APU

Question 1

What’s the purpose of the APU?

Answer 1

Supply the flight crew with an auxiliary source of bleed air and electrical power during ground operations. In-flight, the APU can be used for backup electrical power or used for air for main engine start. APU air cannot be used for pressurization.

Question 2

What indication do you have that air inlet door has opened and APU is ready to start?

Answer 2

READY light illuminates on COP

Question 3

When should you check the APU oil level?

Answer 3

15-30 minutes after shutdown to prevent over-servicing

Question 4

What’s the purpose of the APU load control valve?

Answer 4

It opens to provide bleed air from APU while aircraft is on the ground, as well as in-flight if needed, for main engine start. It’s controlled by the APU BLEED AIR switch.

Question 5

When is APU bleed air available?

Answer 5

After the APU reaches operating speed. If APU was started under cold conditions (i.e. EGT < 149°C), bleed air loading is delayed for 60 seconds unless attempting main engine start in-flight.

Question 6

When selecting APU BLEED AIR switch to ON during ground operations, what valve opens in addition to the load control valve?

Answer 6

Isolation valve

Question 7

True/False: The load control valve is prevented from opening in flight as a function of Weight-On-Wheels (WOW).

Answer 7

True

Question 8

Regarding the APU, two things occur with the right engine lower cowl door open. What are they?

Answer 8

The R COWL OPEN light illuminates on the APU panel and the ECU will inhibit the start while on the ground in order to protect the engine cowl.

Question 9

What function does the ECU perform if APU limits are exceeded?

Answer 9

It automatically shuts down the APU on the ground (non-essential mode) and in some cases, in flight (essential mode).

Question 10

Name the things that occur when APU MASTER switch is selected to ON.

Answer 10

Illuminates ON legend in the APU MASTER switch, performs initialization and APU prestart Built-in Test (BIT), checks APU oil temperature, commands APU inlet air door open, and commands APU fuel shutoff valve open

Question 11

What happens when the APU MASTER switch is selected to OFF?

Answer 11

Switch legend is extinguished, APU fuel shutoff valve is closed and APU, if running, will immediately shut down. The inlet door also begins to close.

Question 12

When does the READY light illuminate?

Answer 12

If all prestart conditions are passed, the ECU commands the signal to illuminate the READY light (within 10 to 16 seconds), indicating the APU is ready to start.

Question 13

Can the APU be started if the READY light fails to illuminate?

Answer 13

Possibly: give it a try

Question 14

What’s the significance of the ON legend in the APU START switch?

Answer 14

When illuminated, it indicates that the starter is engaged. It extinguishes at starter cutout.

Question 15

What happens when the APU STOP switch is depressed?

Answer 15

If APU is running (on-speed), APU loads are shed and APU enters cool-down mode. If on the ground and up to 20,000 feet, ECU decreases APU RPM on linear scale (1/2% per second) for 60 seconds (to 70%RPM), then shuts down APU. If above 20,000 feet, ECU will maintain APU RPM for 60 seconds, then shuts down APU.

Question 16

What happens if APU START switch is depressed at any time during 60 second cool down period?

Answer 16

APU is commanded to 100% RPM

Question 17

What’s the quickest way to shutdown the APU?

Answer 17

Any time the APU MASTER switch is selected off, the APU will immediately shut down (APU fuel shutoff valve commanded closed)

Question 18

When will the APU Generator come on line?

Answer 18

When the APU reaches operating speed

Question 19

Why does the APU GEN switch indicate amber ON?

Answer 19

It represents an abnormal in-flight condition

Question 20

What are the two operational modes of the APU?

Answer 20

Flight (essential mode) and ground (non-essential mode)

Question 21

What’s the difference between essential and non-essential?

Answer 21

In the non-essential mode, there are many conditions which would cause the ECU to shutdown the APU (e.g. ECU failure, loss of speed, APU fire, overtemp, reverse flow, low oil pressure, etc.). If this were to occur, the cool down mode is bypassed, the APU’s fuel shutoff valve is closed, and the APU shuts down immediately. When in the essential mode (WOW air), some protective shutdowns are disabled to allow continued operation.

Question 22

What happens if the APU is operating in the essential mode prior to landing?

Answer 22

The APU will continue to assume the aircraft is in the essential mode until the APU MASTER switch is selected off (or 15 min has elapsed).

Question 23

What’s an amber APU Essential CAS message indicate?

Answer 23

It means it hasn’t shutdown for one of the malfunctions that would have caused shutdown on the ground. Don’t shut it down if you really need it because a subsequent start may be inhibited. If you don’t need it, shut it down quickly by turning off the APU MASTER.