What is the role of the APU accessory drive gearbox?

• A. It energizes the starter motor only
• B. It converts electrical energy to mechanical energy
• C. It transmits mechanical power from the APU to drive accessories such as the generator, oil pump, fuel pump, starting motor, and other components
• D. It stores lubricating oil

Answer: (C)

The APU accessory drive gearbox serves as the power-distribution hub for the APU. The turbine’s rotation is fed into this gearbox, which provides the correct speed and torque to drive multiple accessories at once. In practice, that means it transmits mechanical power to devices like the generator, oil pump, fuel pump, starting motor, and other components that the aircraft needs when the main engines aren’t running. This setup lets the APU supply electrical power, hydraulic pressure, and starting capability through a single drive system.

It’s not about converting electrical energy to mechanical energy (that’s what the generator and motors do) and it isn’t a reservoir for lubricating oil. The gearbox’s job is to take the APU’s mechanical energy and share it with the essential accessories, often with clutches to disengage when not needed.