Fuel Tank Leaks
Fuel Tank Leaks
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. 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 know 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. Its no wonder that the AA-5x have many more fuel leak problems than the AA-1x.
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. Its 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.
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.
Its 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 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.
<To be continued>
