M15.18 APU

Question 1

What is the certification achieved by an aircraft possessing and in flight APU?

Answer 1

ETOPS (Extended twin operational performance) certification

Question 2

In flight APU require a lower starting altitude of what?

Answer 2

Around 15,000-17,000 feet
Capable of operation up to 40,000 feet

Question 3

Why are APU started during aircraft’s landing phase?

Answer 3

Unloads main engines pneumatic burden and allows the engine to keep compressor air for reverse thrust

Question 4

Three sections of an APU?

Answer 4

The power section
The bleed section
The accessory gearbox section

Question 5

The power section of the APU drives what?

Answer 5

The compressor and gearbox

Question 6

The gearbox drives what APU accessories?

Answer 6

Fuel pump
Oil pump
Cooling fan
A-C generator

Question 7

On some APU a separate compressor is installed to support the pneumatic system, what is this called?

Answer 7

The load compressor

Question 8

As with all gas turbine engines, what makes up the power section?

Answer 8

A compressor
A combustion chamber
A turbine

Question 9

On modern APU what type of combustion chamber is used? What about old ones?

Answer 9

Modern is annular type
Can find can type on older

Question 10

The APU has what to monitor start sequence, turbine speed, EGT and oil and fuel supply?

Answer 10

Electrical control box

Question 11

An electronic APU control unit controls the APU in what three different modes?

Answer 11

The start module controls the APU start activation until it’s reached 100% speed
The normal speed mode keeps APU at 100% speed
The APU shut down mode monitors and controls shut down

Question 12

APU start sequence

Answer 12

Manually operated switches in flight deck to supply electrical power to APU from GPU

A test is then conducted to ensure circuits and sensors function correctly

APU air inlet door opened, APU fuel supply prepared, fuel shut off valve opened, fuel boost pump switched on.

Question 13

If APU start sequence fails what happens?

Answer 13

APU doesn’t start

Question 14

Throughout acceleration the fuel metering is controlled by what?

Answer 14

The APU control unit

Question 15

What two signals does the APU control unit use for constant speed control?

Answer 15

The APU speed signal
The torque motor signal

Question 16

What factors affect APU operation?

Answer 16

APU bleed load
Electrical load
Air inlet temperature
Air inlet pressure

Question 17

If APU bleed load increases if what?

Answer 17

The APU bleed valve is open

Question 18

When the bleed load increases what else increases?

Answer 18

APU EGT (Exhaust gas temperature)

Question 19

The APU can only shut down after it has operated for a sufficient time without what?

Answer 19

A pneumatic or electric load

Question 20

Cool down period for APU shut down?

Answer 20

Varies between 0-120 seconds. Adjusted during maintenance

Question 21

Where is the APU fuel normally taken from?

Answer 21

The aircraft’s main engine fuel feed system

Question 22

Main components of APU fuel feed system?

Answer 22

APU fuel boost pump
APU fuel shut off valve (known as low pressure fuel shut off valve)
APU fuel feed line
Pressure switches

Question 23

What does the pressure switch do in the APU fuel system?

Answer 23

Monitors fuel pressure in fuel feed line directly in front of APU and triggers warning if fuel pressure is too low

Question 24

Having positive fuel pressure in the APU fuel feed line is important why?

Answer 24

prevent cavitation at the high pressure fuel pump of the APU

Question 25

APU air system is divided into what two sub systems?

Answer 25

Cooling air system Bleed air system

Question 26

The cooling air system in the APU supplies air to where?

Answer 26

Oil cooler and APU compartment ventilation

Question 27

Three main tasks of bleed air system in APU?

Answer 27

Bleed supply Surge protection The load compressor control (if it has one)

Question 28

The load compressor does what in the APU?

Answer 28

Controls the amount of air going over the load compressor to meet the bleed demand of aircraft pneumatic system

Question 29

What two conditions are needed to open APU bleed valve?

Answer 29

Bleed stitch in on position APU start sequence must have been completed

Question 30

Why are APU’s with load compressors more fuel efficient?

Answer 30

Airflow across load compressor can be adapted to the aircraft’s bleed air demands

Question 31

As it cannot be turned off, how is the air inlet closed off for the load compressor?

Answer 31

Inlet guide vanes

Question 32

Main components of the load compressor system?

Answer 32

Variable inlet guide vanes Inlet guide vane actuator with mechanical transmission assembly APU control unit

Question 33

When can APU compressors surge?

Answer 33

When bleed load changes

Question 34

How can APU compressor surge be prevented?

Answer 34

By bleeding off the air that is blocking the compressor discharge

Question 35

What surge protection is used on APU without load compressors?

Answer 35

Anti surge system

Question 36

For APU with load compressors what is the method of surge protection?

Answer 36

Surge protection system monitors the airflow in load compressor discharge duct and opens a surge control valve if the compressor surges.

Question 37

As a general rule,what is said about the surge control valve?

Answer 37

If the airflow behind load compressor decreases or stops the valve opens

Question 38

Automatic APU shutdown is always activated by what?

Answer 38

APU control unit

Question 39

Where are the external emergency APU shut down switches?

Answer 39

At the nose landing gear In the main wheel well On the refuelling station depending on aircraft size and type

Question 40

What is the quickest way to shut down the APU?

Answer 40

Activate the fire switch

Question 41

What will cause an automatic APU emergency shut down?

Answer 41

A fire condition when aircraft is on the ground

Question 42

Is the APU compartment pressurised?

Answer 42

No