13. APU Exhaust System (Fig. 3)
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A. The APU exhaust system removes the hot exhaust gases overboard, keeps the APU noise to a minimum and prevents hot exhaust gases from touching the internal surfaces of the APU compartment and airplane skin. The exhaust duct removes the exhaust gases aft and upward from the airplane. The exhaust system has an outer exhaust duct, inner exhaust duct and acoustic liner. The exhaust duct is cooled by secondary air flowing between the outer and inner duct and exiting into the exhaust stream at the nozzle. For a more detailed description of the APU exhaust system, refer to AMM 49-81-00/001.
B. High temperatures can be found up to 40 feet (12.2 meters) from the exhaust nozzle of the APU depending on wind conditions. An APU start must not be attempted within range of flammable materials or fumes. A minimum of 40 feet (12.2 meters) must be clear at the rear of the aircraft to prevent injury to persons or damage to equipment during an APU operation.
14. APU Oil External Distribution System
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A. The APU oil external distribution system gives the primary and secondary engine oil cooling by moving the oil through two external heat exchanger units. The APU oil external distribution system has the air-cooled oil cooler and fuel-cooled oil cooler. For a more detailed description of the APU oil external distribution system, refer to AMM 49-91-00/001.
15. APU Oil Indicating System
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A. The APU oil indicating system monitors the oil conditions in the APU. The APU oil indicating system has an oil temperature sensor, low oil pressure switch, oil level sensor and oil filter differential pressure indicator. For a more detailed description of the APU oil indicating system, refer to AMM 49-94-00/001.
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49-11-00
ALL ú ú 05 Page 15 ú Feb 18/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
16. Operation
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A. Auxiliary Power Unit Operation
(1)
The battery switch on the P5 overhead panel is set to the ON position before an APU start. The APU start transformer rectifier unit (TRU) is used as a primary power source to turn the APU starter motor. The APU battery is the secondary power source for the APU. You set the APU START SOURCE switch on the P461 pilots' overhead maintenance panel to the "TR" position for an external power (APU start TRU) start or to the "APU BATTERY" position for an APU battery start. The APU control switch on the P5 overhead panel is set to the START position and then release to the ON position. The air inlet door actuator opens the air inlet door to the fully open position (45°). If the air inlet door does not open in 60 seconds, the EICAS display shows "APU DOOR" on the status page. During an in-flight operation, the ground safety relay, R263, is energized and the air inlet door closes to the partly open position (15°). The APU fuel shutoff valve opens on the rear spar No. 2 main tank. If ac power is available, the ac aft fuel boost pump supplies fuel from the No. 2 main tank of the airplane to the fuel pump. If ac power is not available, the dc fuel boost pump operates from the airplane main battery. After the air inlet door opens, the APU controller does the APU start sequence and supplies 28V dc power to the starter motor. The starter motor starts to turn the low pressure (N1) and high pressure (N2) turbine rotor assemblies. The speed sensor (N1) measures the speed of the input shaft of the load gearbox. The speed sensor (N2) measures the speed of the oil pump drive. When the N2 rotor speed is 8%, the APU controller energizes the ignition exciter and sends a signal to the fuel metering unit to supply fuel to the combustion chamber. The ignition exciter causes the two igniter plugs to make sparks. The APU engine has combustion and starts to increase in speed. When the N2 rotor speed is 40%, the starter motor and ignition exciter are de-energized by the APU controller. When the N2 rotor speed is 55%, the APU controller sets the inlet guide vane (IGV) demand to a minimum. Two seconds after the N1 rotor speed is 95%, the APU is prepared for pneumatic and/or electrical loads.
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