Definition
Any unwanted foreign substance present in or on an aircraft, its systems, or its operating fluids that degrades performance, safety, or reliability. Common aviation contaminants include water, dirt, rust, metal particles, or microbial growth in fuel; moisture or particulates in oil and hydraulic fluid; and ice, snow, frost, slush, or de-icing residue on airframe surfaces or runways.
Plain English
Something that does not belong there and that can cause problems if left alone. In aviation it usually means dirt, water, ice, or other unwanted material on the aircraft, in a fluid, or on a runway.
Context Anchor
Seen in maintenance, fuel checks, preflight inspections, runway condition reports, and discussions of ice, snow, water, dirt, oil, or other unwanted material.
Derivation
From the Latin contaminare, meaning to make impure by mixing or contact. The aviation use keeps that core idea: a clean substance or surface has been spoiled by something that does not belong.
Why Pilots Care
Contaminants in fuel can stop an engine; contaminants on a runway can reduce braking and increase stopping distance.
Grounding Statement
If something unwanted is on a flying surface, in a fluid, or on a runway, treat it as a contaminant until you know it is harmless or removed.
Intuition Check
Do not think of a contaminant only as dirt or germs. In aviation, a contaminant can be water, ice, snow, oil, fuel, dust, metal particles, or any other unwanted material that can affect safe operation.
Example Sentence 1
During preflight, the pilot drained a fuel sample from each sump and checked it for water and other contaminants.
Example Sentence 2
The tower reported that ice and snow contaminants had reduced braking action on the runway.