I was thinking about getting some unleaded to a Rotax engine, and realized I could ferry some UL94 in the tip tanks of something like a Bonanza. Not that it really matters in real life, but let's say that I were ramp checked on my way out of the airport and I didn't have an STC for said UL94 (although it could be any fuel at all which is not approved), would I be legal or not?

And if the tip tanks are allowed, then what about filling one main tank on, e.g., a Cherokee and putting the fuel selector on the other?

That's not to say this is a smart thing to do in general (be careful of ethanol!), but if I can transport the fuel in a jerry-can in the back of the plane, it's obv. not forbidden to use the airplane to carry tanks of non-approved fuel. And in the specific case of UL94, it's approved for basically all GA pistons, so we can be sure it won't have any adverse affects on the tank sealant.
  • $\begingroup$ Do these tip tanks have a pipe that can be disconnected and both pipe and tank be securely plugged for the trip? Do you have a good safe way to drain that tank at the destination? Can you drain them evenly? $\endgroup$
    – Criggie
    May 16, 2023 at 23:46
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    $\begingroup$ @Criggie excellent questions. Unfortunately, the ferry planes all hypothetical, so I don't have a firm leg to stand on when trying to answer you. You make a very good point about draining the tank in an exhaustive manner. If there were anything left, in theory you might have to get an A&P to investigate and formally declare the airplane airworthy before being able to use the tanks again. $\endgroup$ May 17, 2023 at 2:30
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    $\begingroup$ There seem to be two main areas of regulation regarding fuel. One area is in the plane, regarding fuel separation and monitoring; the other is on the ground, regarding the integrity and safety of the fuel supply. It seems that there is a definite intention that an aircraft pilot must be able to assume everything going into their tanks is valid fuel and the systems on the ground must be secured in order to only put valid fuel into the aircraft's fuel tanks. Any process that ended with non-valid fuel (e.g. water) into a fuel tank would likely fall foul of these regulations on some level. $\endgroup$
    – Frosty840
    May 17, 2023 at 7:44
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    $\begingroup$ Some mid-air refueling tanker planes carry fuel for other planes that is different from the fuel they use themselves. These are designed with entirely separate fuel lines such that it is impossible to accidentally route the wrong fuel into the tanker's engines. $\endgroup$ May 17, 2023 at 16:11
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    $\begingroup$ @Criggie exactly... or a water-bomber. Heck, you wouldn't even need to land to fill up at the refinery, just have them fill a trough and scoop it! "What could possibly go wrong" $\endgroup$ May 17, 2023 at 21:33

2 Answers 2


Don't do it. You would get busted on a ramp check (it's normally just a check for documents and the ramp check official is unlikely to go sniffing your fuel tanks, but if they did, or if they came up while you were doing it, oops) if you had an unapproved fuel in one of the airplane's tanks that is used to supply the engine under the airplane's type certificate.

Approved under the type certificate is approved under the type certificate. There are no exceptions for ferrying unapproved liquids in tanks intended to supply the engine. Why not put diesel, or paint thinner, if you were just using it as a transport container. No difference really, from a strictly legalistic viewpoint.

In any case, the official is unlikely to accept a claim that you were just ferrying fuel, and will assume you intended to burn car gas to save a few bucks.

And to top it all off, if you crash the plane during the "ferry" trip, your insurance company has a Get-Out-Of-Jail free card they can use to avoid paying out your claim if they feel like it.

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    $\begingroup$ Well, when the TC lists "approved fuels" that's it. Unapproved is unapproved. I don't think you're going to find a regulation that says "don't put unapproved fuel in airplane fuel tanks". When a road sign says "one way", they don't include a sign saying "don't go the other way". Beyond that, it would be up to the FAA administrative law judge you go before to plead your case as to how to precisely interpret it. $\endgroup$
    – John K
    May 16, 2023 at 13:39
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    $\begingroup$ @KennSebesta The fact that the fuel is in the fuel tank of your airplane is evidence that you intend to run your airplane on it. As far as I know, there's no rule saying that the court has to disregard that evidence and trust that you're telling the truth. If you offer an explanation for your behavior, and your explanation is strange and implausible and has no supporting evidence, then they're allowed to use their brains and conclude that you're probably lying. $\endgroup$ May 16, 2023 at 20:49
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    $\begingroup$ @KennSebesta The chances of getting dinged for it by an FAA inspector are pretty microscopic. I'd be mostly worried about the insurance impact. My policy makes it clear, any violations of regs associated with a crash event means they may or may not walk. $\endgroup$
    – John K
    May 16, 2023 at 22:52
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    $\begingroup$ Would it change your answer if they physically disconnected the tank from the engine by locking a valve closed or removing and capping a section of pipe? Not saying it's practical but... $\endgroup$ May 17, 2023 at 14:37
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    $\begingroup$ @ScottishTapWater That would be an unapproved modification and therefore little different from the original scenario. If there was documentation that made the change legal, and designated the fuel tank in question some sort of "liquids storage tank" or some such, then you're fine. If you just go do it yourself, welcome to uninsured flying. $\endgroup$
    – John K
    May 17, 2023 at 15:59

Assume you do get busted at ramp check.
Assume you decide to take it to court.
Assume you win.

What do you think the FAA is going to do:

a. ignore your success at circumventing the spirit of the regulation
b. publish new regulations that explicitly forbid anyone from doing this

Use your common sense, so that the FAA doesn't have to legislate things that should be common sense.

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    $\begingroup$ c. withdraw your pilot license $\endgroup$
    – jwenting
    May 18, 2023 at 6:15
  • $\begingroup$ The FAA can't legislate anything they can only regulate within the legislation that governs the FAA.(Legislation is the domain of a legislature, ie congress) $\endgroup$
    – Max Power
    Aug 17, 2023 at 7:13
  • $\begingroup$ @jwenting Do you mean certificate? Licenses are about fee collection and limiting participants. Certificates are evidence of some trait or quality. $\endgroup$
    – Max Power
    Aug 17, 2023 at 7:17

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