Definition
Water that is held in solution within aviation fuel at the molecular level, invisible to the eye and not removable by normal sumping or filtering. The amount of water fuel can hold in solution increases with temperature; as fuel cools, some of this dissolved water comes out of solution and forms free water or ice crystals.
Plain English
Tiny amounts of water mixed right into the fuel itself, so small you cannot see them. You cannot drain this water out at the sump. When the fuel gets cold, this hidden water can separate out and turn into droplets or ice.
Context Anchor
Seen in fuel system icing discussions, especially when learning how water in fuel can create ice even when no visible water was seen during a fuel check.
Derivation
‘Dissolved’ comes from the Latin dissolvere, meaning ‘to loosen apart.’ When something is dissolved, its particles are spread evenly throughout another substance — the way salt dissolves into water. Here, water itself is dissolved into fuel, mixed in so completely that it cannot be seen or drained.
Why Pilots Care
A temperature drop can turn dissolved water into ice crystals that clog filters and lines, risking engine stoppage.
Analogy
Sugar stirred into hot coffee is hard to see once it dissolves, but it is still there. Dissolved water in fuel is similar: you may not see it, but colder conditions can make it show up again as ice.
Grounding Statement
Even fuel that looks perfectly clean and tests dry at the sump still contains a small amount of water hidden inside it.
Intuition Check
Do not assume “dissolved water” means visible water in the fuel sample. Here it means water hidden within the fuel, where it cannot be seen as separate drops.
Example Sentence 1
As the aircraft climbed and the fuel temperature dropped, dissolved water began to come out of solution and form ice crystals on the fuel filter.
Example Sentence 2
Mechanics add fuel additives to keep any dissolved water from freezing during cold-weather flights.