Airframe and Powerplant

Take care of your airplane's tires — and they'll take care of you

With the exception of the bulbous, high-floatation balloon tires that are the norm for bush-style airplanes, tires for light airplanes look pretty much the same as they always have. White-sidewall tires were offered during the 1970s, but they never caught on and soon disappeared.

Today's light-airplane tires certainly aren't sexy nor do they provide zing in the same way a new do-all GPS navigator might. However, you can fly without the GPS, but in most airplanes it's pretty hard to land without the tires.

The TSO standard

Tires, like almost everything else installed on airplanes, must comply with standards. In this case the standard is a technical standard order, or TSO. The FAA's regulatory and guidance library Web site states that TSOs "are minimum standards issued by the FAA for specified materials, parts, processes, and appliances used on civil aircraft." TSOs are issued for common generic items such as airspeed indicators, aircraft fabric, skis, position lights — and tires.

The TSO for aircraft tires is TSO-C62e. One example of a TSO standard is the requirement that light-aircraft tires be able to withstand temperature extremes ranging from minus 40 degrees Fahrenheit to plus 160 degrees F without degradation. The TSO also requires that tires must be capable of withstanding an overinflation of at least four times the rated inflation pressure for three seconds without bursting. TSO-approved tires are very tough. So the first requirement when buying aircraft tires is to buy TSOed tires. All TSO-approved tires have TSO-C62 molded into the sidewall.

In addition to meeting the stringent TSO requirements, the ideal light-airplane tire must provide a smooth, comfortable ride while taxiing and still being able to withstand the violent forces generated by wheel spin-up and sidewall deflection during takeoff and landing.

Who hasn't flared a little too high during landing or failed to completely adjust for drift while attempting to compensate for a gusty crosswind? All of us are guilty of nonperfect procedures, yet light-airplane tires rarely fail us. Modern tires are so dependable and trouble-free that they're often completely ignored.

How they're built

Light-airplane tires are built of alternating layers of rubber-coated nylon ply cords laid across the tread at angles of 30 to 60 degrees — on a bias — to the centerline of the tread. Radial tires — the standard in today's automobile tires — are not yet made for light aircraft.

Light-airplane tires are ordered by specifying the section width (maximum cross-sectional width with tire inflated) in inches and the wheel-rim diameter in inches. The standard main landing-gear tire size for many general aviation airplanes, including the single-engine lines of Cessna, Mooney, Piper, and Taylorcraft, is the 6.00 by 6.

Some aircraft manufacturers list several different tire sizes that are approved for installation in the aircraft parts manuals. Several utility models of Cessna, such as the 180 and 185, list 8.00-by-6 or 8.50-by-6 tires in addition to the stock 6.00-by-6 tires. These larger cross-section tires are installed on the same 6-inch wheels as the 6.00 tires. Other tires and tube installations are approved through the supplemental type certificate process.

The second part of tire ordering is the ply rating. Although it's logical to think that the number of plies of cord must relate to this rating, this is no longer the case. With the introduction of higher-strength fibers, such as nylon, fewer plies are needed to give the equivalent strength the older cotton plies provided. The ply rating indicates the load rating of the tire, or the maximum allowable load a tire is certified to carry at its rated inflation pressure. Typical light airplane tire ply ratings (PR) are 4, 6, and 8, with 4 and 6 being most common.

The Cessna 180, which has a maximum takeoff weight of 2,800 to 2,950 pounds, requires a 6-ply-rated tire. The Cessna 172, which has an MTOW of 2,300 pounds, uses the same-size tire, yet requires only a 4-ply rating.

Some pilots mistakenly reason that if a 4-PR tire is good, then a 6-PR tire is better. This is a false economy for two reasons. The ride when taxiing will be quite a bit stiffer with the 6-PR-rated tire and higher-PR tires are heavier and cost more. The approved tire sizes and ratings are included in every airplane owner's manual or pilot's operating handbook (POH).

TSO-C62 mandates that tire manufacturers mold the following information into the sides of each tire: TSO number, balance marker (a red dot on the tire), brand name, size, load rating, speed rating in miles per hour, skid depth, tire type (tube or tubeless), and serial number. The serial number and the manufacturer date often are combined.

Both Michelin and Goodyear serial numbers begin with a four-digit code that details the production date. The first digit in the sequence identifies the last digit of the year the tire was produced — 2 signifying 2002, for example — followed by the day of the production using the Julian calendar (January 1 is 001 and December 31 is 365 unless it's a leap year). The manufacturer's name and the type of tire also are included. Other information may include a manufacturer's plant identification code. Retreaded tires have additional markings relative to the retreading.

