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I've read that it's recommended to store a plane with full fuel tanks to prevent water contamination. Some small general aviation planes, however, would exceed their maximum gross take off weight with full fuel and just two adult passengers of average weight. One option, of course, is to fly with less than a full fuel tank. This seems like it could be a problem if 50 gallons of fuel need to be dumped which cost the pilot $300. My question is: what does a pilot do with the extra fuel that needs to be discarded before the flight?

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You have a few options,

You can burn it off. Laps in the pattern are great for this. Get to the airport before you need to depart and practice some landings. This may cost you in terms of dollars but at least you are not just dumping it.

You can dump it into gas cans if you have them. Every small GA plane i have ever encountered (mostly pipers) has fuel sumps for testing your fuel during your preflight. Most of these sumps can be latched open and you can drain it into a gas can for use later. you should always test for water before doing this. If you rent you should ask the FBO what their policies are on dumping/reusing fuel. I cant find any specific legislation on it but it seems that its common practice (for the airlines at least) to only put the fuel back in the same plane (or within the same fleet) to prevent cross contamination of things that can build up in the tanks.

The sump on the pipers I fly looks like this

enter image description here (source)

Dont have the plane filled up. If you rent planes like many GA pilots do and you plan to take up 4 friends for an evening sight seeing flight ask the FBO to not fill the tanks if someone is using it before you.

Dont use all the tanks. This will vary plane to plane but some planes have 2 or more fuel tanks which may be independently selectable. You may be able to (weight and balance permitting) keep some of the tanks dry if you know you regularly short hops with big loads.

For the sake of a complete answer I will mention that planes can be flown over gross weight, I don't advocate this nor would I ever attempt it but it does happen here and there, here is a discussion on it if you are curious.

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  • $\begingroup$ I've never seen those kinds of drains on a Cessna. In my plane I just fill to 44 gallons instead of 50 during the summer, or after a flight when I know I have a heavier load to fly coming up. $\endgroup$ – CrossRoads Feb 12 at 18:53
  • $\begingroup$ 'This seems like it could be a problem if 50 gallons of fuel need to be dumped which cost the pilot 300 dollars.' Yes, especially as many '4-seat' planes only carry 50 or 60 gallons to start with! $6/gallon seems a bit high, I suppose it depends where you are. I believe my home airport is 4.19 USD at the moment. $\endgroup$ – CrossRoads Feb 12 at 18:56
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Given the well-known unreliability of GA planes' fuel gauges, I would never accept a rental plane with less than full tanks. That is the only way to be sure how much fuel you actually have.

Burning off fuel in the pattern or draining it into tanks are IMHO only reasonable options for an extremely short flight or if you're only overweight by a few gallons.

The three most useless things to a pilot are altitude above you, runway behind you and fuel on the ground. Every pound of fuel you don't have onboard means fewer options (or "outs") available if something goes wrong in the air.

Far more likely is that I would simply rent a larger plane (e.g. C182 instead of C172) that could carry the desired load with full fuel. This is obviously an easier decision for renters, but even if I owned a C172, I'd still prefer renting a C182 for one trip over giving up fuel/outs. If I need to do that often enough that the cost or inconvenience mattered, then it's time to trade up to a bigger plane.

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    $\begingroup$ No, full tanks are not the only way to know how much fuel you have. With most light planes (at least in my experience) you have (or make) a measuring stick, and check. FWIW, with my Cherokee I almost never fill the tanks completely. At most it's "to the tabs" which is about 40 gallons of the 50 max. At ~8 GPH, even 40 gallons lasts longer than I care to go between pit stops. And every extra pound of fuel means less climb performance, which matters when you fly into a lot of short mountain fields. $\endgroup$ – jamesqf Feb 12 at 20:26

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