Fuel Tank Leaks & Fuel quality

[nomadpete]

My GA-912 developed leaks in both fuel tanks. The most dramatic was three pinholes half way up the aft face of the tank. The largest hole was almost 3mm diameter. Was this a result of a chemical reaction from the fuel? Has anyone heard of similar problems with any aircraft? Has anyone heard of anyone installing fabricated alloy tanks to replace fiberglass ones? Anyone who knows Lightwings would appreciate how much work is involved in replacing the integral fuel tanks.

 

Safe flying,

 

Peter

 

 

[Guest Maj Millard]

Pete, My wing tanks in the GR are Aluminum, are yours in the GA fiberglass ? I have flown GAs but don't recall what the tanks were. I recently had my right hand tank out, to replace the filler assembly, which is another nice job, as the cap had cross-threaded and jammed solid about half way out. Whilst I had the tank out, I noticed a few small leak areas, also on the rear face, and there had always been a fuel smell in the cabin, that I couldn't pin down. The small leaks seemed to be the result of shoddy welding on the tank itself, almost like the apprentice had done the welding that day, it was that bad !. Otherwise thay are a pretty good design, with internal baffels and dual outlets, to allow for pick up in either R or L banked positions.

 

As you know it's a great chance to remove all brass fittings and reseal everything, and renew all the fuel lines etc.( especially the small overhead plastic one). I had my welder touch up some of the suspect welds when he put in the new filler cap (available from your local marine shop) and then prior to reinstalling the tank, I cleaned it real good and externally sealed ALL the welds with PRC fuel tank sealant. This is an aviation grade product, it's expensive but it does the job inside or out. No leaks from that side since, and the smell has gone. I was recently in the LW factory talking to Terry the main fuselage welder, and I forgot to ask him about the fuel tanks !!# I doubt if he had anything to do with the tanks, it's just not his style, and they are probabily done outside the factory, or as I say the apprentice.

 

I still would prefer to have welded alum tanks over anything else, best in a crash and the way they are messing with fuel I don't like the idea of plastic, no matter how good it might be. And I would recommend you get a small container of marine lanolin grease, to put on the filler-cap thread so it won't grow and bind up. I have no concerns with the welding on the LW in any areas other than the tanks. So far my left one is doing ok. Cheers Ross.

 

 

[nomadpete]

Maj,

 

That's curious, I was not aware that ALW used alloy tanks. I assumed that since mine are glassed, then they all would be. The construction is a moulded 'handbasin', and that is pop rivetted (and glued) under the top skin of the wing. So the tank sits between two alloy skins of the wing root torque box. I still have not figured how those twin outlets work - the way I see it, as soon as one pickup sucks air, then the other one cannot suck fuel? I do get an air lock sometimes and that stops one tank from flowing until I shut off the other long enough to suck the airlock through. The whole fuel flow process seems to be more complex than it first appears. Many other a/c use a small header tank to make up for the occasional gulp of air.

 

Peter

 

 

[Guest Maj Millard]

Well Pete, were even, because I didn't realise the GA had glass tanks !. The GA wing is totally different to the GR...more like a Saphire really. I think Howie only built around 14 or so. They have a quicker cruise, still climb like a home sick angle, but can be a bit tricky to land at times, if you let the speed get too slow.(below 45 or so.) I have the same flaps on my GR wing (custom order), except I don't have the reflex position, which is good for a free 6 kts at cruise in the GA. The dual outlets may not be on both tanks, and may not ever be on the GA ?. I know they're on the right tank, as thats the one I had out. It appears there are two separate fuel lines that go down to a 2 in 1 underneath the throttle at the fuel tap. I have never had any problem with sucking air on mine, or any other LW for that matter. One tank will feed more than the other, which can be annoying, but not a biggie.....The best fuel sump set up that I have seen is on the Savannah, totally safe. They have a 10 ltr tubular upright tank behind the cabin that all fuel goes through. It has a light that comes on when the level drops below 10 lts, and then you have 30 mins of fuel left. I would gladly put it on the LW if I could..........Cheers Ross

 

 

[Ross]

In relation to air locks in fuel lines the smaller diameter fuel lines are more likely to carry air bubbles along the line rather than allowing an air bubble to rise back to the tank in a sloping or vertical line due to the bigger effect of the meniscus in a smaller bore hose or pipe.

 

You can see this effect in clear tubes by standing a few short lengths of them on end each of different sized diameters in a dish of fuel.

