Definition
A shaft inside an aircraft engine that transfers rotational power from the engine's crankshaft (or turbine shaft) to the accessory section, where it drives engine-mounted accessories such as magnetos, fuel pumps, oil pumps, generators or alternators, vacuum pumps, and tachometer drives.
Plain English
A spinning rod inside the engine that uses some of the engine's power to run the smaller pumps and devices the engine needs to keep working.
Context Anchor
Seen in aircraft engine systems, maintenance descriptions, and discussions of engine-driven equipment.
Derivation
"Accessory" comes from the Latin accessorius, meaning "something added or supplementary." The accessory drive shaft is the shaft that drives those added-on components — items the engine needs to operate but that aren't part of the main power-producing parts.
Why Pilots Care
Loss of the shaft disables multiple engine accessories at once, potentially cutting electrical power, hydraulic pressure, and ignition.
Analogy
It is like a small power takeoff from the engine: the engine is running, and the shaft passes some of that turning motion to other equipment that needs to spin.
Intuition Check
“Accessory” does not mean unimportant here. In an aircraft engine, an accessory may be essential equipment that the engine depends on or that the pilot depends on in flight.
Example Sentence 1
The magnetos and fuel pump are driven by gears connected to the accessory drive shaft at the rear of the engine.
Example Sentence 2
A sheared accessory drive shaft caused the alternator to stop charging in flight.