Definition
A housing at the rear of a reciprocating aircraft engine that contains the drive gears for engine-driven accessories such as the magnetos, vacuum pump, fuel pump, oil pump, tachometer drive, and on some installations the propeller governor and hydraulic pump. These accessories are turned by gears driven off the engine crankshaft.
Plain English
It is the section at the back of the engine where smaller engine-powered devices like the magnetos and the vacuum pump are mounted and driven by the engine.
Context Anchor
Seen in discussions of engine-driven power sources for gyroscopic flight instruments, especially vacuum pump installation.
Derivation
An 'accessory' is a secondary item attached to a main one, and 'case' here means a housing or enclosure. Together it describes the housing for the engine's secondary driven equipment — the things bolted onto the engine that the engine itself turns.
Why Pilots Care
A problem with the accessory case or its mounted components can cause loss of vacuum, resulting in failure of the attitude indicator and heading indicator during flight.
Intuition Check
Do not read “accessory” as something optional or decorative here. In this context, an accessory is important equipment attached to and driven by the engine.
Example Sentence 1
The vacuum pump that powers the attitude and heading indicators is mounted on the accessory case at the rear of the engine.
Example Sentence 2
During the engine run-up the pilot listens for unusual noises that might indicate wear in the accessory case gears.