Anyone who has ever watched an airliner land and noticed the puff of blue smoke that appears the moment the wheels suddenly spin up from zero to somewhere upwards of 140 miles an hour (to use ground-pounder units) must wonder at the abuse those poor tires must take (as well as wondering why they don’t spin them up to somewhere around that speed, first). Heck, the tires on even our own little bug-smashers take quite a pounding, and I don’t mean from bounced landings, either.
From the standpoint of weight alone, it’s a hard-knock life for those little doughnuts. Using a four-place retractable-gear airplane as an example, the tires support the equivalent weight of a fairly sizeable car, but on one less wheel, with all three taken together having well under half of the rubber-to-pavement contact area. (The tires for the average small general aviation airplane are nowhere near the size of the average automobile.) They must endure much greater extremes of temperature, and of course, the sudden screeching spin-up I’d just mentioned is none too gentle. Nor are the comparatively much longer periods during which the tire is sitting there on the ramp or in a hangar, getting flat-footed, conducive to long life. Although we don’t usually take the aphorism literally, the very fact that the phrase ‘kick the tires and light the fires’ exists at all, even though we may recognize it as one of those exaggeratedly harsh sayings we sometimes use to affect romance and adventure, is because there is an element of truth in it. Tires take a beating, plain and simple.
When it comes to designing airplane parts and integrating them into a whole, the requirements specification for airplane tires is inevitably and inherently going to be at odds with reality. Such a specification would say that tires ‘shall’ be light, to allow a greater useful load, yet small, to reduce drag as much as possible for fixed-gear airplanes (or the needed space in the fuselage or wing for retractable-gear airplanes). 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, unlike the radial tires in our cars, the reinforcing belts for which usually run directly across the tread (which I suppose is why they’re called ‘radials’ after all). Radial tires often look ‘bulgy’ but bias-ply tires won’t betray their too-low pressure condition unless they’re really badly under-inflated. (Also, most car tires these days are tubeless; airplane tires mostly have inner tubes.) Radial construction allows automobile tires 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, and quite happily, fly and taxi around on completely bald tires-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, which I’ll get into shortly). Don’t try that in your family car!
The system for sizing and describing piston airplane tires is thankfully fairly easy to understand (although the one for airliners and business jets isn’t). A so-called 6.00-6 tire has a cross-section or width of six inches and is made to fit on a rim that is six inches in diameter. There is a bit of leeway in those dimensions; such a tire might have a diameter anywhere between just under 17 inches up to about 17 and a half inches, and a width anywhere between just under six inches to about six and a third inches. (The FAA’s Technical Standard Order or TSO number C62 covers tire specifications, including performance standards and requirements.) The ply rating system can be a bit confusing. The number of plies a tire 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 tire at 29 psi might be able to carry 1,150 pounds, and the 8-ply version might be rated for over twice that, using an inflation pressure of 55 psi. One might conceivably opt for the sturdier 8-ply tire, since the more rugged version weighs only a couple of pounds more, but you will 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 tires young? As with our cars, the one biggest thing is, again: proper tire inflation pressure. Under-inflated tires will wear faster on the inner and outer edges (shoulders) of the tread, and obviously bowed-out over-inflated 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 tires.) Under-inflated tires are increasingly susceptible to rotational slippage along the rim, which in the tube tires on which most of us ride, can result in a sheared valve stem…and very shortly thereafter, a flat tire. Tires 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 under-inflated tires also generates greater frictional heating, which accelerates-and I know this sounds creepy-faster decomposition of the tire carcass.
Tire pressures should be measured-big surprise-when the tire is cool. Well, you knew that already, didn’t you? But did you also know that tire pressures specified in most POHs are actually for unloaded tires? Yes; if your airplane is up on jacks, those are the right numbers, but if your airplane is actually going to go anywhere soon, use a number about four percent greater than what it says in the book. (That said, some airplane manufacturers do specify ‘loaded’ values.) Also, the specified inflation pressures are minimums, according to Goodyear. The recommended operating range is more like minus zero to plus five 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 tires, if you’re flying somewhere colder, add an extra one percent of pressure for each five Fahrenheit (or three Celsius) degrees 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’ though, because a tire can be flown at a higher pressure (within limits of course), but should not be flown at a significantly lower pressure than the one for which it is rated (more on that in a moment). The type of tire pressure gauge should be the more accurate dial kind, not the ‘pencil’ type. And eyeballing the tires can be deceiving, because tires that look properly inflated before everyone piles in may turn out to be woefully under-inflated, the next time you look. 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 (and this is actually the term in use) the tire carcass. Even a healthy tire will lose pressure (the commonly accepted value is anywhere between 1% and 5% per day). And to wrap all this tire pressure stuff up, all this advice won’t do you much good if there’s no portable compressor or even an air tank nearby, will it?
I can’t say anything other than that you ought to be mindful of your airplane’s tire pressures, and even if it’s ‘just a rental’ you should still apply the same scrutiny to your landing gear. After all, a tire blowout in any tricycle gear vehicle is going to be significantly more calamitous than when you’re riding around on four of them. Along with that, here are some additional points to keep in mind:
- If you have wheel pants, pre-flighting the tires might take some rolling around, but you should do it just the same. Wheel pants or no, look over the entire circumference of your tires during preflight.
- 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 to re-tire (ouch). And it’s absoultely 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 where hydroplaning could occur, you should have at least one-sixteenth of an inch of tread remaining. And if you see any bulges or cracks in the sidewalls through which you can see the plies underneath, that also means the tire is playground food (or whatever might be its recycling fate).
- Remember that heels are usually for rudder, toes are for the brakes. (I’m not joking; it’s easy to confuse the two, or unintentionally apply braking during rudder applications.) And generally, take it easy on the brakes. Showing off a short-field landing is one thing, and if it’s your money and your tire, knock yourself out. But if it ain’t your baby, you should treat it like one.
- Also, go easy on the brakes during taxiing turns and those impressive about-faces in close quarters near the tie-down spot. (I know; we’ve all done ’em…) Whenever you lock one wheel and pirouette like that, you’re basically turning the tire into a giant eraser, and guess what happens to the rubber.
- Lastly, if you see a low tire on that next rental, squawk it. If it’s a flying club, tell your crew chief. Or, just get yourself a Trak Auto, Pep Boys, or Sears (or some kind) 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.