You may be probably talking about this, right?

The hole at the tail end of the aircraft??

Yeah, this is the exhaust port of the Hidden Powerhouse of the aircraft. This is also known as the Auxiliary Power Unit (APU).

Auxiliary power unit is a small turbine engine located at the tail of the aircraft. It too uses the usual Aviation Fuel from the aircraft’s fuel tanks. The APU is started, when the engines are not running or the plane is not connected with an external power source.

The APU is switched off after the engines are started, as it is no longer needed. Sometimes, the APU is kept on for air-conditioning purposes during takeoff, as engines’ bleed air is not used for AC during takeoff (specially at hot and high operations).

The APU is again started after the plane exits the runway after landing. This is because, immediately upon reaching the gate the external power source may or may not be available and using the engines for running the A/C and electrical system is not at all economical and is totally impractical. Therefore, the APU is kept on in such cases.

On a small jetliner, such as an Airbus A320 or a Boeing 737, the APU uses fuel at the rate of 160–200 kg/h. On the other hand, a big Jumbo Jet, such as a Boeing 747 or an Airbus A380, uses fuel at 400–500 kg/h.

Functions of APU—

• Supply high pressure bleed air to air conditioning packs while on the ground.
• Provides electrical power to the aircraft, while the main engines are switched off.
• Provide hydraulic and pneumatic power to start the main engines of the aircraft.
• Can be used to start engines while the aircraft is in flight.