What is the role of the auxiliary power unit (APU) on an aircraft?

The auxiliary power unit (APU) plays a crucial role in aircraft operation, particularly when the aircraft is on the ground. Its primary function is to supply electrical power and pneumatic air, which are essential for various systems. When an aircraft is parked at the gate, the engines are not running, yet power is still required to run essential systems such as lighting, climate control, and avionics. The APU generates the necessary electrical power and can also provide pneumatic air for starting the main engines and powering the environmental control systems. This functionality eliminates the need to rely exclusively on the main engines while on the ground, thus making it an essential component for ground operations.

While the other options refer to important aspects of aircraft operation, they do not accurately reflect the specific purpose of the APU. The APU's contribution to electrical power and pneumatic air supply is vital for operational efficiency and convenience on the ground.