Inside the little black doughnuts
If the hoary preflight aphorism about "kick the tires and light the fires" is even partly put to use, then aircraft rubber gets little of its due. Consider this: Aircraft tires on a four-place retractable must support the weight of your average luxury car, but they do so on contact patches about half those of an economy car's. They must also endure temperature extremes and be capable of living through difficult duty cycles — that is, from sitting there cozy in the wheel wells to screeching along at landing speed, with little time to spool up. Aircraft tires must equally well endure long periods of sitting on the ramp or in the hangar and punishing landings that would send ordinary tires into rubber dust. And yet tires for light aircraft must accomplish all of these performance goals while still using bias-ply construction and being fitted with inner tubes.
A tire designer has his work cut out for him. Not only do the airframe makers want the tire to be light — any heft wasted here comes out of the useful load, after all — but as compact as possible. In-wing real estate is precious; even for fixed-gear airplanes the tire's profile is important — the smaller the tire, the smaller the necessary fairings and therefore the lower the potential drag. Moreover, in retractable applications, the tire designer must carefully control tire growth — a serious consideration with highly loaded tires — so that the wheels will fit into the wells immediately after a high-speed, squealing-from-the-crosswind takeoff.
To accomplish these goals, aircraft tires are designed to be quite rugged. They employ several sets of reinforcing plies — these are made from special nylon fabric impregnated with rubber — laid under the tread on a bias that is usually between 30 and 45 degrees to the rolling axis. (Radial tires, now the norm in automobiles and motorcycles, employ reinforcing belts that run directly across the tread. Radial construction is generally more effective at maintaining the outer diameter of the tire at high speeds and makes for a flatter tread surface.) Bias-ply tires also employ multiple beads (a stranded cable that helps this portion of the tire to keep its shape), where radials have only one on each side of the rim. Depending upon whether the tire is intended to run tubeless or use an inner tube, it will have either a vulcanized liner to block air seepage or a lighter liner that merely prevents the tube from chafing on the inside of the tire.
Tires come in a variety of sizes, of course; most general-aviation aircraft use tires described by Type III classifications. (Airliners and business jets have different, somewhat more sophisticated sizing terminology.) Our system is pretty simple — a 6.00-6 tire is one with a nominal cross section of 6 inches that's designed to fit on a 6-inch-diameter rim. But there is some slop in the numbers. That same tire, according to Michelin specifications, can have an overall diameter of between 16.8 and 17.5 inches; the sidewall width can range from 5.9 to 6.3 inches. These limits are set down by the technical standard order under which aircraft tires are approved, usually TSO C62b.
Tires are also available in various ply ratings. It's important to understand that the ply rating does not indicate exactly how many plies the tire actually uses; in application, most tires have fewer plies than the rated number implies. Instead, the ply rating is indicative of the load the tire can carry. For example, a Goodyear Flight Custom II in size 6.00-6 with a 4 ply rating is good for 1,150 pounds at a 29 psi inflation pressure. The same model and size with a 6 ply rating can carry 1,750 pounds on 42 psi. An 8-ply version is rated for 2,350 pounds at 55 psi. Why not just run a higher-rated tire on your airplane? Weight is one reason — the 4-ply tire weighs about 9 pounds, compared to 11 for the 8-ply model. Moreover, a light airplane riding around on the heavier tire at the recommended inflation pressure will experience a punishing ride. There is a real possibility that during landing or an encounter with a pothole this more rigid tire will transmit shock loads that could damage the landing gear. Stick with the ply rating called for by the airframe manufacturer.
There's one item on the maintenance list that can help prolong the life of your tires more than anything else — keeping them inflated properly. Walk down the tiedown line sometime and take note of how many airplanes sit with underinflated tires. (And we're not just talking about the derelicts here, either.) Worse, many of these airplanes will go flying with the tires woefully underinflated.
The tire manufacturers recommend checking the pressure once a day for airplanes that make many flights each day, or at every preflight for lower-utilization aircraft. That means getting under the airplane with your tire gauge — preferably one with a dial indicator, not one of those Pep Boys pencil-type specials. But how many pilots do this routine so rigorously? Very few. Many old hands try to remember how the wheel looks on the rim, or how far the inflated tire protrudes from the wheel fairing, to get a rough idea of the inflated pressure. But this isn't always the best method. Tires on a lightly loaded airplane can appear to be properly inflated, only to appear substantially flatter when the passengers, baggage, and fuel have been loaded.
There are specified ways to check tire pressure. All of the inflation pressures are determined for cold tires; that is, a tire that has not operated under load for at least three hours. According to Goodyear service literature, when a tire is going to be moved to a different climate, you should make a slight correction in pressure. Always correct for the worst case — that is, set the tire so that it is at least at the rated inflation pressure for the takeoff or landing at the cooler location. In other words, the tire can be operated at a pressure slightly higher than rated, but should not be flown under the pressure. The skew factor, incidentally, is 1 percent pressure change for every 5 degrees Fahrenheit change in ambient temperature; increase for cooler climes.
