Definition
A washer with a raised rim or flange around its edge, forming a shallow cup shape. It is placed under a bolt head or nut to spread the clamping load over a larger area, protect a soft surface from the fastener, or seat against a curved or recessed part.
Plain English
A washer that is shaped a bit like a shallow dish instead of being flat. The raised edge lets it sit neatly against a curved part or spread the pressure of a bolt over a wider area.
Context Anchor
Seen in aircraft maintenance on panels, covers, interiors, and other places where a screw head needs a neat, supported seat.
Derivation
Called a cup washer because the raised rim gives it the shape of a small cup, distinguishing it from an ordinary flat washer.
Why Pilots Care
A missing or damaged cup washer can allow fuel or oil leaks that lead to engine roughness or fire risk.
Intuition Check
A cup washer is not a fluid container or a sealing cup. In this context, “cup” means the washer has a shallow dished shape that supports a screw head.
Example Sentence 1
The mechanic installed a cup washer beneath the bolt head to protect the fairing from being crushed when the fastener was torqued down.
Example Sentence 2
During the preflight, the pilot noticed a slight seep and later found the cup washer on the fuel strainer had cracked.