Definition
The removable, sealing cover fitted to the opening of an aircraft fuel tank, used to close the tank after refueling and to keep fuel, fumes, and contaminants properly contained during flight.
Plain English
The cap that screws or latches onto the fuel tank opening on top of the wing (or fuselage) to seal the tank closed after fueling.
Context Anchor
Seen during fueling and preflight inspection, especially when checking that each fuel tank cap is present, clean, undamaged, and secure.
Derivation
From 'fill' (to put liquid in) plus '-er' (the place or thing associated with the action), giving 'filler' — the opening you fill through. The cap is what closes that opening.
Why Pilots Care
A secure filler cap prevents fuel loss, water contamination, and vapor escape that can affect engine operation or create a fire hazard.
Intuition Check
Do not think of the filler cap as just a simple lid. In an aircraft fuel system, it is a part that must seal correctly and, on some designs, help the tank take in air as fuel is used.
Example Sentence 1
During preflight, she climbed onto the step and made sure each filler cap was tight before walking around to the other wing.
Example Sentence 2
After refueling the student replaced the filler cap and wiped away any spilled fuel from the wing surface.