Under dry conditions, modern high-speed aircraft are able to land on concrete runways without difficulty. Under wet conditions, they are highly susceptible to skidding and to overshooting the landing strip. The low resistance to stopping is due to the lubricating effect of the water film between the aircraft's tires and the runway. The effect becomes more pronounced as water depths and aircraft speeds increase. The water builds up a resistance to displacement and forms, in effect, a "wedge" under the tires. This wedge tends to lift the tires away from the runway, thereby decreasing the area of frictional contact. Skid-free landings are thus dependent on two main factors: drainage and surface texture. For proper drainage there must be correct surface camber or crossfall to drainage channels of adequate size and spacing. Surface textures should have a high frictional resistance and be uniformly coarse. Texture can be provided in several ways: (1) wire combing. This the most effective and most economical texture and is done while the concrete is still plastic. The broom head of the comb should have a minimum width of 24 inches and should be filled with 32 gauge by one-twentieth inch wire tapes. (2) Diamond drum roughening is the most effective method. To ensure uniform scoring over the entire surface of the runway, the drum should be mounted on a multi-wheeled articulated frame with outrider wheels. (3) Flail grooving can also provide a concrete runway surface with adequate frictional resistance. (4) Sawing provides a satisfactory surface texture, but is frequently considered less satisfactory because a large quantity of water is needed during the operation. (5) Acid etching of a runway is not recommended. (6) The only effective surface dressing that can be applied to a runway consists of an epoxy-pitch binder containing coarse grit. Unfortunately, the high cost of epoxies makes the method prohibitive for large scale use.