Auxiliary Power Unit

Table of contents:
  • Normal Operation
  • Alternate Operation
  • Controls and Indicators
  • Schematics
  • Supplementary Information

Normal Operation

APU Ground Start:
Battery Switch - ON
DC Meter Switch - APU TRU.BATT
APU Oil quanity - 2.25 Quarts Min
Note: The START position is deactivated until the APU door has fully opened and the APU DOOR light extinguished.  The DC ammeter indicates discharge current during APU start.

APU Shutdown:

APU Bleed Air Switch - CLOSE
APU Master Switch - STOP

- FUEL VALVE light illuminates, then extinguishes when fuel valve fully closed.
- APU FAULT light illuminates
- RPM and EGT decrease
- APU DOOR light illuminates at 50% RPM
- APU FAULT light and APU DOOR lights extinguish, verifying APU door is fully closed.

Note:  APU FAULT light failure to illuminate when the APU master switch is positioned to STOP indicates a malfunctioning of the overspeed protection circuit.  However the fuel valve functions normally and the APU shuts down due to fuel starvation and the FUEL VALVE light functions normally.

APU Inflight Start:
If certified and placarded for in-flight start, the APU may be started below 20,000 feet pressure altitude.
Note: APU bleed air may be used below 15,000 feet altitude.

Alternate Operation

APU Starter Power Failure:
If the APU fails to start on battery, the APU start procedure may be repeated with the APU start selector (START SEL) in the TRU position.  If start is not achieved, maintenance action is required.

Auto Shutdown:

Note: An APU shutdown can occur due to a rapid decrease in bleed demand.  This causes a compressor stall and results in decreasing EGT and RPM.  The APU fault light does not illuminate.

Generator Bearing Failure Light Illuminated:
Note: APU generators may be operated with a faulty bearing up to 15hours under no load.

Controls and Indicators

Controls and indicators

APU RPM Indicator:
Indicates APU engine speed in percent rpm.

Oil Quantity Indicator:
Indicates APU oil quantity by increments.  Operative whenever battery bus is powered.

Start Selector Switch:
TRU - APU starter power provided from APU starter TR unit.
BAT - APU starter power provided from APU battery.

EGT Indicator:
The index mark (brown) on the EGT indicator is for maintenance use in instrument calibration.

APU Bleed Air Switch:
- APU provides bleed air to pneumatic manifold
- Provides bleed air for APU inlet anti-icing.
- Bleed air valve closed.

APU Master Switch:
START - Held momentarily spring loads to ON upon release.  Initiates starter motor cranking.
ON - Opens APU door.  Opens APU fuel valve.  Starts AC or DC fuel pump.
STOP - Signals 110% switch for shutdown.  Position to STOP after auto shut down to reset circuits for another start.

Annunciator Lights:

APU Fueld Shutoff Valve Light (Blue):
EXTINGUISHED - APU fuel shutoff valve open or closed.
ILLUMINATED - Valve in transit.

DC Pump On Light (Green): APU DC Pump operating.

APU Fault Light (Amber):
Illuminated - APU has shut down due to:
- Low oil pressure with RPM above 95%
- High oil temperature
- Cooling air valve fails to open
- High EGT
- Overspeed, 110% RPM
- Excessive pressure drop across APU inlet screen
- Duct failure overheat
- Loss of airplane battery power
- Loss of APU sensor signals: EGT, Air inlet temperature, Oil temperature, Oil pressure
- Normal shutdown - the light will extinguish when APU inlet door has closed.

Not Used

Supplementary Information:

The auxiliary power unit (APU) is a self contained gas turbine engine.
It is installed in the tail cone of the airplane, isolated from flight critical structure and control surfaces by a firewall.
The APU provides additional electrical and pneumatic power for systems operation.
The operational certification decal located over the APU module states the APU may be operated in flight.

Fuel System:
Fuel is normally supplied to the APU from the airplane No. 2 main tank.
Fuel can be supplied  from any tank through the crossfeed system.
In the event that AC power is not available a DC operated pump, controlled by the APU master switch, supplies fuel from No. 2 main tank to the APU.
With AC power available, the No. 2 aft main boost pump operates automatically.  When the No. 2 engine fuel feed line is pressurized, a pressure sensor deactivates the DC powered fuel pump.

Oil System:
The APU oil system is a self-contained system consisting of independent supply, pumps, regulator, cooler, filters and indicator.
A quantity indicator is provided on the APU module.

Electrical System:
The APU drives two generators which are identical to the engine driven generators at constant speed.
The APU generators are capable of supplying the entire electrical load of the airplane.

Bleed Air System:
The APU provides bleed air to the airplane pneumatic manifold for operation of pneumatic components.
When electrical load and bleed air extraction combine to raise EGT above normal operating temperature, the load control valve modulates toward the closed position to restrict bleed air extraction.
When large amounts of pneumatic power are required, as during engine start, electrical loads should be reduced.

APU Battery and Starter:
Either the APU battery or the APU starter transformer rectifier unit (TRU) may be selected as a power source of starter power.
To preserve APU battery power the TRU position of the APU start selector switch should be selected when AC bus 3 is powered.
APU control circuits are powered by the main airplane battery.
Battery switch must be ON for APU start and operation.

Remote Stop Switch:

The remote stop switch is mounted on the APU remote control panel located in the right main wheel well.

Pressing the switch a minimum of five seconds shuts down the APU.

Note:  The APU FAULT light will not illuminate when the remote stop switch is used for shut down.

Starting Sequence:
7% - APU fuel solenoid valve opens and ignition energized
50% - Starter cutout.  DC ammeter and DC voltmeter normal.
95% - Ignition cutout, APU starts governing, Load enabling switch closes, Bleed air valve can be opened and generators connected to Sync bus.
100% - Normal operation with APU governing +/- 1.25% RPM.

Pulsating EGT Indication:
The electronic turbine control monitors each of the two EGT themocouple harness circuits of the APU.
If one of the two circuits is open or is presenting a significantly lower temperature the fault will be shown on the APU EGT indicator.
The EGT reading will pulse every 2 - 2.5 seconds from a stabilized (valid) reading to a reading above or below the stabilized reading.
This will not cause an automatic shutdown and the APU is operative if this condition exists.
However corrective maintenance action should be requested as soon as practicable to replace the faulty thermocouple harness.

Auto Shutdown:
The auto shutdown circuits will cause APU shutdown by closing the APU fuel shutoff valve, fuel solenoid valve and deactivating ignition (if oeprating).
The following causes of auto shutdown represent those conditions which are readily monitored:
- Overspeed - 110% RPM
- Fire signal (from the fire detection system)
- High EGT (when EGT reaches maximum allowable for the RPM range).
- Low oil pressure.

Location of APU in tail

APU Control Panel (Top Right of FE Panel)