Fuel Venting from left reserve tank

GMPoland · #1 02-16-06, 07:59 PM

Seems my normally reliable skymaster has developed a leak that has me puzzled. After the last top up the FBO called to inform me that there was fuel leaking from my acft. closer inspection revealed that the left inboard was overfull and leaking from the vent. left main that had been topped of was down slightly. Draining from the reserve stoped the problem only slightly. Next day same thing. I drained the reserve and aprox 15 Gal from the mains and the leak has stoped. this is concerning me greatly as the fuel tank selectors were in the off position during all this. I have the parts and service manuals and I can only think that I have one or both fuel selector valves that have gone bad. My acft is a 337A SN 0127. I drained this fuel from the front and rear fuel gascolators. I dont think this this should be possible with the fuel valves in the off position. My local shop is great but has little experience with the Acft type. Any comment on this?

Ernie Martin · #2 02-16-06, 08:06 PM

I'm not sure I fully understand what you wrote, but I think the leak may well be due to a small crack in the upper part of one of the tanks. This is a common problem, and draining some fuel solves it because the fluid level is then below the crack.

Ernie

GMPoland · #3 02-16-06, 09:02 PM

Sorry for the lengthy description. Basically,with the fuel tank selector valves in the off position can fuel be drained in that amount from the gascolators? IF not then where would this fuel come from? The parts and sevice diagrams for this year are not too clear as far as the selector valves are concerned. My thoughts were that the fuel was draining through the bad selector valve into the lines and reserve tank thus overflowing until it either equalized or was empty. The comment about the cracked tank has me more concerned than the possibilty of bad valves. If it is a cracked tank what would be a viable repair or is replacent the only option. thanks

Ernie Martin · #4 02-16-06, 11:28 PM

I had a '69 337D with separate (inboard) aux tanks and I now have a '73 337G with the long-range tanks, but only one filler cap per wing. I don't know what the gascolators are. If it's the sump at the bottom of the boom, from where you drain fuel to check for water, then I believe that these sumps have fuel even if the tank selector valves are closed; in other words, the valves are downstream from the sumps.

Because these sumps are the lowest part of the wing, even if a leak is on the upper part of a tank, the fuel may appear to come from the sump area.

I believe that tank cracks can be repaired, but I'm not certain; my A&P is out of the country, so I can't ask him, but maybe someone on this board will tell us.

Ernie

scade · #5 02-17-06, 02:43 AM

I had a similar situation with my 1966 337A. After all tanks were topped-off, aircraft was parked for a day. Returned to find a significant amount of fuel was missing from the main tank and the aux tank was over flowing through the vent.

I ran the airplane a few minutes on the ground from the aux tank and exercised the fuel valve. The leaking stopped and so far has not returned. I don’t know for sure but think the fuel valve must have been slightly out of position possibly allowing fuel from the main tank (higher elevation) through the valve into the aux tank causing it to overflow.

Steve

general · #6 02-18-06, 01:28 PM

I have seen this allot with my pre 1973 337/336's. After the tanks are topped-off, aircraft was parked for a day or so. I would returned to find a significant amount of fuel was missing!!!! And then and find flowing through the vent, and not just by the change of temp.

I think the fuel valves might be out of position possibly or some thing is leaking most of the time I have found is that one wing is a few in's lower on the parking ramp.
I do know one thing check you fuel tank each time you fly I have tip them off the day before and found 25 gals or more less fuel "not good if you are flying over water like I do!!! DO YOUR PREFLIGHT LOOK into the Tanks!!!!!!

The General

GMPoland · #7 03-13-06, 09:39 PM

After an extensive look at the fuel system in my aircraft the problem was found. seems that during previous maintenace the plackard on the overhead fuel panel had been installed reversed. Thats right. BACKWARDS. This was an easy fix, However, realizing the potential for disaster during the previous flights over the past 15 months has made me very cautious about checking up after the mechanics have worked on my aircraft. And for all the replies, Thank You for your advice. the information was very helpfull.

hharney · #8 03-14-06, 07:18 PM

That could have been disasterous. I am glad nothing happened because of this mistake.

I am surprised that this could happen. I have a '68 C model and have taken off the placard before for cosmetic repair issues. I did not think that this could be done. As I am looking in the service manual right now I can see by the diagram that it actually looks like it could be reversed. But then you would be reading the words upside down or if it was flipped the words would be backwords.

The D model changed the shape so that it only fits one way.

