What component of the aircraft provides backup power during engine start?

The component that provides backup power during engine start is the auxiliary power unit (APU). The APU is a small engine typically located in the tail of the aircraft and serves multiple functions, one of which is to start the main engines. During engine start, the APU supplies electrical power, allowing critical systems to operate even when the main engines are not yet running. This ensures that systems essential for safety and operation are maintained during the start sequence.

While the main battery does provide power, it is primarily used for starting the APU and powering essential systems before engine start. The fuel boost pump is focused on delivering fuel to the engine and does not provide power. The transformer rectifier unit (TRU) converts AC power to DC power but is dependent on an active power source; thus it is not a standalone backup power unit for engine start. Understanding the role of the APU in providing electrical power helps to clarify its critical function during the engine starting process.