APU Flashcards
What is the primary purpose of the APU
Provide electrical power.
What is the name of the computer than controls APU operation automatically?
Electronic Control Unit (ECU)
When does the APU automatically discharge the bottle in event of a fire?
On the ground after 5 seconds
What valve controls APU bleed output?
Load Control Valve (LCV)
What computer shares control of the LCV with the ECU?
ACSC1 (Air Conditioning System Controller)
What is the critical rotation speed of the APU and why is it important?
4%-30%
Below 4% is safe due to lack of heat that could cause bearing damage.
Above 30% allows sufficient cooling oil to lubricate bearings.
What is the minimum battery voltage required to start the APU
22V
What happens when the PWR/FUEL switchlight is selected on the ground?
PDIGS
Pump, door, in bite, gauges, SOV
What happens when the PWR/FUEL switchlight is selected in the air?
PIGS
Pump, in bite, gauges, SOV
What happens when you press START/STOP switchlight on APU start?
Start:
-DC starting motor engages
- APU START status msg
- White START switchlight on
- Fuel + ignition at 5%
- Starter disengages at 46-60% RPM depending on altitude
- 95% de-energize and SOV open removed
- 99% + 2 seconds green AVAIL illuminates
What is the difference in APU air start vs ground start?
In air the door is opened when pressing START/STOP instead of PWR/FUEL
What happens when you press START/STOP to turn off the APU?
ECU is sent an overspeed signal, Green AVAIL extinguishes, APU SOV closes to stop fuel, RPM/EGT decrease until 25% when intake door closes, Door close displayed on EICAS.
When can you restart the APU following normal shutdown?
12% RPM or less.
Limitations: 2 minutes between cranks, no more than 3 starts per hour.
Which tank does APU draw fuel from?
Left collector tank
Which pump provides pressurized fuel to APU?
APU Pump
If the APU pump were to fail, how does the APU continue operating?
Bypass valve allowing APU gearbox driven pump to provide fuel.
What does green AVAIL mean on APU START/STOP push button?
APU is available to take electrical load.
Where does the APU receive intake/cooling air from?
Eductor on tail pipe uses venturi effect to cool.
3 times APU will shut down automatically in the air?
Fire, Overspeed, ECU failure.
Which factors influence APU door position in flight?
APU RPM, Mach #, Weight on Wheels
Which APU indication is always shown on status page?
Door position
What are the ways the APU can be shut down from the flight deck? (3 ways)
- START/STOP switch
- Fire bottle
- POWER/FUEL switch
What are the ways the APU can be shut down from outside the aircraft? (2 ways)
- External services panel
- Shut off switch in APU compartment for maintenance
Max altitude for APU start?
37,000’
Max altitude for APU operations?
41,000’
Max altitude for using APU bleed air to start engines?
21,000’
What is the surge control valve on APU do?
Relieves pressure during compressor surges.
It is always open above 17,000’ because bleed air is not needed and APU is usually only used for electrical loads.
