APU

Question 1

What does the APU provide?

Answer 1

The APU supplies bleed air for engine starting or air conditioning. An AC electrical generator on the APU provides an auxiliary AC power source.

Question 2

Does the APU require the BAT switch to be on to operate?

Answer 2

Yes because the APU Electronic Control Unit (ECU) is powered off the switched hot battery bus. Moving the BAT switch to OFF on the ground or in the air automatically shuts down the APU because of power loss to the electronic control unit.

Question 3

How long should the APU run after illumination of the APU GEN OFF BUS light before opening the APU bleed valve?

Answer 3

Operate the APU for two full minutes before using it as a bleed air source. This two-minute stabilization is recommended to extend the service life of the APU.

Question 4

How long should a pilot wait after the APU switch is selected OFF before selecting the BAT switch OFF?

Answer 4

NOTE: Wait at least five minutes (MAX) after APU GEN OFF BUS light extinguishes before positioning the BAT switch to OFF to allow adequate time for the APU inlet door to fully close.

NG = 2 Minutes

Question 5

What will cause the automatic shutdown of the APU?

Answer 5

• RPM overspeed conditions
• low oil pressure
• high oil temperature
• APU fire
• Fuel Control Unit failure
• EGT exceedance, and
• other system faults monitored by the ECU. Note also a loss of power to the ECU such as turning off the battery switch.