CO_Skymaster · #9 03-18-10, 12:33 AM

My aircraft finished it's annual about 5 weeks ago, but I performed some fixes on it myself. Today I finished the fixes and filled the main tanks up. As soon as the line person filled up the tanks it started leaking fuel fast. Fuel came out of this small hole in the wing near the outside of the both booms (not the drain valves) and started flowing inside the booms leaking out of the seams and flowed all the way to the passenger windows on both sides and started flowing down there. One side finally stopped and after waiting 15 minutes, the other side slowed down considerable. I plan to go back tomorrow morning to see if it has stopped completely. My question is, when my aircraft overflows because fuel is topped off (1966 C337A), is it suppose to just flow out the vent or have other experienced it flowing all over the wing, boom, and fuselage. I'm guessing the mechanic didn't put something back together right, but I wanted to check with the forum first.

Karl

skymstr02 · #10 03-18-10, 06:24 AM

Before you want to blame the mechanic, was this a routine annual inspection? Was there any fuel tank maintenance performed, ie, fuel quantity probes/liquidometers, stress plates over the tanks removed? Its not normal for the tanks to leak if gaskets and hoses have not been disturbed.
Please provide some insight and additional information.

Dave

hharney · #11 03-18-10, 08:36 AM

As Dave suggested, more information is needed to determine what may have caused this to happen. NO, it is not normal for the fuel to migrate to the booms and inside the wings as you have described. The main tank(s) on each wing are two separate fuel cells connected by three couplers, two lower and one upper. The only other port on the fuel cell is the line that delivers fuel to the fuel sump in the boom where the quick drain is. Without knowing what maintenance was performed it is difficult to determine what may have caused this problem.

Need more info..........

Roger · #12 03-18-10, 09:53 AM

My 80-H was sitting for a month with minimal fuel in it last year and a few minutes after it was topped off all of the fuel started running out the two holes where the fuel supms stick through the bottom of the wing. If turned out to be the cork gaskets on top of the inboard wings(s) that are the tank inspection plates where the inboard fuel guage pickups are located. We sent for new cork gaskets, replaced, and the have both beeb perfeect ever since. Apparently as they age, if they are left to dry out (minimal fuel for an extended period of time) they start to leak.

And yes you can weld the fuel tanks. There is a guy in I believe Deland, FL that is an expert, who also does symaster refurbishemnt and conversions. If anybody needs the name and nuymber I can get it from my IA.

And lastly, this again points out why is is vital to have your fuel guages working, vs just a good fuel computer. Anything that leaks out is not being picked up by the computer.

hharney · #13 03-18-10, 06:22 PM

When I replaced my gaskets at the fuel sensor port I replaced them with a rubber type gasket. I am pretty sure I sourced them from Yingling in Wichita. I don't know if they are any better than the cork style but time will tell. I didn't have any leaking at the time but I was inspecting all the tanks so they got changed at that time.

I had to have one tank welded at the outlet fitting. It was welded by a certified welder in Kalamazoo.

Photo shows the original cork gasket and the other photo is the tank back in place with the new rubber type gasket.

Cloud9sa · #14 03-18-10, 06:27 PM

Yeah, we just had to replace the cork gaskets on our G model. I 'spose it was due to the cold and dry winter air. Tracy

CO_Skymaster · #15 03-18-10, 11:39 PM

Here is more information:

My aircraft went into annual in October 2009. It had empty aux tanks and about half main tanks. Along with a normal annual inspection I had my A&P working on many other items. One of which was my fuel gauges. The Aux gauges never worked and the main gauges were intermitted. As long as I was going to have the fuel gauges refurbished, I decide to have the entire instrument cluster checked and refurbished. The person working on my fuel gauges recommended that I have my fuel senders checked for functionality. After inspection, it turned out that all were either non functional or had some problems. I had all the fuel senders refurbished also. It was the end of January before the aircraft finally emerge with all the new repairs and upgrades. Performing the entire upgrades place me under financial strain and it has taken about 6 weeks to catch up. The aircraft was sitting out on the ramp during this time. Colorado has temperature highs of about 40s and lows in the teens around this time of year. Yesterday was the first opportunity I had to perform final post annual checks on the aircraft and fill up the main tanks. That's when all the fuel started pouring out from the wing airframe. I checked this morning and it finally stopped. I suspect a few gallons on each side poured out of the tanks.

After thinking about it today, I'm suspecting the gaskets on the fuel senders didn't seal and the fuel flowed inboard and leaked out into the airframe until the fuel level became low enough it could not leak out anymore. I called the A&P this morning and they already have it in the hanger. Hope he has some information for tomorrow.

Karl