Fuel Tank Leaks
From Bondline
Contents |
Introduction
Mechanical Difficulty: 
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On the AA-1x series there are only 4 possible leak points from the wing spar tanks. Each end of the spar tube has a machined aluminum plug with an O-ring in a groove around the outside of the plug. The outboard plug is secured in place with 4 AN3 bolts which have special washers known as "Stat-o-seals". The inboard plug is secured in place with either 1 or 2 AN3 bolts, depending on the year model. The same Stat-o-seals are used on the bolts at the inboard end. Deterioration of the O-rings or the rubber covered Stat-o-seals can result in leaks. At the inboard end of the spar tank a block of aluminum is bonded on the bottom of the spar and a hole through the spar and the bonded block allow for draining the sump. A short section of 1/4" rubber hose leads from the bonded block to a cast fitting and the drain valve. This hose can fail and leak the entire contents of the tank.
On the AA-5x series there are 4 access panels on the bottom of each wing that are sealed with a special "low adhesion" sealant such as FlameMaster CS3330B or Desoto/PRC PR1428B. There has been an AD on the sealant used by the factory, and it is normal to require periodic re-sealing of the access covers. The wing ribs have a hole that is slightly larger than the wing spar tube. This leaves a gap of 1/16"-3/32" between the rib and the spar tube that the factory filled with a fillet of sealant. After 30+ years that fillet of sealant may leak as the wing flexes and stretches the sealant. The perimeter of the tank along all the bonded joints has been known to leak in the event of an incomplete bonded section or in the case of the purple glue on the late 74 through early 76 models, at areas of debonding. The fuel level sender on the inboard rib of the tank has a rubber gasket that deteriorates with age and can leak. The tank vent tube comes out both ends of the tank and can leak at the inboard end where some models have a section of rubber hose that connects to the next length of aluminum tubing. The AA-5x series can have tank leaks at the access covers, around the inboard and outboard spar to rib joint, at the fuel level sender gasket, the fuel pickup fittings and at bonded joints that are incomplete or failing. It's no wonder that the AA-5x have many more fuel leak problems than the AA-1x.
Find the source
The first step is to identify the true source of the leak(s). On AA-1x, this is straight forward, as there are fewer leak points, and they tend not to be confused with each other. Leaks at the outboard end of the tank are due to the large o-ring around the outer plug that also contains the filler neck, or around the Stat-o-seals. Leaks at the inboard end that drip at the center of the fuselage are from the large o-ring at the inboard plug, leaks inside the wing are either Stat-o-seal leaks or sump drain hose leaks.
The inboard plug o-ring leaks require pulling the wing panel. All the others can be repaired with the wing on the plane. Late model AA-1B and all the AA-1C have access panels on the bottom of the wing in the area of the drain hose. These can be added to early models.
AA-5x wing tanks are more difficult to identify the true source of the leak(s). Many would be tank sealers have been fooled by the location of the leaks, and have diligently scraped and cleaned the old sealant out of the tank in the area they THOUGHT the leak was located in, and then re-sealed with the proper high adhesion sealant, only to find that the exact same leak reappeared upon filling the tank.
One method of locating the actual leak is to pressure test the tank. This is accomplished by removing all the fuel and then applying air pressure to the tank and spraying soapy water over the area where the leak is suspected to be. This will work around fuel level sender gaskets, fuel pick up bulkhead fittings, skin to rib bonded lap joints, and around the spar to rib joint.
Care must be taken to limit the pressure applied. The large surface area of the tank skin turns even very small pressures into TONS of load on the skin. It's possible to destroy a wing if air from a compressor is applied. The pressure applied for testing is measured in INCHES OF WATER COLUMN, not PSI!
Blowing into the tank vent with lung pressure is one convenient source of air that can not cause tank failures. Some gauges suitable for regulated propane or natural gas pressure, and a bicycle pump are another possibility. The idea is that you will not over pressurize the tank if you have the gauges attached and are manually pumping the air in with the bicycle pump. Hopefully you will get tired of pumping and look at the gauge long before you'd put too much pressure in the tank. Using a compressor air line or a tank of compressed air like you'd use for a tire can over inflate the wing very easily, and it can NOT be strongly enough stated that these are to be avoided.
Fixing the leaks
AA-5x leaks at the fuel scupper drain
Mechanical Difficulty: 
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On 72-78 AA-5x the fuel cap has a "scupper" that catches spilled fuel, rain water, etc. and drains it. This depressed aluminum ring around the cap allows the cap to sit flush with the top wing skin for less drag. The scupper has a 1/4" aluminum tube welded to the bottom to act as a drain. The bottom of the fuel tank has an aluminum block with a matching 1/4" aluminum tube rising from it. This bottom tube creates a "standpipe" that prevents the loss of most of the fuel in the tank should the short section of rubber hose that connects the 2 tubes deteriorate.
A common problem on these planes is fuel leaking from the scupper drain, and water in the fuel tank sump. Both are symptoms of the rubber hose leaking.
It's possible to replace the hose through the filler neck without taking the fuel tank access cover off. A sharp utility knife and a pair of long needle nose pliers, AKA "Stork" pliers from Snap-On or other tool companies, are all the tools required to do this job.
