There should be an airflow from the fuel tank vent to the opening on the top. The opening on the top has low pressure, the fuel tank vent should have static pressure. That airflow could be rather violent.
I agree Timothy but I question the magnitudes.
I don’t know how to calculate it, but I feel that you would need a massive pressure drop above the wing to turn the liquid into vapour (to boil it, basically). Many years ago I used to work in a firm which had a vacuum chamber (for degassing encapsulants for high voltage assemblies) and for a laugh we used to put cups of tea etc in there. They would boil eventually but you had to drop the pressure massively. But that was water… avgas will boil more easily.
If the pressure differential across the upper wing surface (the upper skin) was more that really miniscule, it would rip the skin right off. Sea level pressure is 15psi and I reckon 10% of this would rip the skin off. And if that differential was equalised (by venting the fuel tank to the upper surface pressure) then the whole tank would be boiling in flight and, apart from any other issues, float based fuel level sensors would not work because, ahem, nothing would float in the stuff (that’s how oil rigs sink if a gas bubble comes up from down below).
Hence I reckon that people who saw fuel pouring out of the open filler hole had full tanks, and then you get different mechanisms in play.
There should be an airflow from the fuel tank vent to the opening on the top. The opening on the top has low pressure, the fuel tank vent should have static pressure. That airflow could be rather violent.
My tanks vent to the underside wing surface.
Yes there will be an airflow from the tank vent to the (missing) filler cap hole, but that airflow can still carry away only vapour.
It’s like the engine crankcase breather. There is a net airflow from the piston ring gaps to the breather, and this carries away the oil. The other thing which carries away oil is the oscillation in the crankcase pressure, but that isn’t present in the fuel tank.
Peter wrote:
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?
Yes it is possible. Been there, done that in a BE33 where the filler cap had not been correctly positioned so it did not seal properly. It looks like a CAT jettisoning fuel. Not sure how much fuel vented overboard during the 10 minutes I was airborn, but it looked as if it was not insignificant. There was a very visible mist of fuel trailing.
Absolutely possible.
I´ve had it happen on a Cessna 150 when the fuel cap was not put properly. I lost a good quantity of fuel there.
I´ve actually seen a fuel leak on one of the wing access panels on a Bac 111 years ago. Good old Danair. When I told the crew they shrugged and said something to the effect of that they all leaked and it was nothing new to them…
That’s why the vent is on the underside of the wing….I just met a guy a couple weeks ago flying a high wing ULM in France which just had a plastic tube from the top of the filler cap running down through the wing to the underside…unfortunately the tube was cracked at the filler and drained one tank….he was fortunate enough to switch tanks and restart….
what_next wrote:
Under the fuel in a tank there is no air but solid aluminium.
I am fairly sure that I have seen this happen in both bladders and solid tanks, eg PA23 and PA28. But my memory may be playing me tricks.
I agree with Aviathor that is can be quite dramatic, which is why I would always land as soon as the fire trucks can meet me.
I’ve had it with a poorly seated fuel cap on a PA31. For those who are not familiar the PA31 fuel cap, it would be difficult to spot that it was not seated correctly. We lost about 5 USG of fuel in 15 minutes of flying.
Dave_Phillips wrote:
PA31 fuel cap, it would be difficult to spot that it was not seated correctly
Oh, for sure! 
Yupp, happened to me once in a C206 and that tank was not close to full. Good thing I had some Skydivers in the back to spot it. I do not know how much I lost.
My guess, that the low pressure has nothing to with the fuel being sucked out. If anything a lower pressure should make it harder for the fuel to vaporise. The physics don’t make sense.
What I suspect really happens is a large volume of air is pushed into, around and out of the tank, this pushes the fuel out. The vents would further help. I could see how this airflow could shift quite a bit of fuel, a 50knot breeze on the ocean sends a lot of spray into the air.
So I suspect just as much fuel would come out if the caps where on the bottom of the wing
In addition when the tanks are full, slight pressurisation from the vent or fuel being returned, might also push it out, which is what I suspect happens with poorly fitted caps.
