APU stands for Auxiliary Power Unit and is the aircraft's internal unit that supplies electrical power and air conditioning to the plane. This enables the aircraft to be self-sufficient without running the engines or needing external power. However, most of the time, especially in this time of expensive fuel, external power and air conditioning is used. External power can be supplied by a cable that is connected to the jetway or by a free-standing cart.

As you board, it would typically be off to save on fuel. However, some larger aircraft require the Unit to be running to handle the additional electrical loads the aircraft needs at the gate or to supplement the air conditioning if it is a hot day and the external air conditioning is not sufficient to keep the aircraft cool.

APU Inlet- location of air flow into the APU APU Outlet- location of exhaust air from the APU Air Hose supplies air conditioning to the aircraft Ground Power cable supplying electrical power to the aircraft
APU Inlet APU Outlet Air Hose Ground Power

Of course, this can be true of smaller aircraft, so it is up to the pilots to match the comfort of the passengers to the situation. Newer aircraft have better air conditioning systems.

The APU is used as the primary means to start the engines. After you start pushing back from the gate, you will feel the air conditioning cut off. Everyone seems to react at the same time, hands go up, adjusting the little gasper (eyeball) fans for more airflow! Don't worry everything is alright! Most aircraft cannot provide enough air to supply both the air conditioning system and starting the engines, so the air conditioning is turned off for a few minutes until the engines are up to speed.

While airborne, it can also serve as a backup for either the air conditioning or electrical systems. An operative APU is one of the most critical items that must be working for a flight to be dispatched under ETOPS (Extended Twin Engine Operations), more on this later in another section. Each aircraft model has its' own limitations regarding the APU's use while flying and on the ground. Flying, they are based more on flight altitude than anything else, but can also have run time limitations. Additionally, like any engine; the oil quantity, pressure, and temperature must be within allowable parameters.

APU back to Boarding

APU back to My Airline Flight