15.18 auxiliary power units Flashcards
what is one of the primary requirements to achieve ETOPS certification
the aircraft’s possession of an in-flight APU
when are in-flight APU’s often started
during an aircraft’s landing phase
the APU is split into three main sections, what are they
the power section
the bleed section
the accessory gearbox section
what speed are APU’s designed to run at
constant speed
what does the power section of the APU drive
the compressor and the gearbox
as on all gas turbines engines, what does the power section have
a compressor
a combustion chamber
a turbine
how is the start sequence operated for an APU
manually by switches on a panel in the flight deck.
what happens if the pre-start test fails
the APU will not start
what monitors and controls the correct acceleration until 100% speed is reached
the APU control unit
what is one of the most important signals for the APU control unit
the speed signal
The APU control unit uses two signals for constant speed control, what are they
The APU speed signal
The torque motor signal
what does the APU have to monitor start sequence, turbine speed and EGT
a electrical control box (ECB)
how does the APU control unit maintain a constant speed
by comparing the actual APU speed with the necessary speed
where is the fuel for the APU normally taken from
the aircraft’s main-engine fuel feed system
the APU air system is divided into two sub-systems, what are they
the cooling air system
the bleed air
what are the three main tasks of the bleed air system
bleed supply
surge protection
If the APU has a load compressor, the load compressor control
why can the load compressor not be switched off
because it is fixed tot the rotor shaft
what is the air inlet to the load compressor closed by
the inlet guide vanes
what can compressor surging in the APU be prevented by
bleeding off the air that is blocking the compressor discharge
where are anti-surge systems installed
on APUs which do not have a load compressor.
what is the automatic shutdown always activated by
the APU control unit
In case of an APU emergency shut-down, what happens to the apu
it must be switched off immediately without any cooling-down period
where are The external emergency shut-down switches located
At the nose landing gear
In the main wheel well
On the refuelling station depending on aircraft type or size
when is an automatic APU emergency shutdown activated
in a fire condition when the aircraft is on the ground.
