The fuel system beyond the selector valve
Ostensibly the aircraft fuel system has remarkably simple tasks to perform — along the lines of an elevator operator working an automatic lift. Mainly, fuel tanks must hold the power-producing liquid in a safe and orderly fashion within the airframe, and they must carry the liquid in sufficient quantity to the firewall-forward fuel delivery systems. Sounds straightforward, sure, but it's amazing the number of different ways airframe makers have devised to carry out these requirements.
Wing tanks, fuselage vessels, tip casks, baggage-bay puncheons — these are some of the ways airframe designers work the compromises of fuel-system placement. Wing tanks predominate because the space inside the wing would otherwise be wasted. Engineers love this kind of design elegance. Save for structural members, the wing represents vast untapped real estate. Such a location also provides some measure of safety. In a crash sufficiently severe to separate the wings from the aircraft, chances are good that the fuel will go with them. This is an improvement over the fuselage tank, which has historically presented much more of a safety risk. (Then again, it's been some time since header and fuselage tanks have been common as part of the original design. Newer aircraft like the Diamond Katana and Zenith CH2000 have fuel carried behind the cabin for reasons of simplicity, and new testing standards presumably assure the crashworthiness of these fuel vessels.) Cessna, at the introduction of the 310, suggested that the twin's main tanks, comprising intentionally frangible tip tanks, would reduce the likelihood of post-crash immolation. Such a placement forces tradeoffs, however, like long fuel lines and the dynamic effects of carrying 300 pounds on a wingtip. Still, tip tanks are popular for add-on fuel capacity and do have some structural advantages, such as reducing the bending moments of the wing.
Even with the basic placement of the fuel tanks locked in by design, there remain substantially different ways of keeping the fuel inside. Three types are most common: Drop-in metal tanks, wet wings, and rubber bladders. (There are other variants, like the American Aviation/Grumman Yankee's tubular main spar that doubles as a fuel vessel.)
Drop-in tanks are, as the name implies, built-up aluminum tanks that are placed in the wing and held down by straps. Because the tank does not carry flight loads, it is free from most of the flexing that disturbs wet wings. Overall, the metal tank system has been extremely reliable. The main drawbacks include relatively complex construction needs — you are essentially building a box within a box — and there is the potential for the hold-down hardware to wear through or create dissimilar-metals corrosion for the aluminum tanks. Also, there is a slight weight penalty, compared to other tank setups, especially when the tankage becomes quite large.
If simplicity is the key to all goodness, then the wet-wing tank ought to be the ideal. It turns out this is not always the case, although success varies widely in this style. A wet wing is simply the inside of the wing surface — ribs, stringers, and all — sealed by a thick, fuelproof substance to keep the fuel from leaking out through rivet heads and past the end ribs.
In theory, this system ought to be mighty rugged for sheer paucity of parts. And in some cases it is. Reportedly, the cantilever- wing Cessnas, all of which have wet-wing tanks, hardly ever have leakage problems. Even then, they're usually confined to the access panels and don't require a trip inside the wing to fix.
Not so with the Mooneys. Indeed, weeping wings on Mooneys — early models in particular — are much-discussed problems. Depending upon age, use, and whether the airplane is hangared or tied down, the typical Mooney seems to live happily for 10 to 15 years before the sealant breaks down. (The situation has improved greatly with advances in sealant compound, however, and it's likely that Mooneys built in the last decade will fare better.) The most obvious clues will be the wing's beginning to weep along the rivets in the upper surface and the appearance of black flecks in the fuel samples. Depending upon the degree of leakage and location, this may or may not be a safety of flight issue. In any case, the appropriate maintenance manual will spell out the go/no-go guidelines. Needless to say, a hefty puddle under each wing or a solid line of blue fluid running down the wingwalk is grounds for further investigation.
Is there a fix? Yes, but it pays to find a shop well versed in the intricacies of resealing a Mooney tank. Hangar talk suggests that cut-rate jobs stand little chance of lasting and may require even more effort (and money) to remedy. Best bet for Mooney owners is to contact the Mooney Aircraft Pilots Association (210/525-8008) for recommendations on shops with experience in this messy, challenging job.
