APU

Primary functions

Heating, Bleed Air, Electrical Power

Dual gas turbine APU's

Provides electrical power and bleed air for engine start and heating the aircraft on the ground.

Electrical power

Supplied by a 25 KVA generator installed in either APU.

APU battery

If it checks good, the pilots ring the bell (approximately 24 volts).

APU battery disengagement

Pilots can disengage the battery and connect to the aircraft battery if the APU battery doesn't check good.

Surge damper

Dampens shocks from hydraulics to protect the system.

APU fire detector test failure

If the FIRE/DSBLD switch is pressed, the fire/dislbd light doesn't come on.

Operating Modes

Gen Only (150 pph), Normal (250 pph), EWO (275 pph).

APU fuel source

DC powered Boost Pump in Engine #2.

APU Control System

Controlled by an Electronic Sequencing Control Unit (ESCU) which governs start-up, temp limits, on-speed, shutdown, etc.

APU Doors

Hydraulically operated inlet and exhaust doors, can be operated manually.

Operating Temps

Cold weather (OAT below 40 degrees F or 4 C and visible moisture), Hot weather (time limitations associated with operations in NORMAL mode).

Ready light

Comes on when the APU reaches 100% in normal or EWO mode, goes out when downgraded to Gen Only mode or when a shutdown is initiated.

Fire Protection System

Each APU has its own fireproof compartment, emergency stop switch closes fuel valve in case of fire/overheat condition.

Dual loop Fire detection sensors

Activated when a fire is detected, initiates shutdown sequence and discharges fire extinguishers.

What battery voltage is at or below 21?

If battery voltage is at or below 21 volts, the APU fire extinguisher may activate during the detection system check.

Energized duration

The fire protection system remains energized for 5 minutes after APU shutdown.

What is EWO?

Operation in this mode decreases the APU engine life since they operate at a higher maximum EGT limit (1300 degrees F) than in normal mode

Used when starting of two aurplane engines simultaneously underscrable conditions.

Can I start one engine with 2 APU’s?

Using two air sources may provide adequate air pressure for normal engine start while masking a ruptures or leasking bleed air duct

Could lead to a blow duct in flight, causing hydraulic, electrical, engine, and / or flight control problems.

Rule of thumb one air source per engine

Found in Insufficient N2 RPM during start

Can I downgrade from normal to gen only with both APU’s?

You have to manually stop the APU without the generator installed before selecting generator-only mode

Can I start APU with no gen in gen only?

Yes, However you will not get the effects of what the generator would give you such as electrical power.

Does the Emergency stop the air flow immediately

The inlet and exhaust doors of the affected APU will close when RPM reaches approximately 5%.

What does the bypass valve do? How does it work?

Inside the boost pump it contains a bypass port to allow suction feed if the pump fails.