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Field length is balanced when…
  • A

    one engine acceleration from V1 to VLOF and flare distance between VLOF and 35 feet are equal.

  • B

    calculated V2 is less than 110% VMCA and V1.

  • C
    one engine inoperative take-off distance equals one engine inoperative accelerate stop distance.
  • D
    all engine acceleration to V1 and braking distance for rejected take-off are equal.

Refer to figure.

BALANCED FIELD LENGTH
When the Accelerate-stop Distance Available (ASDA) is equal to the Take-off Distance Available (TODA), this is known as a Balanced Field Length. Likewise, when the ASDA and TODA are different lengths, this is known as an Unbalanced Field Length.

BALANCED FIELD TAKE-OFF
A balanced field take-off is performance based and is a condition where the Accelerate-stop Distance Required (ASDR) is equal to the Take-off Distance Required (TODR) for the aircraft weight, engine thrust, aircraft configuration and runway condition.
The balanced field length is the shortest field length at which a balanced field take-off can be performed.
  • A Balanced field length is used to simplify the field-length take-off mass. When applying this concept, TOD = ASD and V1 = V1 Balanced . And this means that we are using the minimum requirements in case of engine failure, thus increasing safety margins. When using stopway or clearway, extra distance will be available and V1 will vary.

BALANCED V1
The first graph on the attached figure, helps understanding the impact of increasing or decreasing V1.
Analyzing it, we can conclude the following:

  • A Balanced V1 is a V1 speed which results in TODR equal to ASDR. Often called the “Ideal V1” as it gives the minimum field length required for a given weight and optimum performance.
  • Any lower V1 (V1 < V1B) would increase TODR, as it would take longer for the aircraft to accelerate to V2 with OEI. As a result, the total field required increases.
  • Any higher V1 (V1 > V1B) would increase ASDR due to a higher energy absorption by the brakes. As a result, the total field required would also increase.

=> In essence, Balanced V1 improves field length limited take-off performance.
=> Unbalanced V1 allows for higher take-off weights by taking advantage of any runway and clearway or stopway available in excess of balanced field length.

Summary and extra info:

  • If TOD = ASD, V1 = BALANCED, Balanced condition.
  • If TOD > ASD, V1 < V1 BALANCED, Unbalanced condition.
    Clearway, no stopway = higher mass and lower V1 speed
  • If TOD < ASD, V1 > V1 BALANCED, Unbalanced condition.
    Stopway, no clearway = higher mass and higher V1 speed.

TORA (Take-Off Run Distance Available) => RUNWAY only (no STOPWAY or CLEARWAY)
TODA (Take-Off Distance Available) => TORA + CLEARWAY
ASDA (Accelarate-Stop Distance Available) => TORA + STOPWAY

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