Auxiliary Power Unit Flashcards
What is the primary and secondary function of the APU?
The primary: operate the accessory gearbox mounted 30 KVA generator.
secondary: supply bleed air for air conditioning and engine starting.
What is the maximum operating altitude of the APU?
37,000 feet
Does the APU vary its RPM?
The APU is required to run at a constant 12,000 rpm to maintain a frequency of 400Hz.
Where is the APU located?
The APU is located in the aft equipment bay inside a fireproof titanium enclosure.
What controls and monitors all APU operations?
An automatic electronic control unit (ECU) controls all of the APU operations.
Is the APU ECU DC or AC powered?
The ECU is DC powered.
What is the maximum altitude to start the APU?
30,000 feet
What is the maximum altitude to use APU bleed air?
15,000 feet
What is the maximum altitude to start an engine with the APU?
13,000 feet
(VGD)
variable geometry diffuser
How is the flow of air through the compressor controlled?
The air is controlled by the variable geometry diffuser (VGD).
What is the function of the APU VGD?
The VGD helps to protect against surge and stalls and regulates the discharge airflow from the compressor impeller.
The VGD opens as the air needed increases during electrical and bleed air loading.
What accessories are mounted on the APU gearbox?
1) The AC generator
2) fuel control unit (FCU),
3) oil cooling fan
4) DC electric starter
5) oil pump
Where is the APU air intake located?
upper left rear side of the fuselage
How many positions does the APU intake door have?
It has three positions, open, mid, or closed
How does the APU door position is determined?
by the ECU and is based on aircraft altitude and true airspeed.
What can be done if the APU door actuator fails and the door is not operable?
Maintenance can mechanically position the door to any of the three positions. This allows the aircraft to be dispatched.
What is the limitation if the APU intake door is failed open or if the door position is unknown?
The airspeed is restricted to 300 KIAS or the APU must remain running then there will not be a restriction.
How many feet is the APU exhaust danger area?
15 feet
The variable position APU intake door is controlled by which system? Door position is depended on:
ECU controls the door position via a DC powered actuator.
Door position is dependent on weight-on-wheels, APU rpm and Mach number.
How is the APU lubricated?
The APU has a self-contained lubrication system.
There is a gearbox mounted oil pump, oil cooler assembly, and an integral oil tank.
What is the purpose of the screened duct on the left rear fuselage?
Air is drawn in through the screen by an APU gearbox driven fan to provide APU oil cooling and ventilation of the APU enclosure.
Where does the APU normally receive fuel?
The APU receives fuel from both wing tanks by the APU/XFLOW pump.
How many pounds an hour does the APU burn?
Approximately 120 pounds an hour with electrical and bleed air loading.
What is the purpose of the XFLOW/APU pump in the operation of the APU?
This pump draws fuel from both wing tanks and sends the fuel to the APU.
How will the APU receive fuel should the XFLOW/APU pump fail?
APU negative gravity relief valve, which is pressurized by the right engine fuel feed manifold.
What is the purpose of the APU fuel feed shutoff valve (SOV)?
It is used to control the fuel supply to the APU.
How is the APU SOV opened?
By pressing the PWR FUEL switch light on the APU control panel.
When will the APU SOV close?
1) It will close manually by pressing the PWR FUEL
2) APU FIRE PUSH switch light.
3) when an APU fire is detected.
Which APU SOV is not on the fuel status page?
The fuel solenoid SOV that is located between the APU FCU and the fuel nozzles.
How is the APU fuel solenoid SOV opened?
The APU ECU at 4%rpm opens it.
FCU
APU Fuel Control Unit
Once the APU is stabilized, what will influence the fuel scheduling?
EGT
RPM
generator
bleed air demands.
How is the APU shut down normally?
By pressing the APU START/STOP switch light will close the fuel solenoid SOV. This will stop the flow of fuel
Is the APU fuel solenoid SOV DC or AC powered?
It is DC powered.
When the APU PWR fuel switch light is pressed, what is occurring
It initiates the APU pre-start sequence.
What is the APU pre-start squeeze
1) The APU IN BITE status message appears on the status page for 5 seconds and then disappears which means it is performing a self-test.
2) The RPM AND EGT gauges for the APU are displayed on the status page.
3) The APU intake door opens and the door status appears on the status page.
4) The APU fuel shutoff valve is opened. The APU SOV OPEN status message appears.
5) The XFLOW/ APU fuel pump starts.
What happens when the start/stop switch light is pressed during the APU start process?
The ECU will perform the following:
1) The APU START on the status page and the START light in the switch light illuminates. The starter motor is energized
2) At 4%rpm fuel and ignition is introduced. Fuel is introduced
by the fuel solenoid SOV.
3) The starter motor is de-energized by the ECU at 50%rpm, and
the START in the switch light extinguished
4) The green AVAIL light in the START/STOP switch light illuminates at 99% rpm plus 4 seconds. The APU LCV OPEN status message appears.
What does the green AVAIL in the APU START/STOP switch light indicate?
The light means that the APU is ready to accept bleed air loading.
