ARE THE FUEL TANKS IN A MOONEY A DESIGN FLAW AND A PAIN IN THE BUTT & WALLET TO MAINTAIN?

Let's see about that.....

The Mooney uses "Wet Wings" for holding fuel. What is accomplished by sealing off the Bay joints between wing ribs are properly sealed fuel tanks, but more than that; wet wings are the safest most efficient way of carrying stored fuel in my opinion. Less parts equate to less maintenance and less opportunity for something to go wrong. Are you aware that most airliners, the Grumman series of singles, the Commander singles, and others employ the use of wet wings? Many Cessna's, Pipers, and Beechcraft airplanes use bladders. The Cherokee series of Piper uses metal tank inserts that have been known to cause spar corrosion behind the tank. The fuel tanks on a Beech Skipper for instance use an integral metal fuel tank and should those begin to leak; you usually have to find a used wing to replace in order to achieve a fix.

I have had a bladder burst on a Beech Queen Air that I once had in stock and virtually all of the fuel within that tank gave loose all over the ground. How would you like that to happen in mid air? That also happened in a Mooney 201 we had in the shop for an annual, but that plane had the bladder conversion and a connector came loose allowing fuel to spill. The upside is that most of the sealer of the original tank bay helped hold in the leaking fuel, but that can be a cause of its own problems. Some Mooney owners have installed the after market bladder kit by O & N, but is there an advantage to doing that? Experience proves the answer to be NO! Those in the know do not understand why the FAA allowed a bladder STC in the first place and the reason why is that there are a series of bladders to contend with; not just one. Each bladder has to drain its fuel into the next bladder until it reaches the fuel pick up port mounted near the bottom of the fuel bay nearest the fuselage. The problem there does not address the one time AD that was issued on each Mooney fuel tank, in fact it re-introduces the potential problem that the original AD had addressed. That AD had each Mooney owner open each fuel bay inspection panel to ensure that some mechanic had not at one time or another slopped the sealer around the tanks while accidentally closing off the small baffle holes at the bottom of each wing rib. The reason for that one time AD was an unfortunate Mooney owner; having strained the fuel prior to flight and not finding water ending up killing his engine and him just after takeoff at the first bank the wings made in the turn. Water had been caught and held back by a closed rib until the plane banked at low altitude having the trapped water run down to the fuel pickup and thus starved the engine of fuel. So the reason for the concern over bladders is that they effectively removed the reasoning for the original fuel baffle hole openings by having a small "hole" for passage of fuel from one bay to the other and while aged or improperly installed bladders can distort in areas large enough to hold an appreciable amount of water later to work its way down to the fuel pickup and into the engine. While carbureted engines may run rough when water is introduced until the water dissipates, the fuel injected engine is not so tolerant; in fact it will quit running all together until all collected water passes through the system. Mooney "wet wings" being made of the metal structure will never distort, so that's one less thing......

As in everything concerning aviation is a trade-off; the bladder kit includes a more advanced fuel cap system which is less susceptible to lightening strikes, but I don't think that any Mooney ever exploded from a lightening strike to the fuel area, but the bladder cap remains better grounded for such events. Also with bladders you are less likely to pick up some of those pesky but airworthy weeps and seeps as found on any airplane including airliners that employ the use of wet wings. But is that worth the major things that can go wrong with bladders? You'll have to decide that for yourselves, but before you answer; consider this question:

Two different cars are parked side by side. One is built for simplicity like the old VW Bug, and one is built with more complicated mechanisms such as power steering, electric windows, power everything. Which vehicle are you going to have more upkeep upon? Duh right? So to me it makes sense that the more complication (and/or parts) we add to our air machines, the more stuff will eventually go wrong; right? What are you actually gaining by having rubber fuel bladders? Basically only the occasional and pesky weep or seep. The cost of the bladder system is about on par with a proper resealing of the tanks, and they last no longer than a properly done reseal. Those that install the standard bladder kit on F, and J, model Mooney's will lose about 6 gallons capacity. Those installed on the B, C, E, and G models will gain some. For more money you can add the extra capacity bladder and get back to the 64 gallons usable you once had in the F and J models.

The real downside to bladders in the everyday world (without the what-ifs) is the weight. You are adding the weight of additional parts that you don't need subtracted from your precious useful load, and you have changed the original design of the airplane.

While I wouldn't discount a particular Mooney for purchase consideration solely because it has bladders; I would just consider it to be one of those things you have to give up on the downsides of any given aircraft for purchase consideration. There is certainly no perfect airplane, so your purchase decision should always include weighing the upsides vs. downsides, and the weight of importance each downside may carry vs. the upsides.

