Definition
The rubber or synthetic gaskets fitted inside an aircraft fuel tank cap that form an airtight, watertight closure when the cap is secured. Their job is to prevent fuel from escaping, water from entering, and — in vented systems — to maintain the designed pressure relationship inside the tank.
Plain English
The rubber rings inside the fuel cap that keep the cap sealed tight so fuel can't leak out and rain or water can't get in.
Context Anchor
Checked during the preflight inspection when the pilot looks at the fuel caps, fuel openings, and fuel quantity.
Why Pilots Care
A damaged or missing seal allows fuel to evaporate, water to contaminate the tank, or pressure imbalances that can interrupt fuel flow to the engine.
Analogy
They work like the rubber ring in a jar lid: the lid may be on, but the soft ring is what helps make the closure tight.
Intuition Check
Do not assume a closed fuel cap means the tank is properly sealed. The cap can be closed, but if the gas cap seal is damaged, water or dirt may still get in.
Example Sentence 1
During preflight, the pilot noticed the gas cap seals were cracked and dry, so the airplane was grounded until they were replaced.
Example Sentence 2
A worn gas cap seal allowed rainwater to enter the tank overnight, requiring the fuel to be drained and replaced.