How Auxiliary Power Units Work | Part 1 : Starting

Summary

The Boeing 777 Auxiliary Power Unit (APU) is a crucial gas turbine engine located in the tail cone, providing electric and pneumatic power for ground operations, main engine starts, and emergency backup during flight. The video details the APU's start-up procedure using aircraft batteries, its electric start system, and the intricate combustion process involving centrifugal compressors. With a peak temperature of 2000°C in the combustion chamber, the APU's self-sustaining combustion ensures reliable operation. As it reaches 100% RPM, the APU is ready to power aircraft systems and assist in starting the main engines, highlighting its vital role in successful aircraft operations.

Takeaways

• 🛩️ The Auxiliary Power Unit (APU) in the Boeing 777 is a gas turbine engine located in the tail cone, providing essential power for independent operations.
• 🔋 The APU can function solely on the aircraft's batteries when no external power or engine is running.
• ⚡ The main battery supplies DC power to the electrical load management system, allowing for the activation of the APU.
• 🔄 To start the APU, an electric starter is used due to the lack of pneumatic power, utilizing a dedicated battery for this purpose.
• 🌬️ The APU features two-stage centrifugal compressors that draw in and compress air, preparing it for combustion.
• 🛢️ Fuel is supplied from the wing tank to the APU and is injected into the combustion chamber through atomized nozzles.
• 🔥 At 7% RPM, the ignition system activates, sending high voltage pulses to igniter plugs to ignite the fuel-air mixture for combustion.
• 🌡️ The combustion chamber can reach temperatures of up to 2000°C, necessitating air flow injection to cool and stabilize the combustor.
• 🔄 The ignition system remains active until the APU reaches half of its rated speed to prevent flameout during start-up.
• 🔋 At near 100% RPM, the APU provides electric and pneumatic power for aircraft systems and assists in starting the main engines.

Q & A

What is the primary function of the APU in the Boeing 777?

The APU provides electric and pneumatic power to operate the aircraft independently on the ground, start the main engines, and supply backup power during in-flight emergencies.

How does the APU start when the main engines are not running?

The APU starts using the aircraft's two batteries as the only available power sources when no external power is connected.

What role does the load management system play in APU operation?

The load management system activates multiple components when the APU switch is turned on, including the APU fuel pump, fuel feed valve, APU controller, and air inlet door.

What happens when the APU reaches 7% RPM during the start process?

At 7% RPM, the APU controller commands the fuel shut-off valve to open, allowing fuel to flow into the combustion chamber, and activates the ignition system to generate a spark.

Describe the combustion process in the APU.

The fuel is atomized and injected into the combustion chamber, where it mixes with compressed air. The ignition system generates a spark that ignites the fuel-air mixture, causing combustion.

What is the maximum temperature the combustion chamber can reach?

The combustion chamber can reach peak temperatures up to 2000°C, which is managed by using a significant portion of the compressor's airflow to cool the combustor.

How does the APU ensure efficient combustion?

The combustion chamber is designed with air injection holes that stabilize the flame, improve combustion efficiency, and protect the combustor liner.

What components are involved in the turbine section of the APU?

The turbine section consists of a three-stage axial flow turbine where each stage includes a stator that directs gas flow and a rotor that harnesses kinetic energy to increase engine speed.

When does the ignition system deactivate during the APU start process?

The ignition system remains active until the engine reaches 50% of its rated speed, at which point the ignition unit is switched off.

What occurs when the APU reaches close to 100% RPM?

Once the APU approaches 100% RPM, its electric and pneumatic power becomes available for selection to operate the aircraft systems and start the main engines.