Effects of tread wear on the wet runway braking effectiveness of aircraft tires

The importance of a good tread design in improving the braking effectiveness of an aircraft tire on a wet runway has been well established. It is of equal importance, however, that the tread be maintained in good condition and replaced when tread wear becomes excessive. To explore in detail the effects of tire tread wear on braking effectiveness, a series of wet runway braking tests was run at the Langley landing-loads track in which smooth and dimple tires were used to represent completely worn tires. Five evenly spaced circumferential grooves were then cut into the tires to depths representing varying degrees of tread wear. Two types of tire wear were simulated: uniform wear with all grooves cut to the same depth, and nonuniform wear with the center groove wearing completely smooth while significant depths remain in the outer grooves. A gradual degradation in braking effectiveness was experienced up to about the 80% worn tire condition, at which point the wet runway friction coefficients dropped markedly. The completely worn tire was observed to develop, at the higher speeds, only about one-half the braking effectiveness of a new tire.