# How fast can fuel be siphoned out if you leave the cap off?

It is a dreaded scenario; forgetting to secure the fuel cap.

I understand the low pressure area generated above the wing will siphon fuel out of wing tanks, how fast would this happen in a smaller GA airplane with wing tanks (something like the Cherokee perhaps) if the fuel cap was to be completely forgotten (rather than just not tightened)? Also, would it be able to suck the tank completely dry or would there be some limit to the amount of fuel that might be lost this way?

I just had X-plane do this to me while doing some IFR practice and it reduced my fuel with about 2 gallons/minute per tank until they were both completely empty and I was wondering if this is realistic.

• In this situation, the tanks were completely empty resulting in a crash. Dec 29, 2014 at 22:56
• Digression but it might amuse, quoting an old song collected by Oscar Brand: "He filed her up with high octane / He left the gas cap off again / I'm flying now on dirt and rain / All honor to the Line Boy!" Dec 30, 2014 at 17:12

It can happen very quickly.

I don't have exact numbers, but an acquaintance had this happen in a Cessna 172 and the entire tank was emptied in just a few minutes - the amount of time it took to do one circuit of the pattern and land. The fuel cap was not in the filler neck on departure, and was hanging from the chain, banging against the wing. The moral of that story is: never assume the fuelers put the cap back in.

I've personally flown an older Cherokee 160 that had a loose fuel tank gasket. When doing slow flight, the pressure above the wing was so strong that it would start to ooze fuel from the cap. Fuel would flow in a rather frightening amount back along the wing, and that's with the cap still on.

• The specific rate of fuel loss would vary based on a number of factors (angle of attack & speed being the big ones) - The 2gal/minute number above sounds reasonable for a missing cap in cruise. Dec 30, 2014 at 0:22
• Don't aircraft have some reserve fuel tank, separate from the primary tank, in case something like this happens Dec 30, 2014 at 7:21
• @Firee That sounds like a potential new question :) Dec 30, 2014 at 13:26
• As much as I don't like climbing up to the wing, I always dip the tanks after the filler's done to see for myself how much fuel I actually have before taking off. Obviously I'd detect an open fuel cap at this point. That might be a little paranoid, but I think it's better than unscheduled glider practice. Apr 9, 2015 at 1:32

I was once passenger/safety-pilot in a Cessna 150 where the gasket between the tank and the wing skin had failed. Looked over my shoulder before the first turn onto crosswind and discovered that the rear windshield was utterly covered in fuel - easily a couple of litres a minute. And that was with the cap firmly in place (the first thing we checked after shutdown!).

It was a long time ago, but I think the engineers said it had been sucked out through the flap tracks and we probably had a wingful of fuel vapour. Nice...

• I've actually had this happen to me too. It started spewing out fuel if I did an aggressive slip towards the wing with the faulty seal, not while coordinated though (slow or otherwise). It was a low wing aircraft, with the caps in plain view, so at least I knew it had stopped so I didn't declare an emergency (and I knew I had enough fuel in the other tank). Needless to say, I rather abruptly turned towards the closest airport for a precautionary landing. Sep 15, 2015 at 8:56

A friend of mine once flew about 2.5 hours with a friend in a 1975 172M after topping-off and forgetting to cap one of the tanks. The flight included some IFR practice maneuvers- turns, constant-speed climbs and descents, etc.

Upon landing and refueling, he discovered, in horror, the uncapped tank. He was surprised to find that he only needed to add about one gallon more than his expected fuel burn should have used up, so if it was siphoning out, it was only a trickle.

The chained cap must have been banging against the wing, though, because it left some marks.

Fuel starvation is obviously a critical situation, and I'm sure it's not the same in all aircraft, but the 172 did okay with one missing cap.