Disposing of sumped fuel
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Open
I know there are myriad opinions on this matter, but I am trying to find authoritative answer.
For decades pilots have dumped sumped fuel on the ground or into containers on the ramp. 
My thought is that t can by put back in the fuel tanks if it is clean. By design, the fuel you are sumping should be the worst in the tank; you look at that sample as representing the contents of the entire tank and determining its suitability for flight. If, based on that sample, you determine that the fuel in the tank is "clean" and safe for flight, then how is that sample not clean and safe for flight? I have heard the argument that you could reintroduce contaiminants. If the contaiminants are so scarce that you can't detect them in the sample, then how could they make a difference compared to what it in the tank?
8 Replies
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Do not like being the first reply, but...  I learned to fly in 1978 and at the time we just dumped the fuel from the sample cup into the grass.  When I was instructed to do so after questioning the procedure, I was somewhat surprised anyone would do it.  When you considered how busy the airport was (It was an urban area field), I thought the act of doing so was careless.  Fast forward to 2017 when I got back into flying and now I use the club GATS jar to sample fuel.  I return the sample to the tank if I see nothing amiss.  It does make me check (For sure) fuel levels in tanks.  The GATS jar has a screen which is supposed to filter out the fuel so you can recycle it.  So I do.  Look up GATS jar...
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Dumping fuel samples on the ground is a violation of the Federal Clean Water Act of 1987.  Enforcement varies from state to state, but in many states (Florida and Virginia, to name two), it's enforced strongly with $500-1000/ dump penalties (multiply that by 13 for the 13 drains on a C-172R/S -- just ask Embry-Riddle Aeronautical University about that).  In those states, airports/FBO's are required to provide dump cans on the flight line in which you can dispose of your fuel samples ($10K fines if they don't provide them)  So dumping it on the ground isn't just a bad idea, it's against the law.  The GATS jar Christopher mentioned is a great way to check fuel and separate out both particulate and water contamination as you return the fuel to your tank safely,

https://www.mypilotstore.com/mypilotstore/sep/4110?qryrmv=1&ppcs=google&ppcg=4-37&keyword=gats%20jar&gclid=CjwKCAjwi_b3BRAGEiwAemPNU5BiBRWPQbdtSbjdA4-6fTZH_dKHkSCOC7VTMAVVaPMMateNybeT5RoC42MQAvD_BwE

Also available from many other internet pilot/aircraft supply outlets.

Otherwise, find the dump can at your airport and put the samples there -- helps keep the airport grass mowers running (Embry-Riddle no longer has to buy gas for their mowers).
I don’t put it back in the tank and have always told my students to never do it.  My reasoning is if God forbid I pour the fuel back into the tank and have a contamination issue.  The first thing the FAA will hear is someone saying that I had a tendency to pour the sumped fuel back into the tank.  Good luck convincing them you didn’t recontaminate the tank.  I keep a gallon gas can in the hangar and use it in my chainsaw.  I don’t go through enough to run my mower.  I’ve seen some airports leave a can near the pump so you have some place to put it.  They use it in their mowers and tugs.
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If you're using a classic sump drain device, I agree.  But there aren't any significant contaminants which can get through the screen on a GATS jar.
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For many years we did off-airport operations that required re-fueling in the field. No matter how clean the dedicated fuel cans were, we  ALWAYS strained the fuel going into the the aircraft through a large funnel lined with a sheepskin chamois. Jump ahead thirty years, and whenever I sump my airplane (read; before the first flight of the day, or after re-fueling), the fuel sample is checked for contaminates. Clean fuel is stored in new "100LL Only" plastic gas can, and when enough has accumulated to make it worthwhile, returned to the airplane tank via the funnel and chamois method. Contaminated fuel is stored in a waste products can, along with dirty solvents and other used VOC fluids, and recycled along with waste oil. Thomas Stein, Ragwing Flyers, Katy, Texas
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WRT slumping after re-fueling, I'm told that this will not help because any water or contaminants will still be suspended for a time, and wouldn't have had time to settle into the sump.  Comments?
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Richard Campbell:
WRT slumping after re-fueling, I'm told that this will not help because any water or contaminants will still be suspended for a time, and wouldn't have had time to settle into the sump.  Comments?

Pretty much true.  You need to wait at least 15-20 minutes after fueling for things to settle out.

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We normally just dump it in our tug in our hangar