Definition
A soft copper sealing ring designed to be deformed (crushed) when a fitting or fastener is tightened against it, conforming to small surface irregularities to create a leak-tight seal. Copper crush gaskets are typically used once and replaced when the joint is opened, because the deformation that made the seal cannot be reliably repeated.
Plain English
A thin copper washer that gets squashed flat when you tighten a fitting onto it. The squashing is what makes the seal. Once it's been squashed, you throw it out and use a new one next time.
Context Anchor
Seen during aircraft maintenance on items such as engine drain plugs, fittings, and some spark plug installations.
Derivation
Copper is used because it is soft enough to deform under pressure but strong enough to hold a seal. 'Crush' describes what actually happens to the gasket during installation -- it is intentionally deformed by the tightening of the joint. The name describes the material and the mechanism of sealing.
Why Pilots Care
A properly crushed copper gasket prevents leaks that could lead to fire or loss of engine oil or fuel pressure.
Intuition Check
Do not read “crush” as accidental damage. In this term, “crush” means a controlled, intended flattening that helps the gasket seal.
Example Sentence 1
After removing the spark plug, the mechanic discarded the old copper crush gasket and installed a new one before torquing the plug back into the cylinder.
Example Sentence 2
Never reuse a copper crush gasket after removing an oil line; the once-flattened ring will no longer seal reliably.