There are two more skew factors to keep in mind: Some airframe manufacturers specify tire inflation pressures loaded, and some call out the unloaded figure. A fully loaded tire should be inflated to about 4 percent above the rated unloaded pressure. Finally, your tires should hold pressure well. A loss of more than 5 percent a day is cause for further inquiry. A newly mounted tube-type tire can be expected to lose some of its air initially because there is slight growth of the tire during the initial takeoffs and landings, and because any air trapped between the tube and the inner liner of the tire will eventually be released through use.
If a tire has persistent air loss, chances are it comes from one of several maladies. Either there's a leak at the valve stem or core — a condition that can be verified by soaking the area with water (or soapy water) and looking for bubbles. Or there may be a fissure in the tube itself; sometimes wetting the bead area will disclose that as the culprit; most often, though, you'll just have to dismount the tire to inspect the tube.
The perils of rolling around on underinflated tires are well known by the tire makers. They include:
Creeping or slippage on the rim. For a tube-type tire — which most of us have — severe creeping will result in a sheared valve stem and a flat tire. Sometimes this happens after the takeoff and you don't realize that the tire's flat until you hear funny noises on the landing. (Or, worse, can't get the gear down.)
Uneven tread wear. An underinflated tire will quickly scrub the tread from the shoulder areas, greatly reducing the useful life of the tire. You may still have enough tread down the center line, but the tire is showing cords near the edges. (Don't confuse uneven wear from underinflation with particular wear patterns caused by airplane idiosyncrasies. Cessnas with the spring-steel gear, for example, tend to wear the outer shoulder of the main gear first because the gear splays outward and scrubs the tire sideways as it takes up the airplane's weight. You can simply turn the tires around, put the more worn shoulder to the inside, and still get some use from the rubber.)
Crushing of the sidewalls. The air in the tire acts as a springing medium, and when the tire is underinflated, the tire load paths become more critical. Moreover, an underinflated tire will flex the sidewalls more radically, leading to stress fractures of the sidewalls. A hard landing committed on an underinflated tire can crush the sidewalls between the runway and rim, cutting or shearing the side of the tire.
Excessive heat. A tire that is allowed to flex greatly under rolling stresses will create tremendous internal heat. This will promote carcass degradation and could lead to inner-tube blowout. Moreover, this heat will be transmitted into the brakes and wheels, where it will surely do no good.
Conversely, you should not routinely fly with the tires overinflated. High inflation pressures can degrade ride quality and traction, as well as make the tire more susceptible to cuts and bruises.
Usually, aircraft owners become concerned about tires only when the maintenance shop says that it's time for new ones. (Really, so soon?) The criteria for retiring tires, so to speak, are pretty straightforward — breaking down into two main categories, tread wear and carcass condition.
Tires should be retired when the tread, according to Goodyear literature, "has worn to the base of any groove at any spot." If hydroplaning is a concern, the tire manufacturers recommend replacing tires when they have worn to a nominal tread depth of 2/32-inch. Also be watching for uneven wear, flat spots, and any evidence of the reinforcing plies' seeing the light of day. Living with flat spots is the pilot's decision, save for when one of those brake-locking events uncovers the fabric; only then must you replace the tire. Be looking for bulges in the sidewall and excessive sidewall cracking caused by ozone intrusion. For a great many airplanes, this sidewall deterioration determines the life of the tire, given that it doesn't get enough landings in a year to wear off the tread. The rule here is that any reinforcing plies showing through cracks in the sidewalls call for tire replacement. Finally, watch for evidence that the tire is making contact with the airframe (in a retractable) or fairings (for a fixed-gear), because these chance encounters can lead to damaged treads and sidewalls and could leave you with a flat at the worst possible time.
The pilot should consider a few of these recommendations for improving tire life.
Go easy on the brakes. Heavy braking after touchdown heats the brakes, wheel, and tire considerably, leading to premature deterioration. Moreover, the heavy brakers stand a better chance of flat-spotting a tire, which at the very least is bad form. Stay off the brakes until you really need them.
Use caution on braking turns during taxi. Those impressive tight turns in the tiedown spot show that you are the master of your airplane, but they are sheer hell on the tires. When you make turns with one tire locked (or nearly so), its tread is swiped across the pavement pretty violently. Not a kind treatment of the tread.
Jack the airplane during periods of non-use. Tire manufacturers say to jack an airplane if it's not going to be used for a week to two weeks; more practically, you should be certain that any airplane that will sit for so long will either get jacked (to relieve the weight from the tires) or will need new tires when you return it to service.
Mind the inflation pressure. Tire manufacturers make this point over and over. Enough said?
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