Cooling Air Fan
A. The cooling air fan provides a positive cooling airflow through the oil cooler and the generator. The fan is an axial flow type mounted in a T-shaped housing (Fig. 1). It is driven by the accessory drive section. One arm of the housing provides the support for the fan bearings and the mounting flange that attaches the housing to the accessory drive casing. The other arm forms the air inlet duct to the fan and includes an air inlet screen. The leg of the T-shaped housing is the fan air outlet and provides the mounting for the cooling air crossover duct.
6. Cooling Air Crossover Duct
A. The cooling air crossover duct provides airflow from the cooling air fan to the oil cooler and ac electrical generator. The duct is V-shaped (Fig. 1).
7. Operation
A. When the APU is operating, cooling air is supplied by the cooling air fan. It is driven by accessory drive section gear train. The fan draws static air while on the ground, or is supplied by ram air in flight, from the torque box through the diffuser air duct. The cooling air shutoff valve is held open by the pneumatic actuator. The actuator obtains the pneumatic pressure from APU bleed duct upstream of the APU bleed valve. The cooling air from the fan is distributed to the ac electrical generator and the oil cooler through a cooling air crossover duct. From the manifold, the air is forced through the generator and oil cooler, and exhausted inside the APU shroud. The exhaust air is circulated inside the shroud to cool the APU engine and engine accessories, then exhausted overboard. When the APU is shut down, the cooling air shutoff valve is closed by the actuator. The valve serves as a flame stop in event of an APU fire, thus preventing the fire from spreading into the torque box in the fuselage.
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49-51-0 Page 2 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Feb 1/76
550 APU Cooling Air System Component Location
Nov 01/74 Figure 1 (Sheet 1) 49-51-0
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
APU Cooling Air System Component Location 535
49-51-0 Figure 1 (Sheet 2) Aug 15/72
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
APU COOLING AIR SYSTEM - MAINTENANCE PRACTICES
1. General
A. The maintenance practices described herein deal specifically with the cleaning of the cooling air inlet screen attached at top of cooling air fan housing. The cooling air fan screen cannot be removed from the engine while installed in airplane. It can however, be checked and cleaned using a portable vacuum cleaner with a minimum diameter of 1.0 inch flexible nozzle. Access to the screen is through the inspection tube attached to the cooling air inlet duct.
NOTE: Sufficient blockage of the screen will cause an automatic shutdown of APU due to high oil temperature.
2. Examine Cooling Air Fan Screen
A. Position BATTERY SWITCH and APU MASTER SWITCH to OFF and placard that APU maintenance is “IN WORK".
B. Open stabilizer trim actuator and stabilizer trim control circuit breakers on panel P6.
C. Open circuit breaker on M280 module -APU accessory unit on E3-3 electrical shelf.
D. Install the stabilizer trim lock assembly on the stabilizer trim wheel. (Fig 201)
(1)
Turn the trim wheel to put the handle at the top of the wheel.
(2)
Adjust the height of the trim lock to set the trim wheel handle tightly in the bottom of the yoke.
(3)
Install the pin through the yoke and install the safety pin.
E. Gain access to compartment immediately aft of pressure bulkhead.
F. Disconnect the electrical connectors from the stabilizer trim electric actuator, and the autopilot stabilizer trim servo.
G. Remove cap assembly from viewing tube.
H. Using a flashlight, inspect screen for accumulation of oil and lint or other debris.
3. Cooling Fan Inlet Screen Cleaning
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