Ted wrote:
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?
It would help but I guess the vent cannot supply enough air through say a 3/16" or 1/4" tube to offset the reduced pressure of the large diameter of the open filler hole…
AnthonyQ wrote:
It would help but I guess the vent cannot supply enough air through say a 3/16" or 1/4" tube to offset the reduced pressure of the large diameter of the open filler hole…
OK but how does that mechanism work when the caps are on as in the example provided by dave_phillips?
Ted wrote:
I am saying that low pressure above the wing, is not the cause! A reduction of pressure by itself does not make sense.
I am saying it is….
The PA31 has the same caps as Beechcraft and Mooney etc and have a seal on the central shaft which is prone to deterioration and leaking… I don’t know whether Dave’s observations were with or without caps but it doesn’t change the basic fact that lower air pressure over filler can cause some level of reduced pressure inside the tank in question…if there is another tank connected it can cause crossfeed….or if the tank is a bladder it can be squeezed….. these two mechanisms are well known….
Of course there may be other causes of fuel overboarding such as structural leaks, or over pressures due to blocked vents during climb, or inadvertently cross feeding to overfull….etc… but the original topic here was a discussion about fuel loss due to leaking or missing fuel caps…
AnthonyQ wrote:
I am saying that low pressure above the wing, is not the cause! A reduction of pressure by itself does not make sense.I am saying it is….
With respect I think you have misinterpreted what I said.
I said “A reduction of pressure BY ITSELF does not make sense”, i.e. there needs to be some other mechanism at play, I gave two possible mechanisms and you have provided at least another… Specifically that comment related to C-172 example, perhaps the real reason for the fuel loss was fuel consumption was greater than what was estimated 
Time for a break.
Timothy wrote:
It’s a full emergency, as far as I am concerned.
REally ????
Like “MAYDAY, MAYDAY WE HAVE AN EMERGENCY, I FORGOT TO TIGHTEN THE FUEL CAP, NEED TO LAND NOW !!!!!”
Seriously, a missing or loose fuel cap is NOT an EMERGENCY, “full” or even partial for that matter .
There is a difference between treating something as an emergency and making a fuss… :-)
If you want to be technical – given nobody is about to die, a PAN-PAN may be more appropriate than MAYDAY,
But I don’t think Timothy was writing about R/T phraseology – but about the mindset.
In an emergency, you decide you have to do something – in this case: land – urgently. If a flammable fluid that I need to keep flying is pouring out of my wing, I consider the appropriate action to be “land as soon as possible”; not “as soon as practicable” and certainly not “as soon as convenient”, which would be the destination…; the only thing it is NOT is land NOW (which is the case when you are on fire).
And I certainly won’t use my time trying to figure out if the tank is going to be emptied or not – I am with Peter that in a sealed-metal-box type tank I will lose a few gallons, but not everything, and it is one tank only anyway – but I will land the basis that no sane person would take off with a fuel cap missing, so I won’t continue to fly without it.
Would I use the M- or P- word? When this happened to me (where is the embarrassed smiley when you need it?) the tower actually told me that fuel was streaming from my wing, and I simply asked for a circuit back to land, so given they knew was going on and I got what I needed, I didn’t use any emergency phraseology. But if they had sent me to fly holds for half an hour while they had lunch, or if I had to break into a conversation to get attention, I certainly would have.
To clarify a couple of PA31 specifics:
The fuel vents are indeed ‘above’ the tanks. Each tank has it’s own dedicated vent.
There is no link between outer and inner tanks – there is a simple Outer/inner/off fuel valve in the cockpit. Crossfeed plumbing is in the port wing but should not have a bearing the phenomenon.
PA31 inners are normally filled first.(ie, I cant imagine an aircraft that was partially fuelled across tanks with the inners not being full). However, due to fuel flow (approx 1.2l/min for each tank) and the dihedral, there is a noticeable gap between the fuel Cleveland the filler cap pretty soon after engine start.I would certainly not be able toes the fuel level through filler cap after about 20 minutes of flight.
I’ve seen whacky fuel venting scenarios before. One of the most bizarre was a Chipmunk which had recently been painted.There is a pinhole vent connected to the main vent underneath the aircraft.In this scenario, one of the pinholes had been blocked by paint. The problem became apparent when an entire tank of fuel syphoned from the main vent underneath the aircraft, overnight in the hangar.
Cobalt wrote:
Would I use the M- or P- word? When this happened to me (where is the embarrassed smiley when you need it?) the tower actually told me that fuel was streaming from my wing, and I simply asked for a circuit back to land, so given they knew was going on and I got what I needed, I didn’t use any emergency phraseology. But if they had sent me to fly holds for half an hour while they had lunch, or if I had to break into a conversation to get attention, I certainly would have.
Agreed, but this is far from a “Full Emergency” .
I would classify it as a “issue” similar say to an incorrectly latched door (Lancairs excepted, where a canopy issue is indeed an all out Emergency!) .
Use of E, M or P words IMHO, would be over doing it by a wide margin.
Dave_Phillips wrote:
I’ve seen whacky fuel venting scenarios before. One of the most bizarre was a Chipmunk which had recently been painted.There is a pinhole vent connected to the main vent underneath the aircraft.In this scenario, one of the pinholes had been blocked by paint. The problem became apparent when an entire tank of fuel syphoned from the main vent underneath the aircraft, overnight in the hangar.
Yes, I can see how that could happen with a Chippy….the pinhole (at a level above the fuel level) serves to break the siphon effect…the tanks must have been pressed full and one of the two NRVs must also have been passing to allow cross-feed…one tank would have completely emptied via the opposite tank’s vent…the siphon would have finally broken when the tank feeding was finally empty!
To add to the above, the pinhole vent is on top of the wing just behind the filler cap….it is only a pinhole so that the lower pressure on top of the wing does not overpower the main vent’s role in ensuring ambient or greater pressure inside the tank….
Michael wrote:
this is far from a “Full Emergency”
If you are happy to have fuel streaming out within a metre of 1600°F exhaust pipes, then you are very, er, brave.
