What is APU on Boeing 737? (Auxiliary Power Unit)

The Auxiliary Power Unit (APU) is an essential component of the Boeing 737 aircraft. It serves as an independent power source that provides electrical power and pneumatic energy during various flight phases. The APU plays a crucial role in the operation of the aircraft, ensuring its reliable performance and enhancing operational efficiency.

The APU is a self-contained unit located at the tail section of the Boeing 737, between the main landing gear. It consists of a gas turbine engine, a generator, and an air compressor. The gas turbine engine runs on jet fuel or kerosene, similar to the main engines of the aircraft. However, the APU is significantly smaller in size and power output compared to the main engines.

How does the Auxiliary Power Unit work?

The APU operates independently of the main engines and is started before the aircraft’s engines are powered on. It provides electrical power for various aircraft systems, such as lighting, air conditioning, and flight instruments, when the main engines are not running. Additionally, it supplies pneumatic energy for engine starting, air conditioning, and pressurization.

When the APU is turned on, fuel is supplied to the gas turbine engine, igniting the combustor. The combustion process generates high-pressure and high-temperature gases that spin the turbine, which is connected to the generator and air compressor. The generator produces electrical power, while the air compressor supplies compressed air to the bleed air system.

Advantages of the Auxiliary Power Unit

The APU offers several advantages for the Boeing 737 aircraft:

  • Reduced reliance on ground power: The APU allows the aircraft to operate independently at the gate or in remote locations without the need for external power sources. This enhances flexibility and reduces turnaround time.
  • Backup power source: In case of an in-flight power loss from the main engines, the APU can provide electrical power to essential systems and instruments, ensuring the safety and control of the aircraft.
  • Engine starting: The APU supplies compressed air to start the main engines. This eliminates the need for external air carts or other equipment to initiate engine start-up.
  • Environmental control: The APU powers the air conditioning and pressurization systems, maintaining a comfortable and safe environment for passengers and crew, especially during ground operations.

The use of the APU also improves fuel efficiency by reducing reliance on the main engines during ground operations. When the APU is running, the main engines can remain shut down, resulting in fuel savings. The APU is designed to provide sufficient power for essential operations, reducing the idle time of the main engines and minimizing fuel consumption.


The Auxiliary Power Unit (APU) is a vital component of the Boeing 737 aircraft. It functions as an independent power source, supplying electrical power and pneumatic energy during various flight phases. The APU enhances the operational flexibility, reliability, and efficiency of the aircraft. Its contributions to powering essential systems, reducing reliance on ground power, and improving fuel efficiency make it an integral part of the Boeing 737 experience.

To learn more about the Auxiliary Power Unit and its role in aviation, check out this article by Boeing.

For More: What is TDZE on Boeing 737? (Touch Down Zone Elevation)