Definition
A chemical additive blended into aviation fuel to prevent water dissolved or suspended in the fuel from freezing into ice crystals at low temperatures. The ice crystals would otherwise block fuel filters, lines, or injectors and interrupt the flow of fuel to the engine. FSII works by combining with the water in the fuel so that it does not form ice as the fuel is cooled to subfreezing temperatures encountered at altitude.
Plain English
A liquid mixed into the fuel that stops tiny amounts of water in the fuel from freezing and clogging the fuel system when the fuel gets very cold.
Context Anchor
Seen in fuel specifications, refueling procedures, aircraft operating manuals, and cold-weather fuel system discussions.
Derivation
"Inhibitor" comes from the Latin inhibere, meaning to hold back or restrain. Here it holds back the formation of ice in the fuel system. The everyday word "inhibit" carries the same idea -- to prevent something from happening.
Why Pilots Care
Without FSII, ice can form in the fuel at altitude and clog filters, leading to fuel starvation and potential engine failure.
Grounding Statement
If water is present in fuel and the temperature gets low enough, FSII helps keep that water from becoming ice that can block fuel flow.
Intuition Check
FSII is not a cure for all icing. It is aimed at ice forming from water inside the fuel system, not ice on wings, windshields, or engine air intakes.
Example Sentence 1
Before departure on a cold day, the pilot confirmed the FSII had been added to the fuel at the correct concentration during refueling.
Example Sentence 2
Preflight checks confirmed the correct concentration of FSII in the turbine fuel.