Michelin, Goodyear, and McCreary are the main light airplane tire manufacturers in the United States. Each company offers at least two different tire grades. There is a wide range in prices, with the Michelin Air and the Goodyear Flight Custom III at the top of the ladder with the 6.00-by-6 4-PR tire costing from $135 to $145 each at well-known suppliers. McCreary's top-of-the-line tire — the Super Hawk — is offered only in a 6-PR ($90) in the 6.00-by-6-size tire. Midrange offerings are the Goodyear Flight Special II ($95 to $100), the Michelin Aviator ($100 to $105), and the McCreary Air Hawk ($45 to $50). Michelin also manufactures an economy tire under the Condor brand. Buying 6-PR tires increases these prices by a few percent.

The retread question

In years past, shoptalk went like this: Sell customers new tires and put retreads on the training airplanes in the company's fleet. This airport legend had another maxim: Never put retreads on the landing gear of a retractable-gear airplane. The thought was that the quality control of retreads was so marginal that the tires were often oversized and would get stuck in one or all wheel wells upon retraction. This is possible, especially if the retread is not tested for clearance at the correct pressure before being returned to service.

All tires, especially the bias-ply tires used on GA airplanes, must be serviced with the correct air pressure and allowed to stretch for at least 12 hours after tire/wheel buildup. Observing this stretch time period, and maintaining the correct tire pressure, is especially important when conducting gear-well clearance checks. The tires will expand if the tire pressures are too high.

A study of tire wear conducted by University of North Dakota — Grand Forks involving 58 tires installed on Piper Arrows and Warriors concluded that the Goodyear Flight Custom III (FCIII) tires averaged nearly 40 percent more landings than a Michelin Air and the Goodyear Flight Special II (FSII) they were tested against. A similar study on the same three types of tires installed on Cessna 172 trainers by Embry-Riddle Aeronautical University did not find such a wide discrepancy in wear rates with the FCIII, averaging approximately 20 percent more landings than the FSII and the Michelin Air tires before being worn out. The Michelin and Goodyear tire care manuals (available on the companies' Web sites) both emphasize that tread wear is seldom the reason light-airplane tires are removed from service. This concept is backed up by a report in the May and June 2004 issues of Aviation Consumer, which showed that it took at least 200 simulated landings before tread wear was measurable. If landings don't wear out tires, what does?

Taking care of those black donuts

Both of the major airplane-tire companies emphasize that maintaining the proper tire pressure is the single most important thing owners can do for their tires. Here's why this is important: All light-airplane tires require the use of tubes because the split-half wheels that are used on almost all small airplanes lack provisions to prevent air leakage.

The tubes, being made of natural rubber, leak air. Both Michelin and Desser Tire, of Los Angeles, a major tire supply and retreading house, sell types of tubes that slow this leakage. Tubes also stretch during use. Experience has taught that new tubes should be installed whenever a new tire is installed. If this is not done, the oversized tube will fold over on itself, which results in abrasion points and premature wear. Cost of a good-quality tube for a 6.00-by-6 tire ranges from $40 to $60.

Tire pressures are critical because overinflation increases the tire size, and because low tire pressures result in increased sidewall flexing that generates additional heat, and heat results in accelerated aging of the tire. Goodyear says that a tire/tube can lose as much as 5 percent of the inflation pressure in any 24-hour period and still be considered normal. Tire pressures should never be adjusted when the tire is hot. Again, information on tire inflation values is included in every owner's manual or POH.

According to the tire manufacturers, high-speed taxis, sharp turns, and maneuvers that create side loading should be avoided, especially during cold-weather operations. Tires should always be inspected for tire damage or slippage relative to the wheel following tire/wheel lockup, or any time heavy braking is applied during balked landings or short-landing procedures.

The chemicals in engine oil, grease, hydraulic fluid, and engine-degreasing agents have a deteriorating effect on rubber. These contaminants should be wiped off with denatured alcohol and then further cleaned with soap and water. Goodyear also cautions against using automobile-type tire dressings, saying that these products may remove antioxidants and antiozonants. Exposure to sunlight does reduce tire life to some degree. Using tire covers to reduce sunlight, oil, and grease exposure is one preventive option for airplanes that are tied down outside.

Tires are worn out when the tread is worn to the bottom of any groove at any spot. Because of the landing gear geometry of some airplanes the outboard portion of the main tire treads wear faster than the inboard tread. Some owners, especially those who are proficient at owner-performed preventive maintenance tasks, can extend the life of these tires by remounting the tires on the wheels so the portion that is less worn is exposed to the high-wear position.

The skid depth is the distance between the tread surface of a new tire and the bottom of the deepest groove. By comparing the skid depth with existing depth of a groove, an estimate of existing tire life can be made.

GA tires do not have a life limit. Nor does weather checking (a series of small cracks that can be seen on the sidewall of a tire or at the bottom of the grooves) automatically mean a tire should be removed from service. Only when the fabric matrix of the nylon plies can be seen because of a cut or weather checking should the tire be removed from service.

Rules of thumb for tire selection are that high-performance, heavy airplanes such as Cessna 210s are probably best served by a top-of-the-line tire; the most economical choice for a flight school airplane is often a retreaded tire; and the rest of us will be ably served by using a name-brand midrange tire or retread, but only if the correct tire pressures are maintained.

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