They’re round, dirty, probably have a growing bald spot and are under loads of pressure; and while I could easily be describing an aviation journalist, I am, in fact, talking about aircraft tires. Alas, the lowly aircraft tire is one of the most important, yet under-appreciated parts of any airplane. Regardless of the weather or the length of the flight, pilots and passengers rely on tires to perform flawlessly and deliver them safely to their destination.

And while many technicians and pilots don’t realize it, there’s a lot of rather advanced technology spinning around at the end of the landing gear. At the heart of tire advancements have been the ongoing efforts to reduce weight while getting more landings out of each tire. No doubt, with the escalating costs of fuel, those efforts are only going to increase.

But, what types of changes can the tire manufacturers make? More than you think. "We play with mold shapes and materials," explains Robert "Rob" Robson, product support manager, general aviation for Goodyear Aviation Tires. "For example our Flight Custom III line uses Kevlar belts. While adding belts usually raises the tire’s overall weight, in this case, Kevlar keeps the added weight to a minimum. The performance gains more than make up for any additional weight. These Kevlar belts, combined with an improved mold shape and two-groove tread pattern, are technical improvements that really increase the number of landings that pilots can expect from this tire."

Another evolution in tire design has been the move away from bias-ply tires towards radial-ply tires, or as Robson calls it, the "radialization" of the industry.

Both types contain fabric plies that provide casing strength; on a bias tire, the plies crisscross at alternating angles roughly 45 degrees from the tread centerline, while a radial tire’s plies run straight from bead to bead.

"A radial tire contains fewer casing plies inside and more belts on top," Robson says. "It reduces the amount of material in the sidewall of the tire, so the radial construction runs a bit cooler and therefore, lasts longer. Because of this, a radial tire also weighs less than a bias-ply tire. An easy way to reduce weight and still gain more landings is to go to a radial tire."

But like most things in aviation, the simple solution isn’t always the case. "The reason pilots haven’t flocked to radial tires is that all these advantages become less apparent on smaller tires," he adds. "Radials, unfortunately, don’t deliver any real benefits for smaller aircraft."

One problem is certification. "When you look at a single-engine piston airplane, the vast majority uses the same size and type of tire. You can take any qualified tire of the correct size and ply rating and install them on these airplanes," Robson says. "On the ‘big stuff,’ it’s an entirely different story."

When a business jet or airliner is designed, the tires are specifically developed for its airframe. The tires are assigned a specific part number; therefore, operators or technicians don’t have an option to easily switch from bias to radial tires. Since wheels and landing gear reacts differently to bias and radial tires, it becomes very difficult to re-certify a radial tire in place of a certified bias tire.

Along with the switch to new-generation materials like Kevlar and tire designs like radials, Robson says that manufacturers like Goodyear have switched from natural rubber to butyl rubber for tire tubes.

"Butyl rubber provides more than 10 times better air retention than natural," he explains. "While a natural rubber tube might lose a pound or two [of air] each day, butyl tubes retain inflation much longer."

Butyl rubber is a synthetic or elastomer compound, which is impermeable to air and delivers exceptional flex properties. It’s found in everything from inner tubes, to adhesives, to chewing gum. So next time you’re out of gum, grab a slice of an old inner tube — the flavor lasts and lasts.

Weight Loss is a Bad Thing

Earlier on Robson remarked that an aircraft’s bias-ply tire was similar in structure to an automobile tire. While an aircraft tire and a car tire contain the same basic materials — rubber, fabric and steel — that’s where their similarities end. For example, let’s look at a typical nose tire on a 737. It’s roughly the same size as the tire on a passenger car, but it’s rated for almost 10,000 lbs. load and 200 lbs. per square inch, and it’s speed rated up to 225 mph. Just don’t try putting the tires on the family Buick through those types of extremes.

"It seems that most aircraft technicians get their aviation tire ideas from their car tires," he says. "They stick air in it and forget about it for a month. They don’t realize that aircraft tires are vented to reduce pressure build-up within the casing, which means they lose pressure on a daily basis."

"It’s slight," Robson continues, "but it’s a lot more than a car tire. Tubeless tires on business jets often lose 1 to 2 percent pressure each day."

What’s the big deal about a percent or two of pressure? When dealing with bizjets it’s quite a bit. "They’re allowed to lose up to 5 percent a day, and it doesn’t take much under-inflation to damage a tire," he says. "In fact, we recommend — as do our competitors and the FAA — operators should remove a tire from service that’s been run at more than 10 percent under inflation."

"Even if you lose only 2 percent of inflation a day, in less than a week, you may need to replace the tire," Robson says. "Since the damage is evident inside the tire, the harm of under-inflation often goes unnoticed."

According to Robson, under-inflation shows up as tiny cracks in the tire’s innerliner right where the tire flexes the most. Of course, under-inflation causes the tire to flex even more than it was designed, and the added deflection generates increased heat, which ultimately leads to visible damage to the tireís interior lining.

"The innerliner, developed to improve air retention, cracks with the added heat," he explains. "So the tire loses pressure faster and that makes the whole problem worse."

Unfortunately, once a tire reaches this advanced stage, the operator is facing more than just the risk of a flat tire. "Eventually the plies will break, and at that point, the tire comes apart," Robson says.

"The tire will often come apart in three pieces." In non-technical terms, the tire will fail. Not something you want to happen at landing or take-off speeds in a bizjet. Heck, a tire failure when landing a Cherokee 140 will get your attention.

With what’s at stake, even a "math wiz" truly can figure out that losing 5 percent a day makes tire pressure vigilance a priority. "Maintaining the correct tire pressure is critical. Some pilots and technicians check tire pressure once a week, whether it needs it or not," Robson states. "I can’t overstress that you must check tire pressure daily."

And by checking tire pressure, he doesn’t mean the cursory glance you routinely give the tires during a preflight. "Good news sir, the tire is only flat on one side..." That doesn’t cut it. "By the time you can see that the tire is over-deflected (low), it’s well under-inflated," Robson says.

Robson stresses the need to use a calibrated pressure gauge. "I know the ones the airline guys use are calibrated, which of course, works until the first time you drop it," he notes. "As far as the little push-up types you get at the auto store, those can be used on small aircraft with 30 pounds of pressure in the tires. They can be accurate, but it’s a good idea to check it against a known source."

"For business jets, the tires are inflated at 200 psi, and you need a special gauge for that," Robson stresses. "At those levels, inflation pressure becomes even more critical."

Of course, there are folks out there who will want to play it safe and over-inflate the tires a bit — an extra pound or two for the wife and kids so to speak. Not a good idea. Over-inflation can cause uneven tread wear, reduced traction and make the tread more susceptible to cutting. Plus, the added inflation increases stress on the wheel itself. The only acceptable solution is to take the time to check tire pressure daily — which, as Robson has stressed, is the most important preventive tire maintenance step you can take.

One thing we haven’t covered is what you want to fill your tires with — garden-variety air or nitrogen. "Nitrogen is required in commercial service tires because it is non-combustible," Robson points out. "It’s also dry, so it’s non-oxidative to the wheel and tire compounds. I’d really recommend it for tubeless assemblies."

"Nitrogen’s advantages in small tube-type tires, however, are minimal," he continues. "In tube-type tires, the differences between air and nitrogen are more theoretical than realistic."

While the simple act of maintaining the correct tire pressure will go a long way to good tire health, there’s more to getting the most out of your tires than you may think. Robson recommends a visit Goodyear’s aviation tire website, www.goodyearaviation.com/tirecare.html for a copy of its Care & Maintenance manual.