Balanced Field Length

During the takeoff run the pilot of a jet transport has the option of aborting the takeoff up to the speed Vj. Above this speed, in the event of an engine failure, the takeoff should be continued. Figure 7.11 (from Ref. 7.1) clearly illustrates these options. A balanced field length is defined as one where the distance to continue the takeoff following the recognition of an engine failure at Vi is equal to the distance required to stop if the takeoff – should be aborted. On Figure 7.11, the field length is balanced if the sum of seginents В and C equals the sum of D and E. FAR Part 25 stipulates the field—length to be the greatest of the accelerate-and-go distance, the ac- celerate-and-stop distance, or 115% of the all-engine-operating distance to a 35-ft height. The stop-and-go portions need not be balanced.

The distance to stop can be found by numerically integrating Equation 7.1. In this case, T will be negative and equal to the reverse thrust. ^ is the braking coefficient and, with antiskid systems, can be as high as 0.6 on a dry, hard surface. Detailed considerations relating to the deceleration of an air­plane will be deferred until the later section on landing distance.