Definition
A small opening or tube that allows outside air to enter a fuel tank as fuel is drawn out, keeping the pressure inside the tank equal to the pressure outside. Without this venting, a vacuum would form in the tank as fuel is consumed and fuel flow to the engine would be restricted or stop entirely.
Plain English
An opening that lets air into the fuel tank so fuel can flow out smoothly. As fuel leaves the tank, air has to come in to take its place, otherwise the tank would create suction and starve the engine of fuel.
Context Anchor
Seen during preflight inspection and in discussions of aircraft fuel system operation.
Derivation
Vent comes from the Latin ventus meaning wind. A vent is simply an opening that lets air move in or out -- here, into the fuel tank to replace the fuel being used.
Why Pilots Care
A blocked vent creates a vacuum that stops fuel flow, leading to engine stoppage even when fuel remains in the tank.
Analogy
It is like pouring liquid from a can with no air hole. If air cannot get in, the liquid may glug, slow down, or stop flowing smoothly.
Intuition Check
Do not think of the vent as just a drain or leak path. Its main job is to let the fuel tank breathe so fuel can keep flowing normally.
Example Sentence 1
During preflight, the pilot checked that the fuel tank vent under the wing was clear of insects and debris.
Example Sentence 2
After takeoff the engine lost power because a blocked fuel tank vent had created a vacuum that prevented fuel from reaching the carburetor.