APU

Question 0

What happens when you press the APU PWR Switchlight?

Answer 0

• APU Start Circuits Armed
• Opens APU fuel shutoff valve (indicated with advisory lights on FPP).
• Self-Test.

Question 1

Give a general description of the APU.

Answer 1

• In the titanium tail cone, with firewall.
• Two clamshell doors below provide access.
• Consists of a gas turbine that runs an air cooled, 28v, 400 amp, DC starter-generator.
• Also provides bleed air to the Packs (engines bleeds off).
• Start, Normal Operations, and Faults managed with designated FADEC.
• Has a designated fire detection and automatic extinguishing system.
• Ground Operations ONLY / CANNOT Operate in flight.

Question 2

When is APU DC Power inhibited?

Answer 2

When the DC EXT PWR switch is selected ON.

Question 3

When is APU BL AIR inhibited?

Answer 3

When either engine bleed switch is selected ON.

Question 4

Describe APU Bleed Air Operation.

Answer 4

• Used for ECS and to hold AFT Safety Valve open.
• Bleed Air automatically reduced if exhaust temperature becomes too hot (gen load has priority).
• Check valves prevent engine bleed air entering APU system and vice versa.
• APU Bleed Valve will close is engine bleed air is turned on.

Question 5

Describe the APU Fuel System.

Answer 5

• Gets fuel from the left wing collector bay through an APU Fuel shutoff valve.
• A gravity fed, APU-driven fuel pump provides positive pressure.
• APU FADEC manages fuel scheduling
• APU Fuel shutoff valve position is indicated on FPP.

Question 6

When will the APU Fuel Shutoff Valve close?

Answer 6

• PWR Switchlight is pushed (always shutoff generator and bleed air first).
• EXTG Switchlight on FPP pushed.
• FIRE TEST Switchlight pushed.
• FIRE
• WOW
• Fault Detected
• GEN OHT

Question 7

When will the WARN Segment off the APU GEN Switchlight illuminate?

Answer 7

• GEN has not been selected ON, with the APU running.
• GEN does not come ON (EXT PWR selected ON)
• APU taken offline (APU GEN malfunction)

Question 8

What indications does an APU GEN OHT produce?

Answer 8

• GEN OHT
• WARN segment of GEN Switchlight
• APU FAIL
• APU Caution Light
• MASTER CAUTION

Question 9

How is an APU GEN malfunction indicated?

Answer 9

• ON segment extinguishes and WARN segment illuminates.
• APU Caution Light
• MASTER CAUTION

Question 10

When does the APU fire detection and extinguishing system monitor the APU hot section and the APU exhaust?

Answer 10

When the right essential DC bus is energized.

Question 11

APU fire indications

Answer 11

• APU FUEL CLOSED illuminates, OPEN extinguishes on FPP
• BTL ARM on FPP
• FIRE on FPP
• EXTG Switchlight on FPP
• APU FAIL on APU CP
• APU Caution
• CHECK FIRE DET Warning
• MASTER WARNING
• MASTER CAUTION

After seven seconds, extinguishing agent should be released, BTL ARM Light extinguishes and BTL LOW light illuminates. FIRE a light should extinguish. APU disabled.

Question 12

APU fire detection test

Answer 12

• 5-1-2-2
• On FPP, FUEL VALVE CLOSED, BTL ARM, EXTG, FIRE, FAULT.
• APU FAIL.
• APU Caution and CHECK FIRE DET Warning.
• MASTER CAUTION and WARNING.

Question 13

When will the APU FADEC automatically shut down the APU?

Answer 13

• Overspeed or underspeed. (95-105%)
• Start / acceleration failure (cranking limits…).
• EGT Overtemp (1890°F Starting, 1324°F Running)
• Oil Overtemp (> 275°F)
• Low oil pressure (< 35 psi).
• Failed Sensors (2 or more).
• Failed valves, relays, or circuits.
• Internal Failure.

Question 14

What are the APU Starter Cranking Limits?

Answer 14

60 seconds On, followed by 5 minutes OFF

60 seconds On, call MX