Definition
The unintended escape or entry of a fluid, gas, electrical current, or electromagnetic energy past a seal, joint, insulator, or shield that is meant to contain it. In aviation, leakage is measured and controlled in systems such as pressurization, hydraulics, fuel, pneumatics, vacuum instruments, and electrical wiring, where small amounts may be acceptable within published tolerances but excessive leakage indicates a fault.
Plain English
Something escaping (or sneaking in) where it shouldn't — a bit of fluid, air, or electricity getting past a seal or barrier that was supposed to keep it contained.
Context Anchor
Seen during preflight inspections, maintenance checks, pressure checks, and troubleshooting of aircraft systems.
Derivation
From the Old Norse 'leka' meaning 'to drip' or 'to leak.' The aviation use keeps the everyday sense but extends it beyond liquids to include gases, pressure, and electrical current escaping where they shouldn't.
Why Pilots Care
Unchecked leakage can cause loss of critical fluids, fire risk, or complete system failure.
Intuition Check
Do not think of leakage only as liquid dripping. In aircraft use, leakage can also mean air, pressure, vapor, oil, fuel, hydraulic fluid, or exhaust gas moving where it should not.
Example Sentence 1
During the preflight, the pilot checked under the cowling for any sign of oil or fuel leakage.
Example Sentence 2
Any fuel leakage detected near the engine nacelle grounds the aircraft until repaired.