APU

Question 1

What does the APU supply?

Answer 1

28v DC power to main feeder bus. (Same as engine starter-gen)

Bleed air to ECS

Question 2

What is the APU fuel consumption?

Answer 2

200 lbs/ hour

Question 3

What are the APU restrictions?

Answer 3

Ground use only

Can’t start during fueling operations

Can’t be operated during gravity fueling

Gen on first before bleed air

Must not be operated unattended with passengers on board

Question 4

What happens if you forget the APU on for takeoff?

Answer 4

Shuts off automatically on liftoff.

Question 5

Can the APU bleed air be on with engine bleeds selected on?

What about a ground air cart?

Answer 5

No - desensitized by WOW

Yes

Question 6

Where does the APU get it’s fuel supply from?

Answer 6

Left tank

Question 7

How long should you wait after starting the APU before you turn on the bleed air?

Answer 7

3-5 minutes

Question 8

What are the APU temperature limits?

….with inlet louveres?

Answer 8

-54°C to 50°C or ISA +35°C

21°C with inlet louvers installed.

Question 9

APU starting limits

Answer 9

1 minute on, 5 minutes off

1 minute on, 30 minutes off and maintenance action required.

Question 10

What is a nice trick to increase success rate in starting the APU in winter?

Answer 10

Press the self test then wait 3-5 minutes before pressing the start button.

Question 11

What precaution must be taken when using external power to ensure that the APU GEN comes on line in the event of GPU failure?

Answer 11

APU GEN needs to be reset or else it doesn’t automatically kick in once GPU is offline. Therefore just cycle the APU GEN off then on after start up.

Question 12

If pressing the APU PWR button does not shut down the APU, what can be done to shut down the APU?

Answer 12

Press the FIRE TEST button on the Fire Protection Panel (APU section is on the left part of FPP).

Question 13

What is the purpose of the APU FADEC?

Answer 13

It monitors normal operation, controls the start and responds to malfunctions. It also supplies fuel for starting, acceleration and speed regulation.

Question 14

When will the APU fuel valve close? (6)

Answer 14

PWR button pushed while running.

APU fire test switch pressed.

A fire is sensed in the tail cone.

Aircraft gets airborne while APU is running.

Illumination of GEN OVHT light.

APU shuts down due to a Fault detected by the FADEC.

Question 15

What is the maximum temperature you can start the APU if the louvers are on?

Answer 15

21 Degrees Celsius.

Question 16

There are two round openings on the bottom fuselage in front of the APU access doors. What are they for?

Answer 16

Composite duct inlet is the one on the left and it provides positive airflow ventilation to keep the APU compartment cool. The other is to provide cooling for the starter-gen.

Question 17

Describe the start sequence of the APU.

Answer 17

Starter cranks till 50% rpm. The start segment extinguishes and at 95% +3 seconds, the gen becomes available.

Question 18

How do you verify the voltage of the APU gen?

Answer 18

Check main busses on electrical page.

Question 19

When shutting down the APU, how long do you need to wait to hit the PWR button after shutting bleed air off?

Answer 19

20 seconds.

Question 20

When the APU bleed air is operating, how is the cabin prevented from pressurizing?

Answer 20

Pushing the bleed air button automatically opens the rear safety outflow valve.

Question 21

What happens if EGT exceeds limits due to high electrical load?

Answer 21

APU prioritizes the GEN, so it removes bleed air in order to take load off of the compressor.

Question 22

When does the APU fire bottle fire after an APU fire?

Answer 22

7 seconds.

Question 23

When must the APU be preheated?

Answer 23

-35 C or colder for more than one hour.

If you fly in temps -35C or cooler for more than an hour and you land in temps below -35C.

Question 24

If the APU fails, how long is a start inhibited for?

Answer 24

2 minutes.