Auxiliary Power Unit
Last updated
Last updated
The APU is a self-contained unit composed of a two-stage centrifugal compressor coupled to a single-stage turbine. It provides electrical power and compressed air during ground operation. It is capable of driving ist standard engine generator and supplying pneumatic air tot he airconditioning packs. Primary control of the unit is achieved through a master switch on the flight engineer’s auxiliary panel. In addition an APU remote control panel is located in the left wheel well. Once the start ssequence is begun, operation of the auxiliary power unit is completely automatic.
The APU is completely enclosed within a shroud and is attached tot he airplane keel beam aft of the main ladning gear wheel well forward bulkhead. APU air inlets are in the wheel wells; exhaust is through louvers, or an electrical operated door in the top of the right wing root. Automatic controls will cut back on bleed air to favor electric power when the APU is reuiqred to support both electrical and pneumatic loads. Fire detection and protection are provided.
(1) APU Master Switch ON: The fuel shutoff valve opens, generator breaker light illuminates. START: The starter and automatic start sequence is energized. OFF: Stops the APU. (2) Generator Field Light Illuminates if generator field is deenergized. (3) Auto Fire Shutdown Switch ARMED: A fire warning or test will automatically stop APU. OFF: Automatic shutdown deactivated. (4) Generator Field Switch CLOSE: Energizes generator field. TRIP: Trips generator field and generator breaker. (5) Generator Breaker Light Armed by APU master switch. Illuminates when generator breaker is tripped. (6) Generator Breaker Switch CLOSE: Connects generator to airplane sync bus, trips external power and engine generator breakers. TRIP: Trips generator breaker. (7) Ammeter Indicates APU or external power load. (8) Exhaust Gas Temperature Indicator Indicates temperature of APU exhaust duct gas. (9) Fire Test Switch: TEST: Lights up FIRE-PULL warning light, bell and disconnects APU starter circuit. RESET: Resets fire test light and bell. Second reset connects starter circuit again.
Fuel is drawn from the No. 2 tank through the APU fuel shutoff valve. APU fuel consumption during normal operation is approximately 250 pph. With the APU master switch ON, the APU fuel shutoff valve opens, the exhaust door opens, and fuel is supplied tot he fuel pump and control unit when the turbine is rotating.
The starting and ignition system consists of a starter motor, a starter relay, an ignition unit and an ignitor plug. When the APU master switch is momentarily held to START, the system functions automatically to energize the starter and ignitor plug in the proper sequence. As the unit accelerates the system automatically deenergizes the starter and terminates ignition, again in proper sequence. Ignition is terminated when the unit approaches full rpm.
The APU has a self-contained oil system. A low oil pressure switch ensures that combustion will not be initiated until a minimum oil pressure is available from the oil pump.
Cooling air is drawn from the wheel well through an air inlet and into the APU shroud. The cooling air circulates around the engine, ist accessories, the oil cooler and the generator. After cooling, the air is vented overboard with the exhaust gas.
Bleed air form the compressor is used for airconditioning packs and engine starting.
The APU generator furnishes 115/200 Volt, 3-phase ac power for ground operation of the electrical system. The generator is the same 40 KVA unit that is installed on the JT8D engines. However, the generator is rated at 60 KVA when installed on the APU because of improved cooling.
The generator hast he following fault protection:
Differential fault.
Exciter ceiling.
Under/over voltage.
Underspeed.
All of the above conditions will trip the field relay and generator breaker except underspeed, which will trip the generator breaker only. The APU cannot be paralleled with the airplane generators.
The following conditions must be met to initiate an APU automatic start sequence:
The groudn safety relay must be in the ground mode.
The battery switch must be ON connecting the airplane battery to the APU controls.
The APU fire handle must be in, and the remote control APU fire handle closed.
The APU FIRE warning TEST-RESET switch must be momentarily placed twice to the reset position if a fire alarm has occurred (either test or actual).
When the APU master switch iss et to ON:
The fuel valve opens and if dc buses are powered the APU light illuminates.
The exhaust door (if installed) opens.
Automatic start circuits are armed.
When the APU master switch is momentarily set to START:
The APU CRANK light illuminates.
The starter is engaged and ignition is energized.
Oil pressure opens the fuel solenoid valve.
The load control bleed valve is armed to open by EGT.
Ignition is terminated.
When the APU master switch is set to OFF:
The APU fuel valve closes.
The fuel solenoid closes.
As the APU spins down, the generator breaker opens and the load control bleed valve is disarmed.
