Definition
Visible discoloration, streaks, or residue on the exterior of an aircraft caused by fuel that has leaked from a tank, fitting, fuel cap, or vent and then evaporated, leaving behind dye and other non-volatile components. Fuel stains are an external indicator of a current or recent fuel leak and are checked for during preflight inspection.
Plain English
Marks or streaks left on the outside of the airplane where fuel has leaked out and dried, showing the pilot that fuel may be escaping from somewhere it shouldn't.
Context Anchor
Seen during the preflight inspection, especially around fuel caps, wing surfaces, drains, vents, and the lower areas of the aircraft where fuel could run or drip.
Why Pilots Care
Fuel leaks create fire hazards and risk of fuel loss in flight; any stain must be traced to its source and corrected before departure.
Intuition Check
Do not assume fuel stains are just old dirt or cosmetic marks. On an aircraft, they can be evidence that fuel has escaped and the source needs to be checked.
Example Sentence 1
During preflight, the pilot noticed blue fuel stains trailing aft from the right wing fuel cap and decided to have a mechanic check the seal before flying.
Example Sentence 2
Fresh fuel stains on the ramp led the mechanic to tighten a loose fitting on the fuel vent line.