Do You Like Your Ribs Wet or Dry?


October 2009


This month’s ShopTalk will take the reader into the Mooney fuel tank leak issue.

Mooney M20 aircraft have wet wings for fuel tanks. Between the main and forward wing spars the ribs, spars and wing skins, sealed with fuel tank sealer, form an internal fuel tank. The ribs in between the wing root and outboard end of the fuel tank are open with lateral drain holes and lightening holes to allow fuel and water to flow to the lowest feed and drain points. The reason for the lateral drain holes is so any fluid (fuel/water) that enters the tank can ultimately flow down to the wing root. Water from contaminated fuel, leakage at the filler cap or condensation accumulates at the tank sump and is removed via the wing quick-drain valve.

The Mooney wing is built dry and riveted together dry. Once completed, sealer is applied to the inside of the tank area. Because the main spar is connected to the main landing gear the main spars are subject to impact loads from hard landings or worn out landing gear shock discs (see ShopTalk, May 2009). During taxi and flight, the wing bends and twists (less for Mooney than many other aircraft). Because of this constant flexing the fuel tank sealer has to be flexible. However, over the years the sealer tends to naturally break down and shed off material into the inside of the tank. You may see this in your fuel sampler cup as small black specks. These specks also get caught in the wing quick-drain valve and cause it to drip. As the Mooney fleet ages the fuel tanks are patch repaired and so you have a 40 year-old airplane with patch repairs on top of patch repairs inside of your fuel tanks. One thing to remember about wet-wing fuel tanks is not if but when the tank will leak. Even the best tanks only last twenty to twenty-five years.

What are your options as a Mooney owner to control your airplane's incontinence as it ages? That Depends. If the leak is small and isolated, a patch can be considered. However, a patch repair may not be possible if the tank has been patched multiple times or even if the airplane is older. If you are seeing a lot of black specs in your tanks when you sample the fuel, the sealant is probably breaking down and a patch repair won’t adhere properly to a deteriorating surface. All A & Ps can patch a fuel tank but not all A & Ps should. There are a handful of companies that will patch repair a tank. Don Maxwell Aviation Services of Longview, Texas does a good job but a patch repair can cost anywhere from $700 to $1500 per tank. At Don Maxwell's if it is not realistic to patch repair then he’ll just close up the tank and send you on your way to choose other available options.

Another option is a complete strip and reseal of the fuel tank. On older aircraft, this process or installing bladders may be your only options. There are a handful of companies that do a complete strip and reseal of Mooney tanks. Wet-Wingologist East and Willmar Air Services along with Midwest M20 Sales and Service are three of the better ones that come to mind. There may be more but I know these three offer a warranty with their work. Wet-Wingologist and Willmar completely strip the old sealant out and reseal your tanks the way the factory did it when it was new. Midwest Mooney has an STC for a polyurethane coating that is applied to the inside of the tanks after striping, cleaning and priming. Strip and reseal can be done on each wing separately to help keep down the initial cost. With bladders, both wings must be done at the same time. So let’s talk about cost first and then we will talk about the down time. The last part of this article will be the nuts and bolts of installing an O&N bladder kit.

Typical cost for complete strip and reseal will range from $3,700 to $4,300 per tank. The polyurethane tank reseal is the most expensive at $4,300 a tank. It takes two to three weeks as the sealant has a two to three day curing time. Then the shop will want the aircraft full of fuel for three to four days to insure there are no small leaks. This job is very labor intensive and that adds to the down time. For one tank, maybe two weeks of down time sufficient. For both tanks, plan for three weeks of down time.

Warranties vary from seven years (Wet-Wingologist) to three years. If there is a leak only Willmar will send someone out to take care of it. Other companies will do the warranty work at their facility. The owner must add in the cost of delivery and pick up of your airplane when doing a complete strip and reseal or maybe needing warranty work. If you are based close to these companies then that cost is minimal compared to the cost of delivering and picking up your airplane if you live on the west coast.

What about the other options the O.&.N. bladder kit? According to Myron Olsen O.&.N. owner the first bladder kit was installed twenty-four years ago. O.&.N. has an STC for Mooney models M20C through J and all the parts in the kit are PMA approved. The first O&N kits were 54 gallons 6 fuel bags 3 per wing. Now they have a 64 gallon 8 fuel bags 4 per wing kit STC approved and with over six-hundred kits installed and flying around they are a proven product. Those54 gallon kits can be updated to 64 gallon kits by installing two more fuel bags but for this article we will concentrate on the 64 gallon 8 bag fuel kit we recently installed on a customer’s 1968 M20F but first the cost and down times.

The 64 gallon 8 bag fuel kit will set you back $7,750 plus shipping and it is estimated to be installed in about 55-60 man hours. The two bag add on kit for the 54 gallon kits will set you back $2,450 for the kit and we would guess about fifteen man hours to install. For those of you that only want a 54 gallon 6 bag fuel kit it is $5,850 plus 45 man hours to install plus shipping. The O&N factory also does installations at a flat rate and because they do them all the time it may be more cost effective if your airplane is based anywhere near Factoryville Pennsylvania. Again one must figure in the cost of moving your aircraft back and forth as part of the overall cost of this upgrade.

