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Aircraft Tyres

Anyone who has ever watched an airliner land and noticed the puff of blue smoke that appears the moment the wheels spin up from zero to somewhere upwards of 140 mph must wonder about the abuse those poor tyres take. Heck, the tyres on even our own training airplanes take quite a pounding, and I don't mean bounced landings, either.

From the standpoint of weight alone, it's a hard-knock life for those little doughnuts. Using a four-seat retractable-gear airplane as an example, the tyres support the equivalent weight of a fairly sizeable car, but on one fewer wheel, and all three taken together have much less rubber-to-pavement contact area. They endure much greater extremes of temperature, and those sudden screeching spin-ups are none too gentle. And the comparatively much longer periods during which the tyre is sitting there on the ramp or in a hangar getting flat-footed aren't exactly conducive to long life.

Airplane tyres need to be light to allow a greater useful load, and they have to be small--to reduce drag as much as possible for fixed-gear airplanes, and the amount of storage space for retractables. Many have several sets of nylon-impregnated fabric under the treads in a bias-ply construction arranged at angles between 30 and 45 degrees to the rolling axis. The reinforcing belts for the radial tyres on our cars, however, usually run directly across the tread.

Radial tyres often look "bulgy," but bias-ply tyres won't betray their too-low pressure condition unless they're really badly underinflated. Also, most car tyres these days are tubeless; airplane tyres generally still have inner tubes. Radial construction allows automobile tyres to maintain a more consistent outer diameter at high speeds, which is something that car designers don't actually have to worry much about--but which designers of retractable-gear airplanes do. On the other hand, we can legally fly and taxi around on completely bald tyres - just as long as there is the same amount or degree of baldness, all around (though there's a little more to it than that). Don't try that in your family car!

The system for sizing and describing piston airplane tyres is thankfully fairly easy to understand. A so-called 6.00-6 tyre has a cross-section or width of six inches and is made to fit on a rim that is six inches in diameter. There's a bit of leeway in those dimensions; such a tire might have a diameter anywhere from just under 17 to about 17.5 inches, and a width anywhere between just under six to about six and one-third inches.

The ply rating system can be a bit confusing. The number of plies a tyre literally has is usually less than the rating, which is more to indicate the load that the tire is capable of carrying, and at what tire pressure. A Goodyear nine-pound, 6.00-6, 4-ply tyre at 29 psi might be able to carry 1,150 pounds, and the 8-ply version might be rated for more than twice that amount, using an inflation pressure of 55 psi. One might conceivably opt for the sturdier 8-ply tyre, since the more rugged version weighs only a few pounds more, but you'll probably notice a bumpier ride down the runway--and if you happen to catch a pothole, the harder ride will transmit proportionally greater shock loads to your landing gear and the rest of the airframe.

What can you do to keep those tyres young? As with our cars, the biggest factor is proper tire inflation pressure. Underinflated tyres will wear faster on the inner and outer edges (shoulders) of the tread, and obviously bowed-out, overinflated tires will experience greater wear on the center tread area. Cessnas with spring steel landing gear will scrub the outer shoulders more; the temporary fix is to just rotate the tyres. Once airborne, leaf springs automatically become a bit bowlegged, and upon landing, the aircraft's weight causes them to bend outward again.

The first parts of the tyres to feel it are the outside edges. As the airplane's weight transfers from the wings to the wheels, the tyres get scrubbed sideways like erasers, and like erasers, they get worn down.

Underinflated tyres are increasingly susceptible to rotational slippage along the rim. In the tube tyres on which most of us ride, that can result in a sheared valve stem - and very shortly thereafter, a flat tyre. Tyres lacking sufficient pressure experience greater load paths and flexure along the sidewalls, which can lead to stress fractures, and in the extreme case of a very hard landing it can crush them and shear the tire right off the rim. The excessive flex and rolling stresses of underinflated tires also generate greater frictional heating, which accelerates decomposition of the tyre carcass. Overinflating also stresses the tire and makes for harder landings.

Tyre pressures should be measured - big surprise - when the tyre is cool. But did you also know that tyre pressures specified in most POHs are actually for unloaded tyres? Yes; if your airplane is up on jacks, those are the right numbers, but if your airplane is actually going anywhere soon, use a number about 4 percent greater than what it says in the book. (Some airplane manufacturers do specify "loaded" values.) Also, the specified inflation pressures are minimums, according to Goodyear. The recommended operating range is more like 0 to 5 percent above the stated number.

If you took high school physics, you probably remember Charles' Law relating volume to temperature. When it comes to your tyres, if you're flying somewhere colder, add an extra 1 percent of pressure for each 5 degrees Fahrenheit (or 3 degrees Celsius) cooler that you think it's going to be there, wherever it is you're going. And the reverse might apply if you're flying someplace significantly warmer. Always correct for the "worse case," however, because a tyre can be flown at a higher pressure--within limits--but should not be flown at a significantly lower pressure than the one for which it is rated.

The type of tyre pressure gauge should be the more accurate dial kind, not the "pencil" type. And eyeballing the tyres can be deceiving, because tyres that look properly inflated before everyone piles in may turn out to be woefully underinflated. When it comes to checking for a suspected leak, the filler valve stem or the area around it is one weak point, but there could be a slow leak or crack in the tube itself, or the tire carcass. Even a healthy tyre will lose pressure. (The commonly accepted value is anywhere between 1 percent to 5 percent per day.)

And all this advice won't do you much good if there's no portable compressor or an air tank nearby, will it? So be mindful of your airplane's tire pressures, even if it's "just a rental." After all, a tyre blowout in any tricycle-gear vehicle is going to be significantly more attention-getting than when you're riding around on four of them.

Here are some additional points to keep in mind:

If your airplane has wheel pants, preflighting the tyres might take some rolling around, but you should do it just the same. Look over the entire circumference of your tires, checking the tires for nicks and flat spots. Check the cotter pins holding the wheel hub onto the axle, and brake pad thickness; check for cracks in the wheel hubs, as well as excessive rust and wear.

You can ride around on "baldies," but if any one part of the tire has worn down to the base of the grooves, it's time for a new tire. And it's absolutely replacement time if you can ever see any of the underlying reinforcing fabric. Also, if you even think that you might be landing on a wet runway on which hydroplaning could occur, you should have at least one-sixteenth of an inch of tread remaining. And if there are any bulges or cracks in the sidewalls through which you can see the plies underneath, that also means the tyre is unsafe.

Remember that heels are usually for rudder, toes are for the brakes. And generally, take it easy on the brakes.

Also, go easy on the brakes during taxiing turns and those impressive about-faces in close quarters near the tie-down spot.

Whenever you lock one wheel and pirouette like that, you're basically turning the tyre into a giant eraser.

Lastly, if you see a low tyre on an airplane you're renting, let maintenance know. If it's a flying club, tell someone who can help help fix it. Or, just get a portable compressor. They're easy to run right from a cigarette lighter, and easy to use. You just might get some mileage out of it--or your money back, in lost accident opportunities.

Take good care of the tyres on the aircraft you fly, and they'll take care of you - every time you ask them to accelerate from zero to 60, instantaneously.

Jeff Pardo, AOPA.

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