Definition
A mixture of two liquids that normally do not blend, where tiny droplets of one liquid are suspended throughout the other. In aircraft powerplant systems, emulsions most often appear when water becomes mixed into oil or fuel, producing a cloudy, milky-looking fluid instead of a clear one.
Plain English
Two liquids mixed together that don't naturally combine — like oil and water shaken up so the oil breaks into tiny droplets spread through the water. The result looks cloudy or milky.
Context Anchor
Seen in aircraft engine maintenance when checking oil, fuel, or other fluids for contamination.
Derivation
From the Latin emulgere, meaning 'to milk out.' The word was originally used because an emulsion looks milky — and milk itself is a natural emulsion of fat droplets suspended in water. That image (cloudy, milky liquid) is exactly what a mechanic sees when water has emulsified into engine oil.
Why Pilots Care
Emulsions in fuel or oil can clog filters, lines, and injectors, reducing engine performance or causing failure.
Analogy
Oil and vinegar in salad dressing can form a temporary emulsion when shaken. They are mixed for a while, but they are still two liquids that do not naturally blend into one.
Intuition Check
Do not read emulsion as simply "dirty liquid." It specifically means tiny drops of one liquid held throughout another liquid that it does not normally mix with.
Example Sentence 1
The mechanic drained the sump and noticed a milky emulsion in the oil, indicating water contamination.
Example Sentence 2
Fuel system additives are used to break up any emulsion that forms from water contamination.