So, with bladder talk aside; let's discuss the Mooney wet wing system. How long does a freshly sealed fuel tank last? Well, if the owner that had the tanks resealed bought price and used these guys: Chances are they won't last that long. However with the proper job, they can last indefinitely. I have had some sixties era Mooney's in stock before that have never had a complete strip and reseal done and yet the tanks were in fine shape. I have had newer models whereby the tanks leaked like a sieve. What gives? What gives is the same thing that happened to the early reputation of one of the finest Mooney's (especially for the money) that were ever built. The ever capable and affordable Mooney 231 which is one of the best values in the used plane market. Here is my 20 years experience with that airplane: I don't care whether the engine is the TSIO-GB, LB, LB1, or the 252's MB; about half of them get a TOP overhaul mid way through TBO and the other half do not and sail through to TBO. What single factor involved makes a good engine like the original GB, but all of them really; get a lousy reputation enough to affect the resale value down to where they sell for so much less than their competition such as a turbo Beechcraft; what is that factor that causes some to have half of the engine operating costs than the other guy? The engine OPERATOR! There are those who know some tricks as to how to make things last longer and not everyone is blessed with such talent in those areas. The owner/operator of the Mooney you may be buying or have already bought from can make a big difference in the longevity of the fuel bay sealer. So let's discuss ways in which to help ensure a long life of your fuel tank sealer:

Bare with me here. Mooney had designed its landing gear with a combination of functionality, ruggedness, and simplicity. They opted for the tougher and more landing forgiveness design of the "Trailing Link" style.  The weakness of this design (if any) are the compressing shock disks. They need to be replaced periodically because with all rubber products (bladders included) time takes away their elasticity properties and they can and will collapse with time lowering your Mooney ever closer to the ground and limiting the cushion needed where landings are other than greasers. At some point in time, there will be no further give in the rubber disks and the only cushion remaining is in the tire pressure. My point here is that we're talking about plopping 2-3000 pounds of weight onto a hard runway surface and if you take away the cushion, you can expect something to give eventually. I suppose I went too far in proving my point here; but it is that depending on who flew the plane and how he or she flew it (in this case landed it); makes a difference in how long a seal job can last. Why? Because the main landing gear of a Mooney are mounted onto the the main spar which also happens to be the rear wall of the fuel bays.  An argument for bladders perhaps? Probably not as a hard enough landing can rupture a bladder in some instances. So, based on the above, the pilot has a lot to do with tank longevity.

What else?

Storage: Inside out of the elements is a factor for longevity. Fuel quantity is another. Some pilots like to top off the tanks before putting her to bed so that she'll be ready to go next time. Fuel expands and contracts appreciably with changing temperatures and having tanks too full can place undue pressure on the sealers as the fuel attempts to expand beyond its capacity. I would not put my Mooney away with the fuel line more than 3-4" below the cap level.

Low Fuel: It is the fumes of the fuel that help keep the sealer's elastic properties functioning. If the fuel level is too low, the tank sealer can suffer from dry-out. Try to have at least 1/4 tanks of fuel when putting your bird to bed, but no more than 3/4 full. Also remember that the more fuel you have in your tanks when parking the plane, the harder it is on your rubber shock disks due to the extra weight of the fuel.

Fuel is dry. It has no lubricating properties; in fact, why do you have to overhaul the fuel controller or carburetor on occasion? What wears out; the metal parts? Nope. The rubber parts do because they have to endure the harshness of gasoline that has no lubricating qualities to it, in fact it is just the opposite. Have you ever gotten gas on your hands? Once they dry, they look ashy because the fuel sucked out every bit of moisture that used to be there. It is the same with rubber and tank sealer. The sealer can get really chalky looking and if the tanks are opened up, you could run your finger on that sealer and have a chalky residue on your finger. So great zef; how do I counteract that? Well, I am not suggesting that you do this as it would be FAA illegal, but I know for a fact that there are many aircraft owners that run Marvel's Mystery Oil in their fuel to the rate of around 4 oz. per ten gallons. Marvel's has been tested by an aviation consumer magazine some many years back, and it found nothing of determent when used in aircraft engines. So, let's discuss the benefits of having something in your fuel that is really "wet" or has lubricating properties. Marvel's Mystery Oil for instance is volatile, and it will explode just as gasoline will when near a combustible such as a flame so it does burn inside the engine as gasoline does. However the oils that it leaves behind would be of great benefit if only Turtle Wax (Owner of Marvel's) would spend the million or so bucks to get their product FAA certified. Harley drivers know the benefits of such products. The benefits are as stated, the lubricating properties for the fuel tank sealer to keep it younger longer, the rubber in the fuel delivery system, the lubricating on the Lycoming weak point being the valve guides, and the removal of all carbon inside the engine caused by burning fuel and air. If we were just allowed to use that stuff in our planes, it would be all benefit and no drawback; plus Marvel's is cheap to buy! But as I said; for now it is illegal to use in our airplanes, however as I stated; there are those, a good many that use it anyway and I have no control over that except that I wouldn't mind buying my next airplane from someone that knew engines and fuel bays enough to know the benefits of having a lubricant within the system. Shame on you law breakers! The government always knows best...

