The secret of the hole in the fuselage!

The Boeing 737 aircraft are among the most popular and reliable planes in the world, used for both passenger and cargo transportation. Like most modern aircraft, all generations of the Boeing 737 (Original, Classic, Next Generation, and Max) are equipped with an additional engine located in the rear fuselage.

Purpose and Usage of the Auxiliary Power Unit (APU)

This additional engine is usually referred to as the APU (Auxiliary Power Unit).

The first airliner to use a gas turbine engine as an APU was the Boeing 727 in 1963. The APU is a relatively small gas turbine engine designed to generate electricity, provide pressure for the pneumatic system, and ensure proper air conditioning during ground operations or flight.

On the Ground

When the plane is parked, the main engines can be turned off. In this case, the APU supplies the aircraft with the necessary electrical power and air for cooling and heating systems, along with other systems requiring power while the aircraft is on the ground. It also provides the compressed air needed to start the main engines. This makes the aircraft independent of airport ground services equipment.

In Flight

Additionally, the APU can be used to supply extra compressed air during takeoff and landing under certain conditions (e.g., high external temperatures or short runways) and to provide additional electrical power during flight in case one of the main engines, used for generating thrust, fails.

Thus, the APU is a critical element for ensuring the safety, autonomy of aircraft systems, and passenger comfort both on the ground and in the air.

Sources: FCOM Boeing 737