Dave_Phillips wrote:
PS. The tanks are bladders.
Ah…Now that offers an alternative mechanism! I can imagine the bladder contracting and emptying its contents due to the lower pressure at the filler…
AnthonyQ wrote:
Pressurizing the tank will not cause (significant volumes of) fuel to flow out the filler unless the tank is full…and being fed from another tank…
But that is was I suggested, pressurisation can cause significant volumes of fuel to flow out, as you state!
Apparently there is vented cap available for the right handside C-172 because the transfer line can get blocked as well…
Ted wrote:
But that is was I suggested, pressurisation can cause significant volumes of fuel to flow out, as you state!
OK…once more…Apparently no one is that interested in the facts here so at the risk of boring everyone I will say it again: fuel will only be over-boarded if the reduced pressure in the uncapped tank causes fuel to be drawn from another tank OR as Dave Phillips pointed out, a bladder is disbonded and allowed to scrunch up… pressurization by an air vent line cannot on its own force (significant quantities of) fuel out of a tank via the filler cap…
Apparently no one is that interested in the facts here
That’s a bit of a sweeping statement because that’s exactly what I am looking for, and I too don’t believe any of the explanations given so far are what they appear to be on the face of it. I believe them as given literally but the physics is not obvious to me.
For example one small thing: if you take off with a full tank and climb, at say a 10 degree pitch up, you are going to chuck many litres of fuel straight out of the tank, and that would happen just the same if you tipped the plane up on the ground. Even a leaky filler cap seal will result in a big leak. But there is no magic there; no pressure effect, etc.
What would be the mechanism for squeezing a rubber tank liner? One must assume the cavity is very well vented because that would be such an obvious failure mechanism.
Researching things in GA is hard for the same reason that most kinds of “social research” are hard: you need to do a lot of legwork to dig out the relevant factors, and most people don’t do that. And you rarely get the whole story. For example, almost all planes (C152 all the way to a 787) cruise at about 3 degrees UP pitch and that alone (as I say above) will dump many litres. Couple that with other likely factors e.g.
and you end up with loads of stories of “fuel p1ssing out”. There will be loads coming out during the climb and somewhat less during cruise, for possibly quite a long time.
@pilot_dar might know about this.
AnthonyQ wrote:
Apparently no one is that interested in the facts
On the contrary, are interested!
and if you read my post regarding pressurisation of the tanks, I suggested it might apply when the caps are ON, in certain circumstances.
Peter wrote:
What would be the mechanism for squeezing a rubber tank liner? One must assume the cavity is very well vented because that would be such an obvious failure mechanism.
The same mechanism that squashes your plastic juice bottle on descent after you have emptied it at altitude and replaced the cap….i.e. ambient pressure….the pressure inside the wing is greater than the pressure at the filler cap.
And obviously the wing internally is well ventilated….otherwise it would be exposed to over-pressure on climb…
Peter wrote:
Researching things in GA is hard
How did they for example figure out a substantial qty was lost? did they operate full rich, and use the POH handbook to work out the consumption… 
Tilting the tank in the climb/cruise, in addition to the mechanical effects of keeping the fuel at the filler caps/standpipe would allow the effects of a pressure void within the tank to last longer. Plus the volume of fuel may be expanding as it warms up.
The point being small details may make the difference, and more than one mechanism may be a play.
The same mechanism that squashes your plastic juice bottle on descent after you have emptied it at altitude and replaced the cap….i.e. ambient pressure….the pressure inside the wing is greater than the pressure at the filler cap.
OK… what I don’t get is that to squeeze the tank liner, you need a net pressure across it, which (during a climb) would imply a lack of venting in the cavity outside the liner coupled with normal venting of the inside of the liner (the fuel tank). During a descent you need the opposite i.e. a lack of venting to the tank coupled with normal venting to the cavity outside the liner. I don’t know anything about liner tank construction but would expect the outside of the liner to be very well vented via multiple routes, which (assuming a blocked tank vent) should generate a spillage only during a descent, which is not what most people report – most reports are post-departure.
How did they for example figure out a substantial qty was lost? did they operate full rich, and use the POH handbook to work out the consumption
Indeed…
Peter wrote:
OK… what I don’t get is that to squeeze the tank liner, you need a net pressure across it, which (during a climb) would imply a lack of venting in the cavity outside the liner coupled with normal venting of the inside of the liner (the fuel tank). During a descent you need the opposite i.e. a lack of venting to the tank coupled with normal venting to the cavity outside the liner. I don’t know anything about liner tank construction but would expect the outside of the liner to be very well vented via multiple routes, which (assuming a blocked tank vent) should generate a spillage only during a descent, which is not what most people report – most reports are post-departure.
Bladder squeezing (due to the filler cap missing) has nothing to do with climbing or descending….it is simply the fact that the air pressure surrounding the bladder (inside the wing) is greater than the sir pressure at the opening (filler hole)…
AnthonyQ wrote:
Bladder squeezing (due to the filler cap missing) has nothing to do with climbing or descending….it is simply the fact that the air pressure surrounding the bladder (inside the wing) is greater than the sir pressure at the opening (filler hole)…
I don’t get this at all, perhaps I have misinterpreted it.
Would not the normal operation of the fuel vent system keep the pressure in the bladder at or ABOVE the pressure of it’s surrounding, so that the bladder does not collapse as you drain the fuel into the engine?
In the case of the PA-31 it would be interesting to get detailed information on the vent system, I presume that the vent is above the filler… My distant memory of the PA-31 is full mains is the norm.