 

The fuel will rise up the smallest diameter tube the longest distance - this will be the tube size that gives the most problems with air locks.

 

The bad effects of friction and meniscus rapidly reduce once you get to about 1/4 inch diameter tubing or larger with petrol.

 

 

[nomadpete]

Thanks Ross for the comment on surface tension, but the size of the fuel line should not be a problem. It is 5/16 I think. However there is a level section of the pathway up near the tank and I assumed that was the cause. Anyway, it was not a great problem, if I noticed that one tank was going down (standard crosscountry check), and the other one staying full, it was a simple matter to turn off one for a little while. All part of inflight fuel managenent.

 

Still trying to figure out where to put a header tank. Even a small one would make a difference. ALW did caution me not to mess with the fuel system.

 

Biggest issue though is about the really serious hazard of new fuel chemistry eating away at our fuel tanks or other parts of our fuel systems. Check out this site for some analysis. I know it is a boat site and that might be heresey to mention in these forums, but they did some real analysis on so called 'fuel resistant' epoxy. And that is the product that we are hoping will keep our composite aircraft in the air.

 

Go to website for full page BoatUS.com - Seaworthy Magazine

 

 

[Guest Maj Millard]

I would agree with Lightwing that all fuel systems should be kept as simple as posible. Minimum going on, minimum problems. The standard LW gravity-feed system works very well as it is simple......................................................................................................

 

 

[nomadpete]

Yes, sadly these tanks are an integral part of the wings. They sit between the top skin and the bottom skin and you couldn't slide a tank in from the wing root due to the root rib.... pic shows tank with top skin removed. My only option is to build alloy tanks and rivet them to the top skin just like the old glass ones.

 

 

 

 

[Ross]

A local friend is building a Sonex kit which comes with a very nice looking fuel tank;

 

some kind of plastic fuel tank moulded into a complex shape which looks very rugged, not fibre-glass.

 

I have no information on it but it looks very good - the material might be worth researching.

 

Regards

 

 

[nomadpete]

Thanks Ross. I will look the Sonex up. God bless Google.

 

Peter

 

 

[Guest nunans]

 

Gday again Peter,

 

I found picture of my tank in my lightwing.

 

The aircraft was stripped, inspected, frame repainted and aircraft recovered five years ago (by the previous owner) so i'm all good for the airframe inspection.

 

However after all that work was done the tank still leaked and the owner must have just given up and let it leak for the past 600hrs....

 

I'd love to be able to fix it without cutting the covering and having to get somebody to recover the wing (matching paint color, trim color etc) but i'm doubtfull that I will be able to fix it witout getting it out.

 

Mine looks like fibreglass tanks (GR912 by the way) serial 93

 

It's dissapointing, out of the three lightwings i've been in, they all either have had or did currently have leaking wing tanks, and that doesn't include yours and Maj's so actually Every lightwing I know of has crappy fuel tanks both aluminium and glass.

 

 

[nomadpete]

OK. Thanks for the pic. I know of one local GR582 which has a leaky tank, too - they are a removable type in the leading edge of the wing. Yours appears to be exactly the same tank arrangement as my GA-912.

 

Several points:

 

1. Your wing seems to have a alloy skin on the top that goes past from the root rib, to at least the second rib (mine only has a top skin over the fuel tank - from the root rib to the first rib in.

 

2. Your wing does not have a metal skin on the bottom of the wing. Mine does.

 

But then that must be a difference between GR wing vs GA ?

 

I noted that apart from the pinholes that had developed in the sides of my tanks, they both had evidence of attempted repairs to the fuel drain fittings and some of the points where the fuel tubes entered the tank. I would suspect that there was never any semkit around the interface between the alloy tube and the f'glass tank. Nor around the fuel drain threaded fitting embedded in the bottom of the tank. Bound to eventually let go and weep. The insides of the tanks also had signs of resin lifting where things had been glassed in, and disturbingly there were flakes drifting around loose in the bottom of the tanks.

 

The only way to fix it properly is to take the offending tank out. Make sure you do a liquid pressure test overnight before putting it all back together.

 

 

[rankamateur]

Stick some Knead-it on it, it will last for years!

 

 

[aj_richo]

Re Sonex tank.. rotationally molded polyethelene

 

Its about 1/8 -3/16" thick, pretty tuff stuff... but one has to be careful with the moulded in tank fittings, if they are overtorqued they will leak (pulls the fitting out of the plastic)

 

http://www.sonexaircraft.com/kits/fueltank.html