Testing your fuel: What water in aviation fuel looks like

If you were asked ‘what are you looking for when you test the fuel after you fill the tank’, would you be able to answer it?

Each time you put fuel in the tank, regardless of the amount, you should be testing the fuel.

The reason? To check for contamination, which means water and debris.

What does water in the aviation fuel tester look like?

The image below should be a familiar site – clean fuel in the fuel tester. Good to go surf some clouds.

AvGas UL91 fuel with no water
Clean fuel in a fuel tester

The image below, however, shows water contaminated fuel.

AvGas UL91 fuel with water contamination
AvGas UL91 fuel contaminated with water

Water is denser than fuel and therefore sinks to the bottom of the tube leaving the fuel on top. In a rather handy fashion, most fuel is coloured. UL91 is a honey colour, whereas AvGas 100 is green and AvGas 100LL is blue. So seeing the clear colour of the water should be easy to define, as you can see above.

Please note: Images above have been created for the purposes of demonstration and are not taken directly from an aircraft.

What to do if you find water or debris in the fuel tester

If you find water in the fuel tester when you examine it after bleeding some fuel off, do not fly the aircraft and report it to AirBourne. They will then decide on the next action. What they may do is continue to bleed off the fuel off from the tap until there is no more water coming from each bleed. Beneath the fuel tank, there is usually an S-bend shaped pipe. This is designed to collect any water that runs into the tank (hence the above, water is denser than fuel and therefore sinks to the bottom) due to condensation. This helps prevent any water being sent through to the engine and causing serious problems. By continuing to bleed the tank from the tap you should notice all the water gradually disappear. However, that does not necessarily mean it is safe to fly as there could be a more serious problem.

Alongside water, you should also be checking the tube for small particles which may suggest degradation somewhere in the fuel system. If you find small debris floating in the tester, do not fly the aircraft, report it and do not fly it after further bleeding. Remember to check the fuel test tube for pre-existing debris before checking the fuel!

Disclaimer: This article was written for information only and was up to date at the time of writing. Please always ask a qualified instructor for more information and discuss airfield operations and the use of aircraft equipment with a suitably qualified person.