Plain English Meaning
A rubbery sealing compound smeared inside the fuel tank to stop fuel leaking out through joints and seams. When it gets old or damaged, fuel can start seeping through and leave stains on the wing.
Full Definition
A flexible, fuel-resistant compound applied to the inside seams and joints of an aircraft fuel tank to prevent leaks. In integral fuel tanks (where part of the wing or fuselage structure itself forms the tank), the sealant is what actually keeps fuel from seeping through rivet lines, panel joints, and access ports. Over time, sealant can age, crack, or shrink, leading to fuel weeps and stains on the underside of the wing.
Context Anchor
Seen in fuel system discussions, aircraft maintenance records, and during preflight when checking for fuel stains, fuel odor, or of around a tank .