And then there's the bladder-tank system. Simply put, this method uses nylon-reinforced rubber vessels formed to fit within the contours of the wing. In this way, the wing structure and skins need not be fuel-tight; only the physical loads of the fuel need be accommodated. Among the main advantages of the bladder system is ease of manufacturing — the wing itself may be less complicated to build and does not require the messy sealing process of a wet wing.
Bladder problems, however, are well known. The bladders don't last forever, for one thing, and will begin to seep as they deteriorate. The most common failure mode of the rubber bladder — a military-aircraft throwback — is hardening and wear-checking of the upper surface. For airplanes parked outside, this is a particularly thorny problem. Tanks left partially full will expose the upper interior surfaces of the bladders to air, and the heating of the upper wing surface during a hot day will promote breakdown. It's also possible for a bladder that's not correctly anchored to rub against something in the wing until the bladder is worn through.
Usually when a bladder begins to fail, you'll notice a slight seepage mainly when the tanks are full. This is the sure sign that the upper panels of the bladder have deteriorated and are in need of repair. One of the most successful bladder overhaulers is Henry Krug's Aero-Tech Services (310/696-1128). Krug spent 30 years with bladder maker Goodrich and says he has learned from all that experience. Aero-Tech's new bladders are double-walled affairs with a nylon-reinforced outer shell and a pure rubber inner liner. Krug says that this double construction prolongs life and makes the bladders more flexible, thus simplifying installation. Aero-Tech charges between $250 and $350 to overhaul Cessna 182 bladders and $795 for new ones.
Bladder systems create some other concerns for pilots of certain aircraft. The water-holding qualities of the Cessna 182's bladder tanks are well understood but still prevalent. According to the Cessna Pilots Association's John Frank, there were some manufacturing miscues in the 1970s that put the quick-drain ports in the wrong place. When the mechanics installed replacement tanks, it was still possible to load the bladder in the bay and have the quick-drain receptacle appear near the hole in the bottom of the wing. Unfortunately, the misplacement of the nipple created a diagonal crease in the bottom of the bladder, one that many pilots discovered could hold enough water away from the normal quick drain to douse the engine. Usually this happened just after takeoff. An airworthiness directive on the Skylane spells out a difficult and awkward "rock and roll" preflight procedure that's meant to reduce chances of water contamination.
Complicating matters is the fact that many of these bladder- equipped airplanes had flush fuel caps. These have a history of admitting water to the fuel system, even with all their O-rings in the best of condition. For a nominal sum, raised or umbrella caps can be retrofitted to prevent water intrusion. "Anyone who has a bladder-tank aircraft and is flying with flush fuel caps is a fool," says Frank. (CPA's technical center can be reached at 805/922- 2580.)
There is a remedy for a variety of Cessna models with bladder tanks. Monarch Air & Development (503/459-2056) makes a set of fiberglass replacement tanks for the bladder-equipped airplanes. These rigid tanks are made with fiberglass and vinylester resin — a common underground fuel tank material — and are reported to have an almost indefinite life. And thanks to their uniform shape, they reduce the likelihood of water entrapment, especially when used with Monarch's umbrella caps. The Monarch replacements start at $2,295 a set for standard-range tanks and are said to take about 40 hours to install.
Owners can take several steps to prolong fuel tank life. By far the most important is to keep the tanks full while the airplane is sitting. For bladder tanks, this keeps the ozone-checking in hand and even helps the wet-wing aircraft by preventing the sealant from drying out. Keep an eye on fuel-cap condition. Hardened O-rings or gaskets can let water in and promote corrosion in metal tanks. Finally, be alert for signs of seepage and fuel staining. Every shop we talked to said that early detection and repair is better than waiting for a problem to become large — both because a big fuel-tank malady is dangerous, and because the fix is usually more involved and costly. Awareness here is the simple preventative.
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