What’s the APU’s secondary function?
The secondary function of the APU is to supply compressed air as a source of bleed air to power the environmental control system (ECS) and engine starting system.
What’s the APU’s primary function?
The primary function of the APU is to operate the gearbox mounted 40 KVA generator. To provide electrical power.
What is the maximum operating altitude of the APU?
41,000 feet
What is the maximum altitude to start the APU in flight?
37,000 feet
What is the maximum altitude for APU bleed air extraction?
25,000 feet
What is the function of the APU intake door?
To supply ambient (static or ram) air to the air inlet ducting for APU operation.
What positions the APU intake door during normal operations?
The APU intake door actuator positions the intake door based on input from the APU electronic control unit (ECU).
In what position is the APU intake door during ground start?
100% open for start and on speed condition for ground operations.
In what position is the APU intake door during inflight start?
In flight the APU intake door opens with the “START/STOP” switch light and its position will vary with the APU speed and aircraft Mach number.
Why does the APU intake door not open in flight until the “START/STOP” switchlight is pressed?
This prevents early exposure of the APU to excessive amounts of ram air, resulting in “blowout” and APU windmilling.
What is the maximum allowable airspeed if the APU instake door position is open or unkown while in flight?
Airspeed is restricted to 220 knots, or the APU must remain in operation.
Where is the APU intake door position indicated in the cockpit?
On the Status Page.
Which tank normally supplies the APU with fuel?
The left collector tank.
What two switchlights control the APU Fuel Shutoff Valve?
- The APU PWR / FUEL switchlight
- The APU FIRE PUSH switchlight
What is the function and location of the APU Fuel Shut-off Solenoid Valve?
- It is located between the Fuel Control Unit and the spray nozzles
- It modulates to increase or decrease the amount of fuel directed to the fuel nozzles
- It closes (simulates an overspeed) to shut down the APU
What occurs when the “APU PWR FUEL” switchlight is selected?
- Activates the APU Electronic Control Unit (ECU)
- Activates the APU Fuel Pump
- Opens the APU Fuel Shutoff Valve
- Activates the APU EICAS Displays
- Initiates a prestart IN BITE feature
- Opens the APU Instake Door on the ground
What component controls the operation and control of the auxillary power unit (APU) fuel system?
The APU ECU
How is the APU cooled?
- APU compartment cooling is accomplish by the exhaust eductor. On the ground, the venturi effect created by the APU exhaust causes air to be drawn from the air inlet, through the oil cooler and into the APU compartment. The air is then removed from the compartment by the eductor. In flight, the ram air from the air inlet is pushed into the compartment through the oil cooler and then removed by the eductor.
What is the function of the APU Electronic Contrl Unit (ECU)?
The function of the ECU is to control all normal modes and emergency shutdown of APU operation.
When does the APU starter disengage?
Between 46% and 60% rotor speed depending on altitude.
What is the function of the “APU IN BITE” check?
The ECU does checks before the ignition/start sequence. Before the ignition/start sequence can begin, the ECU must satisfactorily complete the prestart built-in-test-equipment (BITE) check of all APU and ECU related components.
What controls the APU ignition system?
The ECU using 28 volt DC power from the DC Battery Bus as its primary source (and the Main Battery Direct Bus as its secondary source) to energize the ignition system during the ignition/start sequence.
What is the minimum battery voltage required to start the APU?
Both the Main and APU batteries are required to start and operate the APU. The minimum battery voltage for both batteries is 22 volts.
What are the power sources for the APU ECU and Starter during an APU start?
- ECU is powered by the Main Battery Bus
- APU Starter is powered by the APU Battery Bus
