What is the primary purpose of the APU regarding engine start?

The primary purpose of the Auxiliary Power Unit (APU) in the context of engine start is to supply bleed air. The APU is a small turbine engine located on the aircraft that provides various functions, one of the most critical being the supply of bleed air for starting the main engines. This bleed air is necessary to start the engines by providing the cold air required to enable the engine’s fuel to ignite and begin the combustion process.

This capability allows the aircraft to start its engines without relying solely on ground power units or high-pressure air sources, enhancing operational flexibility, especially in remote locations or locations where ground support is not available. The APU’s function is crucial for the safe and efficient startup of the engines, as it ensures that there is enough pneumatic pressure available when needed.