15.18 APU Flashcards
What is the simple explanation of an APU?
A small gas turbine (free turbine) turbo shaft engine that supplies pneumatic bleed air and electrical power for different A/C systems
What does the free turbine design mean for an APU?
There’s a constant speed for the generator and compressor is not affected by load changes
What is the purpose of an APU?
(5)
1- starting power for engine
2- pneumatic power for cabin AC
3- shaft power for other pneumatic/hydraulics
4- back up electrical/pneumatic
5- electrical/pneumatics power on ground after engine shutdown
What is the allowable APU starting altitude?
To what altitude is the APU usable up to?
Below 15,000-17,000 feet
40,000 feet
What are the three sections that APU is split into?
Power section
Bleed section
Accessory gearbox
What is in the power section?
Compressor
Compression chamber
Turbine
* some APU may use a separate compressor to provide bleed air known as load compressors
What type of compressor do most AP use use?
What type of combustion chamber do most APU use?
Centrifugal compressor
Annular combustion chamber
What does the electrical control box do in an APU?
Monitors start sequence turbine, speed, EGT , oil fuel and bleed air
Also controls auto shutdown
What are the three operating modes in a ECB?
Start- controls APU from start activation until 100% speed
Normal speed mode- keeps APU at 100% speed regardless of load changes
APU shutdown mode- monitors and controls APU shutdown
What will happen if acceleration speed to 100% is too slow in an APU start sequence?
APU CU will abort start to protect against thermal stresses
What does fuel metering ensure in an APU start sequence?
That the APU accelerates as fast as possible and EGT does not exceed its limit
In a typical APU start is the max EGT limit higher at low speeds or high speeds
Low speed
What happens to EGT during the start of an APU?
EGT increases continuously reaches maximum and then decreases slightly
What are the two signals used for speed control?
APU speed signal
Torque motor signal
How is constant speed maintained in torque motor signal?
By comparing actual APU speed with the necessary speed, the torque motor signal changes the fuel metering
How can APU load be increased?
Opening APU bleed valve
Operating electrical hydraulic pump
* adding more fuel reduces the effects on speed under high loads
When can the APU shut down?
And what is the cool down period range?
After operating for sufficient time without pneumatic or electric load
0 to 120 seconds
What are the two systems that APU air systems are divided into?
Cooling air system
Bleed air system
What does the cooling system supply air for?
Oil cooler and APU compartment ventilation
When will the bleed valve open in an APU air system and how is it closed?
Valve opens when solenoid is energised. Bleed valve is closed by a spring meaning it’s always closed before APU starts.
They are pneumatically or fuel operated can also have electric motors
What is one benefit of having load compressors?
Increases fuel efficiency
Can load compressors be switched off? And what are they fixed to?
No, they cannot be switched off and they are fixed to the rotor shaft
How is the airflow quantity into the load compressor controlled?
Variable inlet guide veins
When would the IGVS need to be fully open?
Four engine starts and Max cabin cooling
When would IGV’s close slightly?
If EGT rises when fully open this is to reduce the load on the APU
What are the two protection systems in an APU?
Surge protection
Auto shutdown
How do surge protection systems protect the APU?
By bleeding air off that is blocking the compressor discharge
What are the systems do APU use that do not have a load compressor?
Anti-surge system
When would the surge control valve open in an APU that has a load compressor?
If airflow behind load compressor decreases or stops
Why would the auto shutdown occur in an APU?
(6)
Excessive EGT
Over speed
Low oil pressure
High oil temp
Compressor surging
Fire
Where are the external emergency shutdown switches located for the APU?
MLG bay
NLG bay
Refuelling station
How can the extinguishers be discharged?
Both manually or autonomously by APU controller
