Definition
A geared section of a turbine engine that uses rotational power from the engine to drive supporting components such as fuel pumps, oil pumps, generators, hydraulic pumps, and the tachometer. In a split-shaft/free turbine engine, the accessory drive is typically turned by the gas generator section rather than the power turbine.
Plain English
A set of gears on the engine that runs the engine's helper equipment — the pumps and generators that the engine itself needs to operate.
Context Anchor
Seen in turbine engine descriptions, especially when explaining how a turboprop engine powers its pumps, generators, and other engine-mounted equipment.
Derivation
Accessory comes from the Latin accessus, meaning 'something added.' In aviation, accessories are the supporting components added to the engine — not the core combustion parts, but the pumps and generators that keep it running. The drive is the geared shaft that powers them.
Why Pilots Care
In free-turbine engines the accessory drive keeps critical systems running from the gas generator even when the propeller is feathered or stopped, preserving fuel delivery, lubrication, and electrical power.
Intuition Check
Do not read accessory as “optional extra.” On an aircraft engine, an accessory may be a required part that the engine depends on to keep running properly.
Example Sentence 1
The fuel control unit and oil pump are mounted on the accessory drive at the front of the engine.
Example Sentence 2
During preflight the pilot confirms that the accessory drive is providing normal oil pressure and generator output.