Definition
The removable, sealing cover fitted to the filler opening of an aircraft fuel tank. The fuel cap keeps fuel inside the tank, prevents water, dirt, and other contaminants from entering, and on most light aircraft incorporates a vent or seals against a separately vented system to allow proper tank pressure equalization as fuel is consumed.
Plain English
The lid that closes the opening where fuel is poured into the tank. It keeps the fuel in and keeps unwanted things like water and dirt out.
Context Anchor
Seen during preflight inspection, refueling, and any check of the airplane’s fuel tanks.
Derivation
Fuel comes from an old word meaning material used for burning. Cap comes from a word for a covering. Together, fuel cap means the covering for the place where fuel is added.
Why Pilots Care
A loose, damaged, or missing fuel cap allows water contamination or fuel siphoning that can cause engine failure or reduced range.
Intuition Check
Do not think of a fuel cap as just a simple lid. On an airplane, it is a safety-critical cover that helps protect the fuel supply, and on some airplanes it also helps the tank admit air correctly as fuel is used.
Example Sentence 1
During preflight, the pilot checked the fuel level in each tank and made sure both fuel caps were tight and properly seated.
Example Sentence 2
After refueling, the line attendant replaced the fuel cap to keep rainwater out of the tank overnight.