Definition
Small valves located at the lowest point of each fuel tank that allow the pilot to draw off a sample of fuel during preflight. They are used to check for water, sediment, or the wrong grade of fuel before flight, and to remove any contamination found.
Plain English
Little valves at the bottom of each fuel tank. The pilot opens them before flight to catch a small sample of fuel in a clear cup and check that it is clean, water-free, and the right type.
Context Anchor
Encountered during the preflight inspection, when the pilot samples fuel from each tank before starting the engine.
Derivation
Drain means to draw liquid off or let it flow out. In this term, the important idea is not emptying the whole tank, but drawing off a small amount from a low point so the pilot can check what is in the fuel.
Why Pilots Care
Removes water and sediment that could otherwise reach the engine and cause power loss or failure.
Grounding Statement
Water is heavier than aviation fuel, so it can settle at the bottom of the tank where the drain can reveal it.
Intuition Check
Do not assume fuel tank drains are mainly for emptying the whole tank. In normal preflight use, they are for taking small samples and removing contamination.
Example Sentence 1
During preflight, she opened each fuel tank drain in turn and checked the sample for water before walking around to the strainer.
Example Sentence 2
After refueling, the fuel tank drains were used again to confirm the new fuel was clean.