The balanced field length is defined as the runway length at which the distance required to continue the takeoff (TODR) is equal to the distance required to abort the takeoff and come to a full stop (accelerate-stop required, or ASDR). Theoretically an aerodrome which has no stop way or clear way bas a balanced field length.

Concept of Balanced Field Length

For an airplane taking off, if an engine failure occurs, the timing of this failure significantly impacts the required distances for a safe takeoff or abort.

Scenario 1 Late Engine Failure: If the engine failure occurs later in the takeoff roll, when the aircraft has already reached a higher speed with both engines running, a greater Accelerate-Stop Distance Available (ASDA) will be required. This is because the aircraft will need more distance to decelerate and come to a complete stop from a higher speed. Conversely, if the engine failure occurs later, less takeoff distance will be required. The aircraft, having accelerated on both engines to a higher speed before the failure, will require a shorter remaining distance to achieve the necessary speed for lift-off and reach screen height.

Scenario 2 Early Engine Failure: If the engine failure occurs early in the takeoff roll, less Accelerate-Stop Distance Available (ASDA) will be required (as aircraft speed will be less). Conversely, more takeoff distance will be required in this scenario. With one engine failed early, the aircraft will need a longer distance to accelerate on the remaining engine and achieve the necessary speed to unstick and reach screen height.

Considering these two scenarios, there will be a critical speed at which, if an engine failure occurs, the takeoff distance required will equal the accelerate-stop distance required. This specific distance, whether it pertains to takeoff distance or ASDA, is known as the balanced field length.

For a given weight and conditions, the balanced field V1 provides the optimum performance.

Unbalanced Field

Not all aircraft have the same performance characteristics, so a runway that is balanced for one aircraft might be unbalanced for another. V1, the decision speed, may be constrained by other critical speeds such as Vmcg (minimum control speed on the ground), Vr (rotation speed), or Vmbe (maximum brake energy speed), which can limit the extent to which the field length can be optimized.

If the takeoff aerodrome does not provide a balanced field length, balanced field data can still be utilized by assuming a balanced field length equal to the lesser of the available Takeoff Distance Available (TODA) and Accelerate-Stop Distance Available (ASDA). This approach allows for a practical application of balanced field principles even when the actual field length is not perfectly balanced, ensuring that safety and performance considerations are maintained within the constraints of the available runway.