Auxiliary Power Unit (APU)

A small gas turbine in the tailcone that gives you DC power and bleed air, on the ground and in flight.

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What it gives you

What it is

Line drawing of the rear fuselage with the APU LOCATION marked in the tail cone below the T-tail, next to a cut-away of the tail cone showing the APU and its exhaust duct.
Where the APU sits: the tail cone, below the T-tail. Tap to enlarge
APU control panel with one rotary switch labelled APU and three positions: OFF, RUN and START. A placard in the lower left reads NO RESTART AFTER ABORTED START / FLT. Below the panel: aircraft 20697 and subsequent and aircraft 20501 to 20696 post SB350-49-001.
APU switch on the 3500: no restart in flight after an aborted start. Tap to enlarge
  • A Honeywell 36-150BD constant-speed gas turbine in a fireproof compartment in the tailcone.
  • Its main job is to drive a 400 A, 28 V DC generator.
  • It also gives bleed air for air conditioning and engine starts. The air goes into the right side of the bleed manifold.
  • When demand is high, the electrical load comes first and the bleed air second.
  • The ECU runs and monitors the APU. It shuts the APU down by itself when a limit is exceeded.
  • APU RPM and EGT show in green on the top half of the left MFD. The ELEC, ECS and FUEL pages show its systems.
Controls
APU switch (centre pedestal)OFF / RUN / START. To start, hold START for 3 s and release. The right battery cranks the starter.
APU GEN (ELECTRICAL panel)Puts the APU generator on line.
APU BLEED (AIR COND/BLEED panel)Selects APU bleed air.
APU FIRE (ENGINE panel)Shuts down and isolates the APU. Arms bottle 2.
APU SHUT DOWN (outside, right side aft of the wing)Remote stop for use on the ground.

What not to exceed

Limits, shutdowns and fire

ENGINE panel with autothrottle (AT) system. Top row: L ENG FIRE, APU FIRE and R ENG FIRE red guarded push switches. Below: FIRE EXT 1 and 2 ARMED switches, EVENT button, AUTO APR, AUTO SYNC selector, IGNITION switch and the L and R STARTER switches.
ENGINE panel (with autothrottle): APU FIRE switch in the centre of the top row. Tap to enlarge
Key numbers
APU operationup to 37,000 ft
APU startup to 30,000 ft in flight, not above 285 KIAS; up to 14,000 ft on the ground
APU GENup to 30,000 ft
APU bleed (engine start and air conditioning)up to 20,000 ft
Battery before APU startabove 20 V
Left engine start on APU airnot on the ground above 9,000 ft
Start attempts on the ground3, each followed by 1 min cooling, then 20 min cooling. 2 more, then no further attempts for at least 40 min
Successive starts6 at 10 min intervals, then 1 h before further attempts
Start attempts in flightafter a failed or hung start, none until after landing
Hung startabort if RPM is below 30% after 30 s
Max RPM108%
Max EGT973 °C for start, 718 °C in normal operation
Fluid de-icing or anti-icingAPU operation prohibited
  • The ECU shuts the APU down by itself for faults such as overspeed, 28 V DC loss, or no flame or no acceleration on start.
  • APU OVERTEMP, APU OIL PRESS LOW, APU OIL TEMP HIGH: on the ground the APU shuts down by itself. In flight the crew selects APU OFF. Either way the APU is not used again.
  • APU fire on the ground: the APU shuts down by itself and bottle 2 discharges 5 s later.
  • APU fire in flight: the APU does not shut down or discharge the bottle by itself. APU FIRE shuts the APU down and arms bottle 2. FIRE EXT 2 discharges it.

The full module

The slides take you through the panel, the start, the automatic shutdowns and the fire drill, then a 10-question test.

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