O&N Bladder Mystery

[Cloudmirth]

So here is where I call on your collective wisdom to solve a mystery.  My '69 C has O&N bladders.  Several times over the past year after returning from a flight I have found streaks of blue fuel on the top of the right wing directly behind the tank location, about 3 inches forward of the flap and stretching spanwise across a distance of a couple of feet.  There has never been any evidence of fuel leakage on the underside of the aircraft nor is there any indication of fuel leakage in the cabin.  A conversation with Mike at Griggs left him at a loss to explain this situation.  As he stated, "fuel does not leak up".  So, the question is how is the fuel getting there?  Obviously where the stains are being found is well aft of the actual bladder location and lacking any evidence of tank leakage, the question is simply how is it getting there.  Is Bernoulli's principle somehow involved.  Note also that we have never seen any apparent fuel package past the fuel caps.  OK, I'm at a loss so over to you!

[DonMuncy]

Yes, fuel can leak up. The lower air pressure on the top of the wing can syphon gas up and out. I do not have bladders and don't know about the filler neck seal, but that is where I would start.

[takair]

Are they in line with the cap or inboard?   Is your fuel cap flange assembly above or below the skin of the plane?  Is it possible that where the flange attaches to the tank is leaking and the fuel puddles on top of the bladder….to be drawn out by low pressure above the wing during flight?   Do you have pictures?  Sometimes helps with visualization…

[Ragsf15e]

I don’t have bladders but I’ve had at least two cases of fuel leaking out the top through screws.  It can happen.  If it’s behind the bladders, is there an inspection plate or two you can pull to look around?  Im sure there are more blue stains inside the wing that could help you track it down.

[Marauder]

Here is a picture of what Rob (takair) is talking about. It might help visualize what may be going on. He is speaking about the fuel cap seal.

In between the top of the rubber bladder and the wing skin, there is a cork gasket. Between the top of the wing and the fuel cap assembly is a layer of fuel resistant sealant. If the bladder cork seal is compromised, I could see fuel being released on top of the bladder and leaking backward to the area you are describing.

Pictures of what you are seeing would help.


Sent from my iPad using Tapatalk Pro

[David Lloyd]

Speaking of bladder mysteries, my C has the 27.4 gallon bladders that go in the same place that formerly held 26 gallons.  How is that possible?  Is one of the bladders in a previously unused part of the wing?  Have wondered for a while but haven't ever seen an explanation.

[Cloudmirth]

All of the comments to this question are interesting and perhaps even plausible.  The question that remains in my mind, however, is that to the best of my knowledge when the bladders were installed none of the old tank sealant was removed.  Therefore, any fuel which may have leaked from the bladders should probably remain in the confines of the old tank space or leak out of the bottom thus leaving stains on the bottom of the wing.  However, what we are seeing are fuel streaks well aft of the fuel tank area far removed from the original containment space.  I would agree that if fuel had somehow migrated from the original tank space to wing bays at the aft edge of the wing then negative pressure in that area could pull fuel up through the seams.  Seems (no pun intended) a little unlikely to me.  I like the idea of fuel possibly leaking from between the bladder and the wing skin at the filler cap but it also would seem that if fuel was leaking from that source there would be streaks on the wing surface leading from the filler cap area back to the aft portion of the wing.  In this case there are no such streaks.  The evidence of fuel we have seen is at the very aft edge of the wing just before the flap and extending perhaps 3 feet in length from about a foot out from the fuselage.  So while all of the comments so far have merit and are much appreciated, I believe I will apply the principle of occams razor and start with the simplest proposal first, i.e. look closely at the filler cap area.  Nonetheless, keeps those cards and letters coming in folks as I am always open to new suggestions!

[takair]

1 hour ago, Cloudmirth said:

All of the comments to this question are interesting and perhaps even plausible.  The question that remains in my mind, however, is that to the best of my knowledge when the bladders were installed none of the old tank sealant was removed.  Therefore, any fuel which may have leaked from the bladders should probably remain in the confines of the old tank space or leak out of the bottom thus leaving stains on the bottom of the wing.  However, what we are seeing are fuel streaks well aft of the fuel tank area far removed from the original containment space.  I would agree that if fuel had somehow migrated from the original tank space to wing bays at the aft edge of the wing then negative pressure in that area could pull fuel up through the seams.  Seems (no pun intended) a little unlikely to me.  I like the idea of fuel possibly leaking from between the bladder and the wing skin at the filler cap but it also would seem that if fuel was leaking from that source there would be streaks on the wing surface leading from the filler cap area back to the aft portion of the wing.  In this case there are no such streaks.  The evidence of fuel we have seen is at the very aft edge of the wing just before the flap and extending perhaps 3 feet in length from about a foot out from the fuselage.  So while all of the comments so far have merit and are much appreciated, I believe I will apply the principle of occams razor and start with the simplest proposal first, i.e. look closely at the filler cap area.  Nonetheless, keeps those cards and letters coming in folks as I am always open to new suggestions!

Keep in mind that the outboard bladder is in a section of wing that was Not previously sealed.  The bladders extend into an additional bay…or two…

[Marauder]

Keep in mind that the outboard bladder is in a section of wing that was Not previously sealed.  The bladders extend into an additional bay…or two…

And without that sealant in place, who knows where fuel can seep.


Sent from my iPhone using Tapatalk Pro

[Cloudmirth]

For those of you who may have followed this thread (or not) an update.  After a considerable amount of head scratching and investigation my friendly local IA had something of a brainstorm.  He asked me if the fuel caps (Shaw Aero) were vented.  I replied that I really did not know but I suspected they were not.  He then suggested that if the caps were not vented and the tank vent was plugged (I have O&N bladders) then perhaps as fuel was consumed the bladder was compressing forcing fuel out somewhere.  While there did seem to be a certain logic to his suggestion I was dubious.  Nonetheless, we checked the tank vent and indeed, a mud dauber or some such insect had set up housekeeping in the vent.  After clearing vent of debris, the problem has not reappeared through a number of tank fills.  So there we are, take it for what it's worth.

[1964-M20E]

Glad that didn't lead to fuel starvation.

[A64Pilot]

Either way thanks for posting the solution, I have bladders too and follow any bladder threads 

[carusoam]

Yowsers!

Great follow-up!

 

Our fuel caps do not get vented…. Until the rubber orings fall apart…. Or somebody mis-adjusts the cap…

 

I’ve had the mud Daubers do their thing…

I didn’t figure out the hints they left for me…

 

1) Mud daubers are really messy near the nest they building…. Be sure to check the vent….

2) Temperature is always changing…. And fuel is always evaporating….until pressure builds in the tank….

3) If your cap seals are in good health…. The small pressure release is barely noticeable…. When you notice it… you should know it isn’t right….

4) The third sign is the oil canning of the nicely curved surface of the top of the wing…. It’s not very long into the flight before the fuel pump will start changing the geometry of the fuel tank….

Oddly, The fuel pump is a pretty powerful device…. 

Double oddly, There isn’t a way to relieve the vacuum while flying… unless you can access the rubber fuel line connector behind the wall panel, while you fly…

 

For the future… if Mud Daubers continue to be a problem… there are methods to keep them out of the tank’s vents….

There are a few spring discussions for Mud Daubers and other pests….

PP thoughts only,

-a-