General Aviation: Don’t Know, Don’t Care: Tires

In the air, tires are just excess baggage. Unfortunately, that’s how many pilots feel toward their rubber doughnuts, and that is not a good thing.

During an annual inspection, tires get a good look, and proper inflation is assured. For most of the rest of the year, tires are at the mercy of the pilot’s preflight, which too often consists of a look and maybe a kick.

Proper tire pressure is the most-important aspect of tire maintenance. Too-high pressures are seldom encountered on private aircraft, however, a highly-loaded tire, underinflated by just a few percent, can fail before the takeoff roll is completed.

In order to ascertain the extent to which improper inflation afflicts small-GA airplanes, I asked about 60 pilots at a recent Sun ’n Fun fly-in if it would be OK to check their tire pressures. Of the 60 airplanes I checked (most were certified designs, by the way), only two were running pressure within five percent of spec. Nearly all were 10 percent or more below recommended pressure.

Don’t immediately blame the pilots. As students, they were told about tires’ importance, but their instructors seldom, if ever, had them put gauge to valve stem. Unfortunately, the tire that was so lightly loaded under their Cherokees will be running at its maximum performance on their VLJs, and their habits won’t have changed. Potential trouble? You bet.

Overinflation

Too much air reduces the tire’s contact patch, reduces traction, and stresses landing gear. A pilot will notice a harsh ride and ineffective braking. Extreme overinflation can cause catastrophic and unpredictable casing or wheel failure.

Underinflation

Underinflation forces the sidewalls to deflect excessively, building heat rapidly. The tire’s footprint decreases as the center of the tread folds inward towards the wheel; braking, wet-strip performance, and resistance to side loads will decrease. Underinflation lengthens the takeoff roll and reduces the tire’s load capacity. Lateral scrubbing increases exponentially as pressure drops below optimal.

Overdeflection increases exponentially as tire pressures decreases below optimum. Because of their different missions, airplane tires are designed to deflect a lot more than car or truck tires — two or three times more, so it is important that they do not deflect more than that.

Bias-ply vs. Radials

The debate may rage on, but the merits are clear: radials offer a lot of advantages, which is why the manufacturers are offering radials on all their new offerings.

Bias-ply tires, by their nature, flex in their tread area. Radials do their flexing on the sidewalls, promoting a cooler tread. Bias-ply tires also "grow" a lot more than radials at speed. (Some takeoff checkoff checklists still include a tap on the brakes, so the tire will stop spinning and fit into the wheel well.)

Radials seem to handle FOD better in tests, too, possibly because their supportive carcass can more-easily conform to irregularities, transferring less stress to adjoining threads.

So, why aren’t all sizes available as radials? Costs of compliance with the governing Advisory Circular (AC 25-22) are prohibitive, even for aircraft as populous as B-737s. That’s why both Goodyear and Michelin make both radials and bias-ply tires. As more new sizes come on line, expect them to be radials.

Experts from Michelin and Goodyear Offer A Lot of Advice

Don’t mount tires with any type of petroleum product. Oil degrades rubber. Oil doesn’t dry, and a slick surface can allow a tire to rotate on the rim, building heat, losing air, decreasing braking and destroying balance. Try a dedicated mounting lube (e.g., Slyde, Murphy’s Black Lube, Tire-Slick). Only in a pinch (bad choice of words) should you use liquid soap.

Don’t wash your tires unless you have to (e.g., oil contamination). They are coated with an anti-ozone slime that oozes out, looks bad and works great. When you wash that off, you invite ozone to attack the rubber, cutting the long-string molecules in the rubber carcass. After repeated washing, you’ll notice black dust accumulating — that’s your former tire. (Interestingly, tire manufacturers use a longer-string molecule in the sidewalls, and shorter molecules in the thicker tread area. It’s the long-string molecules that are degraded fastest.) If you must wash your tires, follow with a proper protectant. When they stop looking slick, reapply the protectant (e.g., Michelin’s Tire Care Spray).

If you’re running near or faster than 200 mph on the ground, inflate with nitrogen only. Nitrogen is relatively inert; shop air expands when heated.

Don’t let your pants get in the way. If a tire is going to lose air (and they all do), using wheel pants to obscure the valve stem won’t help.

Don’t cheat yourself on inner tubes. Good tubes will hold air better, balance better, stand up to heat better, and perhaps resist punctures better. Change the tube whenever you change the tire.

Tires lose up to one percent of pressure a day. Each five degrees Fahrenheit drop typically robs another percent from your tire pressure. Wheels and safety plugs, too, can leak.

Hot tires have higher pressure, so check your pressure before the first flight of the day. Don’t lower the pressure in a hot tire either — unless you’re removing the tire from the wheel or the airplane.

Avoid flight immediately after changing a tire. New tires stretch (thus losing pressure, up to 10 percent) on the first day after inflation; and if you’re using tubes, some air will be trapped for a day or so between the tire and tube on installation. So, be sure to check the tire pressure on the day following a tire/tube change. If you don’t have to fly or even rest the airplane’s weight on the tire that day, don’t.

Valves can get old, are affected by fluids that leak down the gear legs, and need periodic replacement. If you run tubeless tires, replace the valve stems each time you replace tires. Oh — replace the valve core and dust cap, too, for the same reasons.

Look at the tread and sidewalls of all three tires, before every flight and after any landing that is unusual — off-field, wet, short, or sideways.

Not Every Tip is Designed to Take Time or Cost You Money

Mild flat spots, as long as they do not penetrate beyond normal wear limits or cause excessive vibration, may be tolerated.

Don’t buy a higher-rated tire than your aircraft needs. It will be heavier and more-expensive, and it will add nothing to performance.

An otherwise-good tire may show excessive (but within limits) wear around its outside or inside tread, usually from a chronic camber-induced load condition (like a six-place airplane that is often flown light). It’s generally OK to dismount that tire and turn it around, to concentrate wear on the other edge.

Don’t use differential braking; increase turn radius as much as possible; braking is scrubbing; turning is scrubbing.

Avoid turning off at first taxiway — promote low lateral forces.

If you don’t pay attention to your tires, at least operate on grass strips. Grass is much more forgiving than pavement.