How does the APU assist in the start-up of the main engines?

The APU, or Auxiliary Power Unit, plays a crucial role in the operation of an aircraft by providing both pneumatic pressure and electrical power, which are essential for the start-up of the main engines. During engine start, the APU supplies bleed air that is used to turn the main engine’s starter, enabling the initial rotation necessary for ignition. Additionally, it supplies electrical power to the aircraft systems, ensuring that the necessary instruments and avionics are operational during the start-up process.

This dual provision of pneumatic pressure and electrical power makes the APU a vital component in successfully starting the main engines, enhancing the overall efficiency and reliability of the aircraft’s operations. Other options, while related to aircraft functions, do not encompass the critical roles of pneumatic pressure and electrical power needed specifically for engine start-up.