APU System

Question 1

What does the APU do?

Answer 1

• Provides pneumatic power for starting engines
• operating ECS

Question 2

what is mounted on the APU accessory gearbox?

Answer 2

• Hydraulic starter
• output driver for apu fuel assembly, oil pump, air-cooled ac generator

Question 3

how is the APU lubricated?

Answer 3

• self-contained oil system.
• fuel consumption is 150 lbs per hour

Question 4

APU system accessories

Answer 4

• prime/boost pump-used to prime engine or APU fuel lines. provide fuel under pressure to APU during starting and ops at PAs above 8,000ft.
• Hydraulic accumulator (w/ hand pump): provides hyd pressure for driving the APU starter. min accumulator pressure required for starting the APU is 2,650 psi.
  
  • can be recharged by using the hand pump. with ac pwr it is charged by the backup pump

Question 5

APU CONTR switch

Answer 5

• ON: opens APU airframe fuel shutoff valve, sends a start signal to APU electronic sequencing unit (ESU or DESU [digital])
• OFF: removes electrical power from the system, closing the airframe fuel shutoff valve

Question 6

Starting the APU

Answer 6

• FUEL PUMP in APU BOOST. pressurized fuel supplied from R fuel cell by the prime/boost pump.
• APU CONTR to ON initiates start sequence
• ESU/DESU sends signal to open the APU start valve, releasing hyd acumulator charge to the starter.
• ass accum pressure drops below 2,650psi, the APU ACCUM LOW advisory appears. (good dump)
• APU ON appears when APU is on and operating normally.
• placing APU GEN switch ON makes electrical power available.

Question 7

What if APU does not start and the APU ACCUM LOW advisory light is not illuminated?

Answer 7

• a start may be attempted by simultaneously moving the APU CONTR switch to ON and actuating the manual START/OVERRIDE lever (located on accum manifold)
• APU pressure will be dumped to starter to turn the compressor until the APU has reached a self-sustaining speed.