Planning for aviation facilities requires special consideration of the type of aircraft to be accommodated; physical conditions of the site, including weather conditions, terrain, soil, and availability y of construction materials; safety factors, such as approach zone obstructions and traffic control; the provision for expansion; and defense. Under wartime conditions, tactical considerations are also required. All of these factors affect the number, orientation, and dimensions of runways, taxiways, aprons, hardstands, hangars, and other facilities.

Pavements (including the surface and underlying courses) may be divided into two classesrigid and flexible. The wearing surface of a rigid pavement is constructed of portland cement concrete. Its flexural strength enables it to act as abeam and allows it to bridge over minor irregularities in the base or subgrade upon which it rests. All other pavements are classified as flexible. Any distortion or displacement in the subgrade of a flexible pavement is reflected in the base course and upward into the surface course. These courses tend to conform to the same shape under traffic. Flexible pavements are used almost exclusively in the theater of

operations for road and airfield construction since they adapt to nearly all situations and can be built by any construction battalion unit in the Naval Construction Force (NCF).