15.18 - APUs Flashcards
(18 cards)
Characteristics of an APU
Small free power gas turbine engine.
Supplies bleed air and electrical power.
Used on the ground or as a backup in flight.
Compressor is not affected by changes in load on the free turbine.
Designed to run at a constant speed, ensures the generators are at the correct frequencies.
What specific legislation is ther around APUs and ETOPs
APUs are required to be able to operate at 40,000 ft but with a lower starting altitude of 15,000-17,000 ft.
When are APUs normally started in flight
During the landing phase. Unloads the main engines in case of an aborted landing or go around.
What does an APU have instead of a EEC
An ECB (electrical control box)
If it fails the APU auto shuts down.
What are the 3 modes that the APU control unit controls the APU in
Start mode - from start to 100%
Normal mode
Shutdown mode.
APU start sequence
Manually switched on in cockpit, electrical power must be supplied by a master switch.
System test happens, if failed it will not start.
Air inlet doors are commanded to open.
Control unit controls and monitors the acceleration until 100% is reached.
Long operation at low speeds causes high thermal stress
APU control - on speed
Once it reaches operational speed, control becomes simple.
Remains constant at 100% and no excessive EGT is reached.
2 signals for constant speed control:
- torque motor signal
- speed signal.
During normal operation speed can change due to:
- bleed load
- electrical load
- air inlet temp and pressure.
APU EGT increases with load.
APU shutdown sequence.
Master switch to off
APU only allowed to shutdown once it has operated for a sufficient time with no load.
Cool down period 0-120s
After cool down period fuel is cut.
APU fuel
Taken from main engines in case fuel feed system.
Has its own boost pump.
Fuel lines are in inner wall of the fuselage and behind sidewall linings of aft cargo bay.
Positive fuel pressure.
Pressure switch gives warning if it is too low.
APU bleed air.
2 sub systems:
- cooling air system. - bleed air system
Cooling air system supplies air for the oil cooler and the APU compartment ventilation.
Bleed air system has 3 main tasks:
- bleed supply. - surge protection. - load compressor control.
What are the 2 conditions for the APU bleed valve to be opened.
Switched to on
APU start sequence complete (greater than 95%)
When this happens a solenoid is energised and valve opens.
Load compressor control.
Load compressor cannot be switched off.
Inlet to load compressor is controlled by inlet guide vanes.
Engine start requires max airflow so the variable inlet guide vanes need to be fully open.
APU protection systems
Turbine over speed
Excessive EGT
Low oil pressure
High oil pressure
Fire detection
Bleed air delivery duct overheat
Fire detection
Bleed air over pressure
APU surge protection
Surge protection. APU
APUs can surge especially when the bleed load changes.
- anti-surge system
- surge protection system.
APU anti-surge system.
APUs without a load compressor.
Has an anti surge valve that always bleeds air off when surging is possible.
Valve is normally opened during flight to prevent stall.
APU surge protection system
For APUs with a load compressor.
Monitors the airflow in the discharge duct of the load compressor and opens the surge control valve if the compressor surges.
Automatic APU shutdown.
Automatic shutdown is always activated by the APU control unit.
Shuts down without any cooldown period.
Excessive EGT. Over-speed. Low oil pressure High. High oil temperature. Compressor surging. Fire.
APU external emergency shutdown
Shutdown switches can be found in areas easily accessible by ground crew.
Nose landing gear. Main wheel well. Refuelling station.
