Definition
Living organisms — primarily bacteria, fungi, and yeasts — that grow in aircraft fuel, particularly jet fuel, where free water is present. They form dark, slimy, sludge-like colonies at the fuel-water interface in fuel tanks, and can clog filters, corrode tank structure, and produce inaccurate fuel quantity readings.
Plain English
Tiny living things, like bacteria and fungus, that grow inside fuel tanks where water has collected. They form a slime that can corrode the tank, block fuel filters, and interfere with fuel system components.
Context Anchor
Seen in aircraft fuel-system maintenance, fuel tank inspections, sump draining, and fuel filter checks.
Derivation
From Latin 'microbius' (small life) — 'micro' meaning small and 'bios' meaning life. 'Contaminant' comes from Latin 'contaminare,' to pollute or make impure. So literally 'small living things that make the fuel impure' — which is exactly what they are.
Why Pilots Care
Unchecked growth clogs filters, damages pumps, and corrodes tanks, which can lead to fuel starvation or engine failure.
Analogy
It is similar to unwanted growth in a damp container: the problem is not just the growth itself, but also the residue it leaves behind and the damage it can cause.
Intuition Check
Do not think of fuel contamination only as dirt or water. In this context, contaminants can also be living growth and the harmful material that growth produces.
Example Sentence 1
After finding dark sludge during a fuel sample, the technician treated the tank for microbial contaminants and inspected the tank walls for corrosion.
Example Sentence 2
Regular use of a fuel biocide keeps microbial contaminants from forming in the tanks during long periods of storage.