15.18 Auxillary Power Units

Question 1

What are the 3 APU sections?

Answer 1

Power section, bleed section and accessory gearbox

Question 2

What does the bleed section do?

Answer 2

Ensures sufficient bleed air is supplied to pneumatic system

Question 3

How is bleed air suppled to the pneumatic system?

Answer 3

Using a load compressor or APU compressor

Question 4

How is a load compressor regulated?

Answer 4

Can’t be switch off so air regulated by IGVs

Question 5

What are the 3 components of the power section?

Answer 5

Turbine, combustion chamber and compressor

Question 6

What 2 things does an Electrical Control Box (ECB) do?

Answer 6

Monitors starting parameters and controls auto shutdown

Question 7

What does the power section drive?

Answer 7

The compressor and gearbox

Question 8

What does the gearbox drive?

Answer 8

All APU accessories

Question 9

What type of combustion chamber is found on an APU?

Answer 9

Annular

Question 10

What does the compressor do?

Answer 10

Provides air for combustion

Question 11

How does the fire protection system activate fire extinguishers?

Answer 11

Automatically

Question 12

What 5 conditions cause an APU auto shutdown?

Answer 12

Excessive EGT, over speed, low oil pressure,high oil temperature or fire

Question 13

What are the 2 APU air system sub systems?

Answer 13

Cooling air systems and bleed air systems

Question 14

What does the cooling air system on an APU do?

Answer 14

Supplies air to oil cooler and APU compartment ventilation

Question 15

What are the 3 tasks of the APU bleed air system?

Answer 15

Bleed supply, surge protection and load compressor control

Question 16

When are anti surge systems installed? What position are they during flight?

Answer 16

On APUs without load compressors. Open during flight

Question 17

What is the fastest method to shutdown the APU?

Answer 17

Using the fire switch

Question 18

How long is APU cool-down?

Answer 18

0-120secs

Question 19

What is the minimum altitude an APU can start during flight?

Answer 19

15000-17000ft

Question 20

What is the maximum altitude an APU can operate during flight?

Answer 20

40000ft

Question 21

When is the APU start sequence complete?

Answer 21

If speed reaches 95%

Question 22

If the APU bleed valve opens the what happens to bleed load and EGT?

Answer 22

Both increase

Question 23

What are the 3 operating modes?

Answer 23

Start mode, normal mode and shutdown mode

Question 24

When is start mode used?

Answer 24

Until APU reaches 100%

Question 25

What is the function of an APU?

Answer 25

Supplies pneumatic bleed air and electrical power to different aircraft systems

Question 26

Does the free turbine of an APU run at constant speed?

Answer 26

Yes

Question 27

Is the compressor of an APU affected by load changes on the free turbine?

Answer 27

No

Question 28

What 2 signals does the APU use for constant speed control?

Answer 28

Bleed signal and torque motor signal

Question 29

What 2 conditions must occur for the APU bleed valve to open?

Answer 29

Bleed switch ON and APU start sequence complete