What is a fuel leak? In Wet Wings, a fuel leak would be one where the fuel runs ending up in a drip. Fuel stains such as from weeps and seeps up to the size of a half dollar coin are acceptable as airworthy and rather normal. I have seen fuel bay inspection panel screws leak and that is an easy fix buy emptying the tank beyond the screw, removing the screw and putting some silicone that is made for fuel around the threads and screw it back in; wait at least 8 hours until filling the tank beyond the screw point. If you're the type that weeps and seeps get you bugged about, then there is always AeroSeal for those. The late PHD and Mooney owner Norm Smith, came up with a mixture of goop that you can seal weeps and seeps with. You in effect empty the fuel tank and apply about 2" of suction to the tank at the fuel cap opening; add the mixture where you see the leak appear and it is supposed to suck it in along the seep and you then remove the suction and let it sit overnight. Between you and me, I think Norm's sealer concoction was about the same formula as crazy glue. I tried crazy glue once in a pinch using the above application method and it worked!

So what if you have a genuine leak? What are your options? Do you have to fully strip and reseal the tanks every time you get a leak? Not really, but care must be taken in your efforts to repair and area of leakage. Where the fuel comes out of the bottom of the airplane is not necessarily the area where there is a breach in the sealer. You can get a crack in the sealer up to  a foot or more from where it appears  because the fuel may be traveling under and along the sealer joint until it finds a place to escape. But I want you to remember that only the joints of the fuel "box" are sealed. There need be no sealer on the aluminum sides because they are already liquid proof, so only the corners of the box needs sealer. There are many do-it-yourselfers out there that have successfully removed most of the old sealer and resealed their own tanks. Sure it's a job, but a job worth doing so long as you have an A&P to check your work and sign it off and you are absolutely assured that you have removed any and all loose matter from the tank prior to refilling. Some owners have made their own leak repairs in which you have to find the area of breach using a flashlight and mirror, scraping enough old sealer away to ensure getting to the area of breach; and then apply the PRC sealer as directed followed by a thin coat of the sealer membrane protector.

What if I smell fuel in the cockpit? The mandate is not to fly the plane if you smell fuel inside the cockpit. If the leak is bad enough you wouldn't want to even get into that cockpit as your eyes would water profusely and would obviously be a fire hazard. When there is an interior fuel leak it is usually traced back to the wall of the wing at the skin part dead ends into the fuselage. The usual culprit is a leaky fuel line where it attaches to the wall of the wing root due to a bad rubber seal. That is usually an easy fix, but part of the interior has to be removed in order to get to it.

What if I see little red flakes when I test the fuel sample? Don't be alarmed. This can be considered normal to an extent depending on how sloppy the membrane application was, it may come loose some where it hits solid metal just past the sealer it is covering.

What if my fuel tanks don't leak but the existing sealer is very old? If it ain't broke, don't fix it! Remember that treated correctly and under the right sets of circumstances; the original sealer can and will last a very long time and if the logs show that there has been little work done to the tanks i.e. fixing or repairing leaks, then the existing sealer is a good candidate for patch repairing areas that may later open up. It's not always the end of the world to see a seep, a weep, or even an occasional leak. If your landings consistently suck; then expect to have to eventually do some tank work, so work on your landings and ensure that you have shock disks that meet specs. The newer the shock disks, the more shock protection you will have for those Bounce-In landings. Those with the newer "Super-Strech" bodies such as the TLS, Ovation, Acclaim's etc. will have to change shock disks much more often than the medium or short body because the gross weight has increased largely in the newer models so you guys need to get used to changing disks every 4 or 5 years whereby the earlier models can go up to twice the time on a set depending on your landing habits. BTW, a good thing to do for your shock disks is to protect the rubber from the ozone by spraying some Amoral (or the like) tire foam on those and your tires. Again the lubrication qualities in tire foam along with its UV protection will help make them last longer.

So what's the bottom line? There are things that you can do to help prevent premature sealant repair and a good way to start is to try to avoid stepping on the part of the wing walk that is painted over the upper inspection panel. It's easier said than done in order to avoid it because it's near where you naturally step inside the airplane. Enjoy your Mooney. You made the best choice!

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zef    You need THIS and THIS (Package price available) and THIS

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