Definition
A small-diameter line that connects the inside of a tank or sealed component to the outside atmosphere, allowing air to enter or escape so that pressure inside the tank stays equal to the surrounding air pressure as fluid level or temperature changes.
Plain English
A tube that lets a tank breathe. As fuel or oil is used up, air flows in through this line so a vacuum doesn't form. As the tank warms and the fluid expands, air can flow back out so pressure doesn't build up.
Context Anchor
Seen during aircraft system discussions, especially fuel tanks, oil tanks, hydraulic reservoirs, and battery or engine-compartment plumbing.
Derivation
From the Latin ventus, meaning wind. A vent is an opening that lets air move. The line is simply the tube that carries that airflow between the tank and the outside.
Why Pilots Care
Ensures fuel can flow freely without the tank collapsing or building dangerous pressure.
Analogy
Think of pouring liquid from a sealed can. If no air can enter, the liquid glugs or stops; a vent lets air in so the liquid can flow smoothly.
Intuition Check
A vent line is not mainly a drain or a fuel supply line. It is mainly there to let air or vapor move so pressure stays safe and flow stays normal.
Example Sentence 1
During preflight, the pilot checked that the fuel vent line under the wing was clear of dirt and insect nests.
Example Sentence 2
A clogged vent line can cause the fuel tank to collapse under atmospheric pressure as fuel is used.