Definition
A gearbox attached to an aircraft engine that uses power taken from the engine's main rotating shaft to drive the engine's accessories, such as magnetos, generators or alternators, fuel pumps, hydraulic pumps, vacuum pumps, and tachometer drives.
Plain English
A set of gears bolted to the engine that turns all the smaller machines the engine needs to run, like the parts that make electricity, pump fuel, and create suction for the instruments.
Context Anchor
Seen in engine system descriptions, maintenance records, and discussions of engine-mounted equipment.
Derivation
Accessory comes from the Latin accedere, meaning 'to come near' or 'be added to.' The accessories are the supporting components added to the engine — not the engine itself, but the helpers it drives. The gearbox is simply the housing of gears that distributes that drive power to each accessory.
Why Pilots Care
It supplies the correct rotational speeds to critical systems; a failure can cause loss of electrical power, hydraulics, or fuel delivery.
Intuition Check
Do not read accessory as optional equipment here. On an aircraft engine, accessories are connected devices that may be essential to normal engine operation.
Example Sentence 1
During the engine inspection, the mechanic checked the accessory drive gearbox for oil leaks and gear wear.
Example Sentence 2
During the annual inspection the mechanic checked the accessory drive gearbox for excessive play in the drive gears.