I've been taught to always sump the plane's fuel system before going flying to check for water / contamination / proper fuel grade. However, I've yet to go flying in rain. What's the proper procedure to drain fuel during rain? If I took the fuel cap off to dump the fuel back into the plane, I'd be worried about rainwater getting in the tanks.

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    $\begingroup$ I would suggest using an umbrella. $\endgroup$
    – Magnetoz
    Commented Apr 15, 2014 at 17:24
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    $\begingroup$ Is this considered a safe accepted practice? I'd be worried about wind blowing water into the tanks. $\endgroup$
    – Mike
    Commented Apr 15, 2014 at 17:26
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    $\begingroup$ Originally when I did my training in Texas, I was taught to throw the fuel on the asphalt. By contrast, in the northeast, all of my instructors have told me the EPA would go after them for dumping fuel on the asphalt. I was taught to pour the fuel with through a strainer back into the tanks to filter out any contaminants, after verifying that no water was present. I'd still be nervous about using an umbrella if it was raining really hard with strong gusty winds. $\endgroup$
    – Mike
    Commented Apr 15, 2014 at 17:41
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    $\begingroup$ If its raining that hard, and that windy, you will most likely be delaying your flight until conditions improve. And again, a few drops that might make their way in while you quickly remove the cap, dump and replace will not affect engine operations. $\endgroup$
    – Magnetoz
    Commented Apr 15, 2014 at 17:51
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    $\begingroup$ Related (but not duplicate) question: aviation.stackexchange.com/q/1141/520 $\endgroup$ Commented Apr 15, 2014 at 18:09

1 Answer 1


AOPA has a good article about checking fuel:

... check the fuel caps and replace worn, brittle, or cracked seals that can allow rain to seep into the tanks.

You cannot be 100% that the fuel does not contain any contamination. If you risk that by putting the sample fuel back in the airplane (debatable topic) will add more water in the tanks, then simply don't do it.

I guess since many flight schools offer wet rentals, they don't want to waste a drop of fuel. But it is okay to do that, so that their aircraft and your life are safer.


There are many arguments about putting sump fuel back in the tank. Many people say that a GATS Jar is there to capture the contamination and then you can put it back in the fuel tank. In Cessna training material, they use a container where you can put sump fuel. At many places, there is a fine for throwing it on the ground.

So in case of rain, the best option is to put in such a container. However, if it is raining that hard with strong gusty winds, I am sure taking care of sump fuel should not be your problem.

  • $\begingroup$ If not back in the tanks, what's the general consensus on where to put the fuel? Is it common practice to dump it on the asphalt, down a drain, or dump it in a container the FBO provides to safely dispose of it? $\endgroup$
    – Mike
    Commented Apr 15, 2014 at 18:19
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    $\begingroup$ @Mike: See aviation.stackexchange.com/q/1141/520 $\endgroup$ Commented Apr 15, 2014 at 18:27
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    $\begingroup$ This doesn't appear to answer the question about water getting in when the caps are removed during the preflight.... $\endgroup$
    – Lnafziger
    Commented Apr 15, 2014 at 22:15

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