SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/pilot-water-in-fuel-sample

When you drain a fuel sample, where will any water show up?

At the bottom of the sample, because water is heavier than fuel.

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  1. Floating on top of the fuel
  2. At the bottom of the sample ✔
  3. Mixed in evenly

Why: Water is heavier than avgas, so it sinks to the low points of the fuel system and to the bottom of the sample jar. It often looks like clear beads or bubbles. Drain each sump before flight and after refueling.

Huh, didn't know that: Water can get in through worn fuel cap seals in the rain or from the fuel supplier's tanks.

The dad joke
Why did the fuel sample break up with the water? They just did not mix.
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References

Answer: At the bottom of the sample. Water is heavier than avgas, so it sinks to the low points of the fuel system and to the bottom of the sample jar. It often looks like clear beads or bubbles. Drain each sump before flight and after refueling.

  1. FAA Advisory Circular 20-43C, Aircraft Fuel Control www.faa.gov/documentLibrary/media/Advisory_Circular/AC_20-43C.pdf
    it will collect at the bottom of the container and should be easily detected.
    Checked 2026-10-09.

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