Definition
Liquid water present in aviation fuel as a separate, undissolved phase, typically settling to the bottom of fuel tanks, sumps, and lines because water is heavier than fuel. Free water is distinct from dissolved water, which is held in suspension within the fuel itself.
Plain English
Actual droplets or pools of water sitting in the fuel as a separate liquid, rather than mixed into it. Because water is heavier than fuel, it sinks to the lowest points of the tank or fuel system.
Context Anchor
You will encounter this term during fuel sampling, preflight inspection, and discussions of fuel contamination.
Derivation
‘Free’ here means ‘not bound or combined with something else.’ The water is free of the fuel — present as its own separate liquid rather than dissolved into the fuel.
Why Pilots Care
Undrained free water can enter the engine and cause power loss or complete failure.
Grounding Statement
Because water is heavier than fuel, it settles below the fuel in the tank.
Intuition Check
Do not read “free water” as “fuel that is free of water.” It means the opposite: separate water is present and needs attention.
Example Sentence 1
During preflight, the pilot drained each fuel sump and checked the sample for free water before starting the engine.
Example Sentence 2
Any free water left in the fuel system risks being drawn into the engine during takeoff.