Rx for Overactive Vents

A flood of fuel provides a topic of conversation.

Technicalities Fuel Vent

Technicalities Fuel Vent

Aftermarket bladder tanks on a Mooney protect
the vent with a streamlined ice catcher.

(Photo by Robert Takacs)

The day began benignly enough. I flew to Las Vegas in midmorning to pick up a couple of friends and bring them back to Los Angeles. The flight up was routine, cruising at 7,500 feet, 15-knot headwind, but smooth and clear with unlimited visibility.

We put in 20 gallons at North Las Vegas, where the price was a bit lower than at my home field of Whiteman (WHP). During the takeoff roll something went wrong: a change in the exhaust note, a drop in the manifold pressure. I aborted and taxied off the side of the runway.

We quickly diagnosed the problem. A slip joint in an exhaust collector on the right side of the engine had come apart. Nothing was broken. The pipes have lugs that are supposed to be connected with a steel link, but I didn’t think they could move far enough to slip apart, and so I had omitted the link.

We put the pipes back together and taxied over to EGA Aviation, where a very good-natured A&P named Lenny secured them through the link lugs with many turns of heavy safety wire and sent us on our way.

The return trip was quick, cruising at 12,500 feet with a groundspeed of nearly 200 knots, flowing 10 gallons an hour. At Whiteman we rolled the plane into the hangar, locked up and drove off.

Late that night I got an excited call from the airport. Fuel had been flowing out from under the door of my hangar. They had cut the padlock and found the plane listing drunkenly to port, fuel running out at the wingtip. After hauling off multiple jerrycans of avgas, they finally leveled the wings and stopped the leak. Huge stain all over the taxiway. Big mess. No lock.

I promised to come in the morning and see what I could do.

When I arrived I found a great deal of what appeared to be kitty litter in the hangar. Fortunately the fuel had run out the front door, not back inside. The plane was pretty level now, like a dog who tears up your slippers and then tries to look as though he knows nothing about it.

I struggled to figure out what had happened. I had recently added some air to the right strut, and it was a little stiffer than the left. The ­back-seat passenger had sat on the right side and so I had fed more fuel from the right wing than from the left. Evidently after we left the hangar the left strut had compressed slightly, perhaps because of a temperature drop. The tanks run full span and the wings have just three degrees of dihedral, so as the plane tipped, the fuel in the left wing migrated outward, putting more load on the strut. Possibly the overinflated right strut extended a bit. Finally the fuel ran all the way to the wingtip and began to flow out the vent.

This had happened once before. In 2010 I stopped for the night at Prairie du Chien, Wisconsin, on the way to Oshkosh. I fueled the plane and went to a motel. In the morning I came back to find the airplane listing and a huge fuel stain on the ground. I was mystified then, not to mention mortified. I slunk away and banished the horrible sight from my thoughts.

Now that it had happened twice, I assumed it must be a weird vice of my airplane and no other. Some awkward expedient would henceforth be required to remedy it; for example, I might have to carry around a couple of sawhorses to put under the wings when I parked, or a couple of concave blocks to keep the struts from compressing beyond a certain point.

Having reported the bizarre event in my online journal, melmoth2.com, I soon learned that I was not the only person to whom this had ever happened. One visitor wrote to report that he had seen it several times while flying a P210. Once he had lost 25 gallons of fuel.

I also received reminders about airplanes that required special fueling procedures. A photograph, taken at Oshkosh, of fueling instructions posted in the windshield of an MU-2 revealed that the unhappy fueler had to make several trips back and forth as he added limited amounts of fuel to first one side and then the other. It behooved him not to leave a ladder under a wing, which might settle down on top of it. An ex-­Learjet pilot reported the need for similar precautions with the old tip-tank Lears, which some said could even tip over if one tank were filled while the other was empty. Indeed, this was facetiously said to be the origin of the term tip tank.

The common theme, not surprisingly, was airplanes with narrow landing gears, fuel carried outboard along the wings and small dihedral angles. This put me in mind of airliners, which are all of that description, excepting perhaps the dihedral part; but they are single-point fueled, with the right and left tanks communicating with each other and a system to prevent overfilling. A retired Delta pilot reported having seen a daydreaming fueler pump fuel into one wing of a jet as it steadily ran out the other, the automatic shutoff system having failed to operate.

Fuel tanks need vents for several reasons. For one, air has to be able to get in to replace fuel that goes out; for another, excess fuel — from thermal expansion or returned from a fuel-injected engine — has to be able to go somewhere, if necessary overboard. Both functions are critical, and so tank vents must be protected from icing. Airplanes deal with the problem in various ways. One is to put the vent behind some ice-­catching object, like a wing strut; another is to put it a few feet behind an exhaust pipe. Mine are in cavities fed by flush NACA scoops.

Obviously, the vent must leave the tank above the highest fuel level. For airplanes with wing tanks and little dihedral, there is small leeway, and spillage from full tanks is likely on a sloped ramp. One correspondent pointed out to me that Cessna 177s have their tank vents arranged in such a way that the vent for the right tank is at the left wingtip, and vice versa — an ingenious solution.

At some point it dawned on me that, since I have upturned wingtips that rise 6 inches or so above the tops of the fuel tanks, I ought to run the vents up through them. (When I added the tips after years of flying, I left the original vents where they were.) Higher outlets would not provide complete protection, but they would be a great deal better than what I have now. But that led to questions about where to put the outlets to ensure both positive pressure and protection from icing.

It took a nonpilot RC modeler to point out to me that my new vent didn’t have to open at the tip; it could go up the wingtip and then turn around and return to its present outlet. He noted that there are aftermarket vent tubes for high-wing Cessnas that differ from the original ones only in bending upward a little just after they leave the tank.

There is still a slight risk of siphoning with this scheme, but the conditions that could produce it are remote. The nice thing about it is that I should be able to insert the vent loop and seal it in place through a couple of small holes at the outer and inner ends of the wingtip, leaving the load-bearing structure intact.

I have reached the age at which dinner table conversations with my peers revert disagreeably often to medical matters. So far I have been lucky, and I have little to contribute. But soon I will be able to bring to the table a medical adventure of my own: laparoscopic surgery upon my wingtips.

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