Definition
Small valves located at the lowest points of an aircraft's fuel system that allow the pilot to drain a small sample of fuel before flight. Their purpose is to remove water, sediment, and contaminants that have settled to the bottom of the fuel tanks and lines, and to allow the pilot to verify the correct fuel grade and color.
Plain English
Little drain valves at the bottom of the fuel tanks. You open them during preflight to let out a small cup of fuel, check it for water and dirt, and confirm it's the right type of fuel.
Context Anchor
You encounter fuel sump drains during preflight inspection, especially when checking the fuel tanks and fuel strainer before starting the engine.
Derivation
Sump comes from a Middle English word meaning 'a marsh' or 'low place where liquid collects.' That fits exactly here -- the sump is the low point in the fuel tank where any water or debris naturally settles, and the drain is what lets you get it out.
Why Pilots Care
Draining these points removes water and debris that can cause engine power loss or complete failure in flight.
Intuition Check
Do not think of these drains as accidental leaks. Fuel sump drains are intentional openings the pilot uses to sample fuel and remove water or dirt from the lowest points of the fuel system.
Example Sentence 1
During preflight, she used the sampler cup to check each of the fuel sump drains for water and sediment.
Example Sentence 2
After refueling from a truck the pilot drained the fuel sumps again to confirm no new contaminants had been introduced.