When does the AVAIL light will come on?
4 seconds after the rpm is at 99% or more
What does the amber PUMP FAIL in the APU PWR FUEL switch light indicate?
It indicates a failed XFLOW/ APU pump.
When will the APU starter disengage?
At 50% or when starter engagement time exceeds 60 seconds
What are the APU start limitations? with APU battery start
What are the APU start limitations? with APU DC GROUND POWER start
How many igniter plugs does the APU ignition system have?
It has two air-gap igniter plugs.
During the APU start sequence, when is the ignition de-energized?
The ignition is de-energized at 95% rpm.
Which battery is used for starting the APU?
Both the main and APU batteries are required to start and operate the APU.
The APU battery is used to turn the APU starter
the main battery is used to operate the APU ECU
What is the minimum battery voltage to start the APU?
22 volts on both the main and APU battery.
Why do the APU PWR fuel switch light and the APU start/stop switch light have to be pressed within 5 seconds of each other while in flight?
Concept of APU ECU inhibit logic:
The APU door will open and if the APU is wind milling at a speed greater than 8%rpm before pressing the START switch light, the APU starter will not engage.
How long after the APU is stopped can it be restarted?
When the APU is stopped, the ECU initiates a 60-second BITE test.
This test must be complete before the APU can be started again.
What is BITE?
Built-in test equipment
After the APU is stopped by the START/STOP switch light, when can
the PWR/FUEL switch light be deselected?
When the AVAIL light in the START/STOP switch light extinguishes.
A better practice is to wait until 10% RPM.
What is the recommended time between cranking attempts of the APU?
Two minutes because this allows the ECU to reset.
Under what conditions will the APU automatically shutdown on the ground and in flight?
APU automatically shutdown on the ground and in flight?
Fire: APU fire detected
Ground and Flight
APU automatically shutdown on the ground and in flight?
Slow start: engaged more than 60 seconds
Ground and Flight
APU automatically shutdown on the ground and in flight?
Over speed: RPM greater than 107%
Ground and Flight
APU automatically shutdown on the ground and in flight?
DC power loss
Ground and Flight
APU automatically shutdown on the ground and in flight?
RPM signal lost
Ground and Flight
APU automatically shutdown on the ground and in flight?
EGT signal lost
Ground
In Flight
Note 1: APU bleed air is not available in flight when the EGT signal is lost.
Note 2: The APU control logic reverts to the ground mode 60-seconds after landing.
APU automatically shutdown on the ground and in flight?
Low oil pressure
Ground
Flight
Note 2: The APU control logic reverts to the ground mode 60-seconds after landing
APU automatically shutdown on the ground and in flight?
EGT over temperature > 743C, EGT >974C and <50% rpm
Ground
Note 2: The APU control logic reverts to the ground mode 60-seconds after landing.
APU automatically shutdown on the ground and in flight?
Intake Door position signal lost
Ground
Flight: The APU control logic reverts to the ground mode 60-seconds after landing.
APU automatically shutdown on the ground and in flight?
An electrical short detected on a circuit, over current
Ground
Note 2: The APU control logic reverts to the ground mode 60-seconds after landing.
Explain the APU variable geometry diffuser (VGD).
The VGD is used to protect the compressor from surging or stalling and is controlled by the APU ECU.
The vanes, which are located at the compressor discharge, are opened to increase the flow of air through the compressor and closed to decrease the compressor airflow.
The need for different airflow amounts is due to electrical and bleed air loading of the APU.
What switch allows bleed air to be supplied to the 10th stage bleed air manifold by the APU?
The APU LCV switch light located on the BLEED AIR control panel.
What is the primary function of the APU?
The APU’s primary function is operation of the AC generator and has priority over the bleed air service.
The APU LCV will modulate towards closed to reduce the amount of air bled from the APU compressor during combined APU generator and APU bleed air loading.
Explain the two levels of APU compressor stall and surge protection
The engines can provide 10th stage bleed air at a much
greater pressure that the APU. An APU compressor stall or surge could happen if this high-pressure engine air were to reverse-flow to the APU. To solve this problem, a check valve is installed in the APU bleed air duct to prevent the reverse flow.
The APU LCV has an interlock function with the 10th stage bleed air switches. If the left 10th stage bleed switch is selected open, the APU LCV will close. If the right 10th stage bleed switch and the isolation valve switches were selected open, the APU LCV will close. It is all tied to switch position
Can the APU be shutdown from outside the aircraft?
There is a remote cut-off switch located at the external services panel and in the aft equipment bay on the APU
If the APU is shut down on the ground, what will alert ground
personnel?
A warning horn will alert ground personal
If the APU door position is unknown by the APU ECU, what will indicate this?
The EICAS will show a white DOOR message with amber
What are the APU RPM ranges?
- Green 0 -100%.
- Amber 101-106%.
- Red 107% and above, which will cause an automatic shutdown
What are the APU EGT ranges?
- GreenO-712C.
- Amber 713 - 742 C.
- Red 743 C or greater or 974 C <50% during start.
