-ring extruded). 6) Fuel control accelerator limiter crack pressure setting low. 7) Fuel control accelerator limiter pneumatic control lines loose, or leaking. 8) Fuel control accelerator limiter air supply restricted (orifice clogged or line
plugged with ice). 9) Swing check valve or 3-way solenoid valve leaking. 10) Thermostat set too low.
NOTE: Hung starts cause combustor and hot gas nozzle damage.
11) Thermostat stuck open. Item 5. RECORD NO-LOAD IDLE EGT - APU BLEED VALVE SWITCH CLOSED - MINIMUM ELECTRICAL LOAD (280-350° MAXIMUM)
(a)
Low IDLE EGT can be faulty EGT indicating system.
(b)
High idle EGT may be caused by:
1) Dirty compressor
2) Dirty crossover ducts
3) Damaged compressor wheels
4) Restricted inlet
5) Damaged diffuser section
6) Damaged or wrong combustor
7) Damaged torus
8) Damaged hot gas nozzle
9) Damaged turbine wheel
10) Leaking gasket between plenum and case assembly
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11) Bleed air duct leakage - clamp at APU bleed valve, clamp at APU turbine
plenum or slip joint between APU and APU shroud. 12) Damaged generator bearing 13) Any bearing failure causing DRAG
Item 6. NO-LOAD GENERATOR FREQUENCY (405-410 HZ)
(a)
Used as RPM indicator.
(b)
Used to calculate droop under load.
(c)
Over-speed may cause APU shutdown.
(d)
Under-speed will cause 95% switch actuation dropping load. Item 7. SELECT NO. 1 TANK FUEL BOOST PUMP ON
NOTE: If you must use the fuel boost pumps in the center tank, you must have a maintenance person or observer in the flight compartment to continuously monitor the LOW PRESSURE lights. Turn the applicable fuel boost pump to the OFF position if the LOW PRESSURE light for the center tank stays on. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：737 AMM 飞机维护手册 机载辅助动力AIRBORNE AUXILIARY POWER(31)