Fly the fuel off, below the 19 gallon tab on the LR tanks or below the level of the mounting block for the standpipe on the standard tanks, and then reach into the tank through the filler neck and grip the spring clamp at the bottom of the rubber tube and slide it to the bottom of the tank. Just let it sit there, it can't go anywhere, it's still on the aluminum standpipe. Now the top clamp is loosened and slid down, just the same.
Take the utility knife with a SHARP blade and slice the rubber tube lengthwise to that you can peel it off the aluminum tubes, top and bottom.
Remove the hose, cut a new piece the same length. I recommend a dab of Parker Fuel Lube on the inside of each end of the hose. Slide it down on the bottom tube all the way, and then bend the top over as needed to get it started on the top tube from the scupper and then slide it up about even with the bottom of the scupper.
Retrieve the 2 hose clamps and slide them up into place about 1/4" from each end of the rubber hose.
Collect up the tools, fill the tank and confirm the leak is gone.
I've seen a few planes that have had the spring clamps replaced with worm drive type clamps. They are harder to do, but the truly motivated can accomplish it through the filler neck. If yours are worm drive (tiny radiator hose clamps), then I'd suggest taking your new hose to the auto parts store and getting some spring clamps to re-install.
AA-5x leaks at the spar
Mechanical Difficult: 
The most common tank sealant failure on the 4 place wings is at the spar tube to rib interface at the inboard and outboard ends of the wet portion of the wing. The rib has a hole in it that is slightly larger than the spar, allowing them to be slid into place and then attached with a bonded and riveted collar. The collars only cover the sides of the spar (not really any room above or below the spar), and that leaves a curved slot at the top and the bottom of the spar tube with only a fillet of sealant to prevent leaks. There is NO mechanical joint at this location and some relative movement occurs as the wings flex with flight loads. Over time the sealant loses its flexibility and begins to break loose or develop cracks that allow fuel to migrate out of the tank.
Leaks at the fuel pickup fitting
Mechanical Difficulty:
The 2 fuel pickup fittings on the AA-5x tanks are AN832-8D bulkhead fittings with an o-ring and a special washer under the nut, and a fuel strainer screen on the inside of the tank. The bulkhead fitting is inserted into the tank from the inboard side of the first fuel tank rib, then the o-ring, the washer and the nut are installed and tightened to seal the fitting, and finally the "finger screen" strainer is installed on the inner end. It's vitally important that the bulkhead fitting be held still while the fuel lines are installed and tightened. There isn't much room to work, but letting the fuel line turn, even a little, may cause your 30 year old o-ring to spring a leak! Even new o-rings can be tempermental about sealing, and proper torque and careful holding of the bulkhead fitting while servicing the connections may not keep you leak free!
The repair seems simple enough. Drain ALL the fuel out of the tank, take the inboard access cover(s) off as required to reach the forward or aft pickup, hold the bulkhead fitting on the outside of the tank, unscrew the finger screen, then unscrew the nut securing the bulkhead fitting, and remove the special washer and the old o-ring. Replace with new and re-assemble in the reverse order.
Hmmmm, sometimes this doesn't work, and the fitting weeps fuel. There isn't a good way to test for a leak before you've re-installed the access cover with new sealant and let it cure overnight before a pressure test.
I recommend a liberal coating of Parker Fuel Lube on the tank side of the rib where the o-ring seats, on the o-ring, the inside of the special washer, and on the inboard side of the nut that slides on the washer and o-ring. This lets everything move smoothly without binding or scuffing the o-ring.
Grumman Owner's Leak Repair Stories
Bluejay Adametz has constructed a web page to tell the tale of his fuel tank leak and repair, and has graciously offered to share his experiences with Bondline. Bluejay reports that it's been over a year since the resealing, and it is not leaking, so he thinks he was successful in repairing his tank.
Roger Rucker has resealed his fuel tank with assistance and supervision of an A&P friend after multiple unsuccessful attempts by another shop. He shares his story with us.
Some hints on working with the sealer
Ask an RV builder how he did his tanks and get some help...
Some items that will help the job go better...
- Use a digital scale calibrated in grams (cheap at Harbor Freight) to measure by weight - 10 to 1 is the correct ratio.
- Batches of 30 grams or more seem the best to handle.
- Syringes work great, but some of them have too small a hole. Drill out the hole to 1/8 and it will be much easier to dispense.
- Small paper cups are not usually strong enough to handle the mixing action - strong plastic cups are better, and again, the auto paint store has good ones. Cheap plastic cups will melt in MEK.
- Buy good, thin gloves that are resistant to MEK - I found the best at the local auto paint dealer (Harbor Freight is not good for this)
- Get a push top dispenser (like your wife uses for cream) and fill it with MEK for cleaning.
- Cut an old sheet in to lots of 3 inch squares. Use these for cleaning after wetting them with MEK from the dispenser. Then instantly throw them away, since cleaning cloths can spread sticky stuff more than they clean. Use a square to wipe your gloves when they get Sealer on them.
- Paper towels are not the best for cleaning, use the old sheet squares.
- Popsicle sticks and tongue depressors came cheaply in packs of 100 from Wal-Mart - they are called Craft Sticks and are in the Crafts section. They also have a narrower and thicker stick that you might find useful.
