The Realities and Practicalities of Adequate Rubber

Air Safety Week, Feb 21, 2005

Treadliness Is Next to Godliness

Economics is the paramount factor in aircraft tires -- retreads cost 75 percent the price of new tires yet give the same service life. The difference between aircraft tire design, inspection and retread quality and that of car tires is chalk and cheese. Only a percentage of aircraft tires can be retreaded. They are allowed only very limited defects. Automobile tires typically have only four plies of tread and two plies of sidewall cord, but aircraft tires have many more plies and other strengthening features.

An airplane tire must be built to withstand stresses and pressures far beyond those a vehicle would ever experience. Building the tire to withstand those forces results in a robust ply body (the tire minus its tread) that will outlast many treads (see box, "A Tire Cross-Section").

When a tire is worn to its tread life limits, the tire is taken to the recapping unit of a major manufacturer. Residual tread is removed from the ply body and is inspected for defects. If defective, it's discarded. Airlines usually apply a limit to the number of times it will allow a tire to be recapped. The Boeing [BA] 747 main gear tended to experience an increase in the number of tire failures once tires were retreaded more than nine times. Recaps were then limited to eight for that aircraft.

The smoke you see on landing is not where the most tire wear takes place. About 10 percent of total wear happens at touchdown, while 80 percent takes place during stopping. Friction between the tire and the runway surface during the landing roll is where most of the tread wear occurs, although the build-up of black rubber deposits in the touchdown zones seems to indicate the opposite. The remaining 10 percent wear happens when the aircraft turns off the runway and taxis to the ramp. Sharp turns cause excessive tire wear due to tread scrubbing.

In the 1960s, there was a move to make tires that would spin up during the approach phase and limit the speed differential between the tire and the runway at touchdown. The result was a tire that was more expensive and harder to balance. The reduction in wear was just a little less than 10 percent because the fins attached to the wheel did not exactly match the tire speed to the aircraft speed at touchdown. The increased cost was not offset by increased tread life. The fins made a noise that sounded like a siren as the wheels spun up in the airflow and they undesirably increased the all-important noiseabatement decibel count near the ground.

If an engineer or pilot rejects a tire it will be for a cut, flat-spot wear or insufficient tread. The latter is important when considering tire traction on contaminated runways. Runway overruns are becoming very common. Pilots cannot control the runway RCR (runway condition reading that measures friction coefficients) but they can say nay to flying with lousy tread. Traction is a large part of the "finals" solution. Braking on bald tires is like iceskating barefoot. But good tires will still blow, infrequently, normally under hard braking. One of the most common causes of tires blowing on takeoff is newbies who leave their feet up on the toe-brakes (heels off the floor) once they've gained directional control with rudder.

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