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
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).