M15.18 Flashcards
What type of engine is the APU?
A small gas turbine engine
What does the APU supply?
Bleed air and electrical power
When would the APU be used?
When you don’t need the engines
On very large APU’s what may you find two of?
AC generators on the gearbox
What does the power section drive on a APU?
The compressor and the gearbox
Will you find on an APU to make sure that the bleed air is sufficient?
A separate compressor usually called the load compressor
What is the compressor of the power section mainly used for on an APU?
Provide air for the combustion and bleed air
What type of compressors are found in an APU?
Centrifugal flow compressors
Why are centrifugal flow compressors used?
Good performance, long lifetime, resistant against FOD
What type of combustion chambers are used on APUs?
Annular (new) or can-type (old)
What type of turbine will you find on a small APU?
Centrifugal turbines
What type of turbines are found on large APUs?
Axial turbines with two or more stages
What pressure is the air pressure at that comes from the APU?
30-45psi
What are the two different ways a APU supplies bleed air?
- a compressor from the power section
- an individual load compressor
Why are load compressors very fuel efficient on an APU?
Because the load compressor can be switched off if bleed air isn’t needed
Why would the APU need to supply electrical power?
As a back-up source for the aircraft
Where does the electrical power supply come form on an APU?
From generators which are driven via the APU gearbox
How many generators will you find on a large APU?
2
What are the APU doors for?
Servicing and maintenance
How many support mounts are there on the APU?
3
What are the names of the support mounts on an APU?
- right aft mount
- left aft mount
- Forward mount
When would APU mount links be used?
On large aircraft
Why is one of the APU support mounts fixed in place?
For strength and stupidity
How is the APU removed and refitted?
By using hoist equipment commonly know as fish pole hoist
What is the air intake door of an APU also called?
Air intake flap
How many volts is used to operate the APU intake door?
28volts DC
What is the APU air inlet duct often named?
A diffuser
What are the two types of drains on an APU?
- direct drains: drained directly overboard
- indirect drains: drained to a drain tank
The electronic APU control unit controls the APU automatically by using three operating modes, what are these?
- start mode: controls start to 100%
- normal speed mode: keeps it at 100%
- shut down mode: controls shut down
Where would you find emergency shut down switches for the APU?
Anywhere easy accessible for ground personal so landing gear areas
What is the primary fuel system of the APU also known as?
Low pressure fuel system
Where does the primary fuel system (low pressure fuel system) start and finish on a APU?
Starts at the fuel tank and finish’s at the fuel control unit
What is the secondary fuel system of the APU also known as?
High pressure fuel system
Where does the secondary fuel system (high pressure fuel system) start and finish on a APU?
Starts at the fuel control unit and ends at he fuel nozzles of the combustion chamber
Where is the APU fuel boost pump and shut off valve normally located?
The left hand side main wheel well
What is the job of the APU fuel shut off valve?
To shut off fuel supply to the APU if its not running
What is the APU fuel line vent switch used for?
To bypass the APU master switch which allows for easy maintanence
What is the APU air system divided into?
Cooling air and bleed air
How can compressor surging be prevented in an APU?
By bleeding off the air that is blocking the compressor discharge
Why does the APU oil system have a de-oiling valve?
Helps to improve the APU start sequence
What is the APU oil system used to cool?
The APU generator
Where is the APU oil reservoir located?
The lower part of the APU gearbox
What does the APU oil supply pump do?
Supplies oil to the lubrication areas
What are the three types of APU oil system pumps?
- gerotor pump
- gear pump
- vane pump
How does the air/oil separator work?
By using rotating action
What pressures can the APU gearbox be pressured to?
4-5psi
When would the APU gearbox need to be pressurised?
When operating at high altitudes
What is the advantage of pressurising the gearbox of the APU?
It supports oil supply and minimises oil foaming
Where is the APU start relay installed?
The electronic equipment compartment
What type of motors start up the APU?
DC motors (usually a brush type)
What are brushwear indicators used for?
To show how worn the brushes are on the start motor
Why must you wait for the APU starter motor and battery to cool down?
Because they get very hot from the APU start
Why are starter limitations in place on the APU?
It makes sure that the starter equipment does not get over stressed and overheated
What is the APU ignition system usually powered by?
DC power that comes from the APU control unit
What does the ignition exciter convert low DC voltage to?
High voltage for igniter plugs
Where would you find the battery voltage in the cockpit?
Next to the battery switches
When do the indications on the ECAM come available?
When AC power is switched on
