[523] The FAR landing and takeoff field lengths given in table VII contain certain built-in safety margins to allow for unanticipated situations. Brief and somewhat oversimplified descriptions of these distances for dry, hard-surface, level runways in zero-wind conditions are given below.

Landing Field Length

The landing field length is defined by the Federal Air Regulations for transport-category aircraft. Briefly, the landing distance is measured, horizontally, from the point at which the aircraft is 50 feet above the surface, in steady gliding flight at an approach speed not less than 1.3 times the stalling speed, to the point at which the aircraft is brought to a complete stop on a hard, dry, smooth runway surface.^H1 The FAR landing field length is obtained by dividing the measured landing distance by 0.6 in order to account for the possibility of variations in approach speed, touchdown point, and other deviations from standard procedures.^H2 A sketch depicting the FAR landing field length is shown in figure H.1. The landing field length as defined in figure H.1 usually appears in specifications for transport aircraft designed to the criteria of FAR part 25 and is the distance employed in table VII.

Takeoff Field Length

The FAR takeoff field length, often called the FAR balanced field length, contains certain inherent safety features to account for engine failure situations. This takeoff field length is defined in several slightly....

.....different ways and is described fully in reference H1. Briefly, if an engine should fail during the takeoff roll at a critical speed, called the decision speed V₁, the pilot is offered the option of two safe courses of action. He may elect to continue the takeoff on the remaining engines, in which case, the takeoff distance is defined as the distance from the point at which the takeoff run is initiated to the point where the aircraft has reached an altitude of 35 feet. In the second alternative, the pilot may elect to shut down all engines and apply full braking. The decision speed V₁ is chosen in such a way that the sum of the distance required to accelerate to V₁ and then decelerate to a stop is the same as the total distance for the case in which the takeoff is continued following engine failure. If an engine should fail before Vi is reached, the aircraft is usually brought to a stop on the runway; whereas, if an engine fails at a speed greater than V₁ the takeoff is continued. The distances are based on smooth, hard, dry runway surfaces. A somewhat idealized sketch of the FAR takeoff field length is shown in figure H.2.

^H1 "Airworthiness Standards: Transport Category Airplanes," FAR Pt. 25 (FAA, February 1, 1965).

^H2 "Certification and Operation: Domestic Flag, and Supplemental Air Carriers and Commercial Operators of Large Aircraft," FAR Pt. 121, paragraph 121.195 (FAA, June 1974).