I have just read an account of a flight in some high wing plane (Cessna, IIRC) which suffered large and mysterious fuel losses enroute. The eventual conclusion was a bad seal on the filler cap. It was speculated that the fuel was getting sucked out. Being high wing, it would not have been visible from the cockpit.
Is this possible?
I don’t know about the cap in place, but 8 years ago I forgot one filler cap after refueling (my Warrior) and when I realized it after takeoff I saw fuel streaming over the wing. I came back (the fuel cap was on the runway …) and there were about 3 gallons of fuel missing.
One of the most stupd things I have done in flying.
Peter wrote:
Is this possible?
Of course, aerodynamics mean low pressure on the wing surface. A common problem. If you forget your fuel cap or the cap is not tight, you lose your fuel.
I have seen this a number of times on low wing aircraft, so must be possible on high wing.
It’s a full emergency, as far as I am concerned.
Of course, aerodynamics mean low pressure on the wing surface. A common problem. If you forget your fuel cap or the cap is not tight, you lose your fuel.
What is the physics? The fuel is liquid, so the vapour pressure above the liquid will be according to the type of liquid and the temperature. At a guess, the density of the vapour will be of the order of 1/100 to 1/1000 of the density of the liquid. How fast would this vapour need to be removed to lose many litres of the liquid?
This has two stabs at the answer; one is 1/15000 and the other is 1/200 
It doesn’t add up to me.
What I can see is that if you depart with a missing filler cap and a completely full tank, then any sloshing of the fuel will tip the stuff straight out. But once there is enough room in the tank to contain any sloshing (due to e.g. out of balance flight) that loss should stop.
Take a cup of Tea, drive to the next Motorway (Autobahn would be better) and hold it out the window. Cheap tea is fine.
Surely the physics is very simple.
The fuel is at ambient pressure, the air above the wing is at lower pressure.
Fluid will flow to the lower pressure, minus gravity.
The lower air pressure on the top of the wing is sufficient to lift the whole aircraft, that’s how aeroplanes work.
If that lower pressure is enough to lift a 600T A380, it’ll probably extract a few litres of Avgas 
Alexis wrote:
Take a cup of Tea, drive to the next Motorway (Autobahn would be better) and hold it out the window. Cheap tea is fine.
But that is obvious. The opening of the cup is very large and a vortex will form inside which will keep pushing liquid upwards. But in a tank with sufficient depth and a large surface and only a comparatively small hole at the highest point? I don’t see how this mechanism should work there.
I fully understand that a forgotten filler cap will lead to some fuel being sucked out as long as the fuel level is close to the opening. But once it has dropped to something like 10cm below the opening the outflow should stop.
And regarding Peters question (not a forgotten filler cup but a bad seal under the cup!) I cannot see how more than a few drops can come out. I have flown on enough planes with bad fuel filler seals – as could be concluded from the amount of water one had to drain from those tanks before each flight – and never experienced any unexplicable fuel loss.
Timothy wrote:
If that lower pressure is enough to lift a 600T A380
Yes. But it can lift the A380 because there is (higher pressure) air underneath it. Under the fuel in a tank there is no air but solid aluminium. So unless you get some air underneath your fuel you can suck as much as you like and nothing will happen.
One possible exception would be fuel bladders of course. They lie loosely inside the wing and are surrounded by ambient air. Low pressure will cause them to be squeezed, thereby pushing the fuel upwards.
Yes, i do not think that much fuel will be missing when the cap is on. Whenni forgot to put on the cap on my Warrior, the tank was full … so some was sucked out.