The O&N Factory will need one week to complete your fuel tank kit and you must consider the possibility of a warranty claim if there is a leak after you pick up your plane. Fuel tank senders may need repairs or replacement with the bladder kit or a strip and reseal. Each fuel sender can run about $150 to &175 to overhaul and in the F&J model there are four senders total. This will add time and cost to either job. If you want new senders plan on 340 each and availability is spotty at best. Some of the benefits of the wet-wing strip and reseal are no loss in useful load and over all the cost is less than a bladder kit installed and they can be done one tank at a time spreading the cost out making it more affordable upfront. Some of the benefits to the bladder kit are almost any A&P can do this job this puts your airplane closer to home and reduces the delivery and pick up costs. The bladders last as long if not longer than the wet wings. If there is a leak only the leaking bladder need be removed for repair and most A&P’s do this job regularly. Bladders are not susceptible to worn out landing shock discs as wet wings are see ShopTalk MAPA May 2009. O&N also offers a warranty of five years on the bladders and parts they supply in the kit.

So now that you are an educated buyer let’s talk about the basic nuts and bolts of installing an eight bag 64 gallon bladder kit on a M20F. The aircraft must be de-fueled and all fuel tank top wing panels removed. All the fuel sender attachment rings along with the fuel cap receivers must be removed. The old fuel drain attach nut and fuel pick up lines will be removed along with the fuel vent system.

Once all of these items are out of both tanks then an additional fuel tank access hole is cut outboard of each fuel cap and a wing access panel is fabricated from parts supplied in the kit. This access hole is for the fourth fuel bag to be installed in the wing outboard of the fuel cap. A new vent impact air system is located and installed outboard of each wing on the fourth bay and is interconnected to the fourth fuel bag.

The old vent hole and one lightning hole in one rib (each wing) must be plugged with parts supplied in the kit. A cover assembly must be installed in the fourth bay in each wing to cover the aileron push pull rod that runs through this bay. These parts are riveted in place in each fourth wing bay. Each rib between each bag must have interconnected holes drilled into them and these holes must be lined with caterpillar grommet material. Both inboard fuel senders and the rib it goes into are drilled out to 1/4” to accommodate the new mounting screws and insulated washers.

The old fuel pickup hole is enlarged to accommodate the new fuel pickup and fuel bag nipple. A new lower sump drain is located to accommodate the inboard fuel bag. These parts are installed on each lower wing sump. Two holes are drilled into each wing bay in the aft corners to allow any fuel leak to vent out of the tank bays. Once all this is complete then all the tank bays can be cleared of all materials, chips and debris. With the tank bays clean then all rivets and seams stringers and lighting holes are covered with duct tape leaving the tank interconnect holes uncovered. Once everything is covered with duct tape the foam base plates can be installed into each wing base bottom. These foam base plates are what support the bottom of the fuel bags, they may need a little trimming to fit properly and not block the drain holes drilled in each corner of each wing bay.

We installed the fuel cap fuel bag first and located it to line up with the oversize fuel opening cut into the wing during a earlier step. On the right side everything lined fine on the left side the old fuel cap location was a ¼” inboard on the wing compared to the right side this caused the fuel bag outboard interconnect nipple to not stick out far enough into the forth bay and made it difficult to get the correct clamp on the nipple and torque down properly.

The outboard fuel transmitters were also a problem as they had numerous bends in the float wire at the transmitter base with all these bends they tend to interfere with the fuel bag metal ring that holds the nut plates onto the bag. These nut plates are what hold the transmitter in place and secure the gasket in the center of the fuel bag. Once the outboard transmitters are in and installed and tightened down make sure that the float works properly up and down. Use a multimeter to verify that the fuel at full up is at least 29 to 30 ohms as you will probably need to bend or adjust the wire to the float to get the 29 to 30 ohm rating. I strongly recommend sending all four transmitters out for overhaul if they have not been done in the last twenty years, this will save you a lot of grief when you try to get the system up and running properly.

We also recommend to put the fuel tanks in with a little baby powder on the outside so they will not stick to anything as they are fueled and spread out. O&N says in their kit to install all the gaskets dry, but we use Fuel Lube or Easy Turn to coat the gaskets before installation. Fuel Lube or Easy Turn are fuel resistant and they never dry out so having to re-torque the cork gaskets at a later date because they are leaking will not be necessary. Another problem we had was on the installation of the inboard transmitters. These transmitters cannot be grounded to the airframe in any way or the system won’t work properly. When you install it, check it with an ohm meter to make sure it is not grounded to the airframe and test the complete system before you fill the tanks.

Once all the fuel bags are in and the interconnect hose clamps are torqued go back and re-torque them the next day this will save you a lot of grief when you fill the aircraft up as the rubber interconnects will take set and the clamps will be loose the next day. The new fuel cap neck is installed to the wing opening with PRC fuel tank sealant. When you are installing that fuel bag mask off around the area where the fuel cap neck is so when the PRC squeezes out it will be easy to clean up.

Once all the transmitters and the fuel bags are installed the vent can be located along with the anti ice mast. Don’t forget to duct tape over the outboard fuel transmitter as the outboard fuel bag will rest against this transmitter and could be damaged over time. With all the fuel bags closed locate the two screw holes in each wing access panel. These two screws hold the fuel bag up just as snaps on the bags on other aircrafts. Install the wing access covers with the RTV provided with the kit after installing the two screws per fuel bag that hold the bags up.

Fuel up each wing five gallons at a time and look for leaks until the tanks are topped off. We like to see the airplane topped off for two days with no leaks before closing up the interior. Last, but not least, special placards are installed on top of the current fuel gauge dials. The fuel gauges will have to be disassembled to do this job. Other placards are installed at the drain, filler and fuel selector.

To get rid of the fuel smell in the cabin, clean all the little fuel stains using lacquer thinner and re-tape the wing root interconnect areas in the cabin, then apply and rub baby powder into the carpet on the floor. This will help a lot and an air freshener can’t hurt either.

As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.