Boeing 777 APU – Introduction & Start Sequence

17 Q&A flashcards covering the Boeing 777 APU location, functions, electrical and pneumatic start sequence, fuel and ignition systems, compressor and turbine stages, and key RPM milestones during start-up.

Where is the Auxiliary Power Unit (APU) located on the Boeing 777?

In the aircraft’s tail cone.

What three main functions does the APU provide?

(1) Electric power, (2) Pneumatic power for main-engine start and air-conditioning, (3) Backup power during flight emergencies.

When no engines or external power are available, what are the only onboard power sources to start the APU?

The two aircraft batteries: the main battery and the dedicated APU start battery.

What does selecting the MAIN BATTERY switch accomplish?

Closes contactors and energizes the DC battery bus.

Name four actions triggered when the APU switch is moved to ON.

• APU fuel pump runs
• APU fuel feed valve opens
• APU controller powers on
• APU air-inlet door opens.

Why does the APU controller choose the electric start option in this scenario?

Because aircraft pneumatic power is unavailable.

What component actually spins the APU during an electric start?

A DC electric starter motor mounted on the front of the APU.

How is starter torque transmitted to the APU accessory gearbox?

Through a ratchet wheel and pawl clutch that drive the gear train.

How many stages do the APU’s centrifugal compressors have, and what are their parts?

Two stages; each stage has an impeller that accelerates air and a diffuser that increases pressure.

At what engine speed (RPM) does the controller open the fuel shut-off valve and command ignition?

At 7 % of rated engine RPM.

How many fuel nozzles inject fuel into the APU combustion chamber?

Fourteen fuel nozzles.

What voltage does the ignition unit receive and what does it output?

Receives 28-volt DC and outputs high-voltage pulses to the igniter plugs.

What type of combustion chamber does the Boeing 777 APU use and what is its peak temperature?

A reverse-flow annular combustor; peak temperature can reach about 2000 °C.

How many turbine stages are in the APU and what is the purpose of a stator?

Three axial-flow turbine stages; the stator directs gas flow onto the rotor blades.

When does the controller disconnect the electric starter and why?

At 50 % RPM; to stop motor rotation, allowing the pawl clutch to disengage from the ratchet.

What happens to fuel flow around 50 % RPM?

Pump pressure opens an internal flow-divider valve, routing additional fuel through the secondary manifold.

At approximately what speed is APU electrical and pneumatic power available for aircraft use?

Near 100 % of rated APU RPM.