Fuel cap lost

[Nev]

Today I refueled my Bearhawk and replaced the fuel caps. On landing we realized that the left fuel cap was missing. We don't think it had been left at the fuel pump (but if it turns up at the local airfield I'll eat some humble pie and buy a round of beers).

The fuel cap had the O rings lubed with a thin coat of EZ lube. It had been difficult to remove even with the lube and I had problems reinstalling it correctly with the air vent facing forward. Because of this I had removed it again and then reinstalled it. It seemed secure, but may not have been as tight as the right one.

To elaborate further, we searched the (rather remote) landing area for over an hour, thinking it had most likely come off on touchdown. Later, we checked the fuel quantities and realized the left tank (with the cap missing) was showing full, and the right tank (with the cap in place) was showing 60 liters. The short flight had burnt 20 liters, meaning that we were missing an additional 20 liters approximately from the opposite tank. Probably the siphon action from the missing cap has pulled fuel through the fuel selector (in BOTH) to the left tank.

I'm struggling to understand what forces have caused this to happen. I wouldn't think that external forces would be sufficient to pull the fuel cap off. I wonder if some sort of hydraulic action internally has contributed. Any thoughts appreciated. Has anyone else had this happen ?

[Nev]

I've done a thread search and found Marks advice from this thread HERE :

Occasionally we hear this. If using some grease on the o rings with the o rings uncompressed - doesn't allow the gas caps to go in and out OK - then there are a couple things. Fuel Lube is usually NOT recommended for the gas caps because it is slippery that normal fitting gas caps can loosen and be lost on a flight. But if the caps are too tight after grease - this Fuel Lube can make them work sometimes.

Other option is to take .005 off the body of the gas cap where the o rings sit. Mark

This goes some way to answering my questions above. I've misunderstood the advice to use EZ turn and put a thin smear onto the O rings,

[Mark Goldberg]

Bob's technique is to smear a little engine oil on the o rings when he gets fuel. I put a little bearing grease into a little zip lock plastic bag that I keep in the plane - and put some grease on the orings every now and then. Not every time I flew. But perhaps every other time.

The EZ lube or Fuel Lube is very slippery stuff. There have been a few fuel caps lost when using it. But like I said - if the fuel caps are still too tight with grease or engine oil - EZ lube can be the best thing to use. Mark

[Richard E]

Nev.
I have taken to putting the last dribble of fuel out of the pump onto the O rings each time I refuel. Then turn the cap to just start squashing the O rings, and then back off about an eighth of a turn. Push it in and then tighten fully. The fuel makes it easy to push in without being too slippery, and it ends up tight and facing the right way each time. I haven't lost a cap yet ..............

Very interesting commentary on the hydraulics issue. It appears that the missing cap allows the tank to de-pressurize pulling fuel through to the opposite tank. A bit worrying that this would eventually empty both tanks! Maybe a good reason not to fly on BOTH.

[jaredyates]

I have had a similar instance and once I saw the uneven feed in the sight gauges, I switched the selector to the full tank and it worked great. I think there may be some low pressure on the open tank side, but there is also definitely positive pressure on the capped side due to the ram air vent. I'm not sure to what degree each factor contributes.

[Nev]

The local engineer said he saw an aircraft get airborne once with the fuel cap off, trailing on the chain. He said it looked like a geyser as the fuel shot out the filler neck.

[Nev]

Very interesting commentary on the hydraulics issue. It appears that the missing cap allows the tank to de-pressurize pulling fuel through to the opposite tank. A bit worrying that this would eventually empty both tanks! Maybe a good reason not to fly on BOTH.

In theory it probably would. I guess in practice if this happened you could then select either L or R tank prevent a further transfer.

If selecting the fullest tank (the one with the missing cap) I'd want to be flying high enough to cope with an engine failure as I suspect the fuel might be reluctant to flow down the fuel lines due to the large pressure differential.

When I thought back on yesterday's flight, I did remember doing a fuel check before landing and thinking "that's odd" when the sight gauges were uneven with the ball centered. In hindsight it was the first indication of an issue, but we landed shortly after that. I think if we'd been on a longer flight, the miss-match between sight gauges, and the miss-match between the total fuel (from the sight gauges) vs the fuel flow transducer, would have alerted us. If I see that again then I'll switch to one tank and prevent further transfer.

One thing I would definitely do differently before getting airborne, having landed out and needing to fly home with a fuel cap missing (thank god for gorilla tape) and a fuel imbalance, I would select BOTH and raise the full wing to allow the fuel to flow into the tank that had the cap (and vent) on.

[Utah-Jay]

Would it be a reasonable idea to carry a spare fuel cap in your tool kit?

[Nev]

Yes, I've now got a spare in the kit. I think a locking cap will be the solution for the longer term.

[Gerhard Rieger]

I had caps made for me and added the vent to the filler, because of the aux tanks made them 4 inches high to help not to spill fuel when parked

[Utah-Jay]

Yes, I've now got a spare in the kit. I think a locking cap will be the solution for the longer term.

Is the BH fuel cap system generic so we can use other brands?

[jaredyates]

I did, and needed one twice due to unrelated factors.

[robcaldwell]

I bought 2 extra caps from Mark that I keep onboard. Hasn't happened yet, but...

[Bcone1381]

Difficulty in removing a loose cap is one of the root issues in this event.

When I loosen my cap the "squeeze pressure" on the O-ring is relaxed, and pressure that holds it in place is released and it should easily come out. But when my caps are "released" it is still quite difficult to remove like Nev's. I dont like it.

I see myself dealing with this on a ladder and I like it even less. Comparison of the tolerances of my caps against the caps engineering diagram including the o-ring in on my todo list. If mine is right, then I've thought about experimenting with a different o-ring. I've often wish I had a different cap, but if I can fix it I will be happy and accommodative. If its always sticky to remove I'll never like it. Extra caps from Mark seems essential.

[mbaerobat]

I have had a few caps disappear in flight.
The method that seems to be working for me is the application of some fuel to the O rings when placing the caps in the neck and then tightening the cap tightly using a tool that I made.
Finger tight doesn’t seem to do the job.
The tool is made from UHMW plastic but my original was made from hardwood.
Haven’t lost a cap since. Still carry a spare cap just in case….

[Nev]

It does seem that the fuel caps are coming off with a higher frequency than I first imagined.

On mine, I've struggled with them on many occasions. It's probably "operator error", but the design doesn't really lead to intuitively getting it right. One compounding factor is that it's an awkward position - braced on the strut and reaching out with one hand to fit the caps. So far it's resulted in an inconvenience to us as the operator, but there could be a more serious incident.

I do wonder if we should be collectively working on a better solution.

[jaredyates]

It does seem that the fuel caps are coming off with a higher frequency than I first imagined.

On mine, I've struggled with them on many occasions. It's probably "operator error", but the design doesn't really lead to intuitively getting it right. One compounding factor is that it's an awkward position - braced on the strut and reaching out with one hand to fit the caps. So far it's resulted in an inconvenience to us as the operator, but there could be a more serious incident.

I do wonder if we should be collectively working on a better solution.

Where's Kevin? Guest

[robcaldwell]

Someone at Oshkosh last year was talking about redesigning the fuel caps. I haven't heard anything about that since, but it has been on my mind....