This entry gives the total number of airports with unpaved runways (grass, dirt, sand, or gravel surfaces) by length. For airports with more than one runway, only the longest runway is included according to the following five groups - (1) over 3,047 m (over 10,000 ft), (2) 2,438 to 3,047 m (8,000 to 10,000 ft), (3) 1,524 to 2,437 m (5,000 to 8,000 ft), (4) 914 to 1,523 m (3,000 to 5,000 ft), and (5) under 914 m (under 3,000 ft). Only airports with usable runways are included in this listing. Not all airports have facilities for refueling, maintenance, or air traffic control. The type aircraft capable of operating from a runway of a given length is dependent upon a number of factors including elevation of the runway, runway gradient, average maximum daily temperature at the airport, engine types, flap settings, and take-off weight of the aircraft.