Definition
A thin, flat metal disc with a hole in the center, placed under a bolt head or nut to spread the clamping load over a wider area, protect the surface of the part being fastened, and provide a smooth bearing surface for the fastener to turn against.
Plain English
A simple metal ring that sits between a bolt or nut and the surface it's tightening against. It spreads the squeeze of the fastener over a bigger area so the surface doesn't get dented or damaged.
Context Anchor
Seen during aircraft maintenance and inspection when hardware is installed on brackets, panels, engine mounts, and other bolted parts.
Why Pilots Care
Distributes load to prevent surface damage and helps keep fasteners secure under aircraft vibration.
Analogy
It works like putting a small plate under a table leg on soft ground: the weight is spread over a wider area instead of pressing into one small point.
Intuition Check
A flat washer is not meant to lock the nut in place. Its main job is to spread pressure and protect the surface; a locking washer or other approved locking method is used when locking is required.
Example Sentence 1
The mechanic placed a flat washer under the nut before torquing the bolt to spec.
Example Sentence 2
Always replace a damaged flat washer during reassembly to maintain proper clamping force.