Definition
An outer covering or sleeve that surrounds another component to contain, protect, or channel something around it. In aircraft systems, a jacket is most commonly the outer shell of a liquid-cooled engine cylinder that holds coolant in contact with the cylinder walls, or the protective outer layer of a wire, hose, or cable.
Plain English
A jacket is a covering wrapped around something else. It either holds a fluid against the inner part (like coolant around an engine cylinder) or protects what's inside (like the rubber covering on an electrical wire).
Context Anchor
Seen in aircraft maintenance descriptions, especially around wiring, hoses, cables, and engine parts.
Derivation
From the Old French jaquet, meaning a small coat. The aviation use keeps the same idea: something that wraps around and surrounds the part underneath, just as a coat wraps around a person.
Why Pilots Care
A damaged jacket can expose conductors or allow chafing that leads to electrical shorts or control cable failure.
Intuition Check
Do not read jacket here as clothing. In aircraft use, it means a protective covering around a part.
Example Sentence 1
The mechanic inspected the cylinder jacket for cracks before refilling the coolant system.
Example Sentence 2
Chafed insulation jackets on the engine harness caused intermittent electrical faults during flight.