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See these questions.

What is balanced field length?

What are runway declared distances?

My multi-engine jet airplane uses a balanced field length concept. Most airports served by airliners have declared distances.

Obviously, my takeoff distance can't be longer than the runway length. Does it have to be lower than all the declared distances? TODA? TORA? ASDA? LDA?

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  • $\begingroup$ Actually, your takeoff distance CAN be longer than the runway if you use an overrun or a clearway. Takeoff distance includes the distance required to climb to 35 feet above the ground in a jet. $\endgroup$
    – Lnafziger
    Oct 25, 2015 at 3:08
  • $\begingroup$ @Lnafziger If you are using unbalance field length calculations, I would agree. The question is about balance field length calculations. $\endgroup$
    – wbeard52
    Oct 25, 2015 at 20:57

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Actually, the take off distance given in runways will be greater than the runway distance as it will include the distance to overcome a predetermined obstacle.

Take off distance

Source: tc.gc.ca

The balanced field length is dependent on the aircraft configuration among other things and is usually available in AFM. The airfield lengths, on the other hand are made available so that operators can decide which aircraft to operate based on the runway lengths available.

In general, the ASDA should give you the available distance for taking off as it gives the required field length available in case you decide to abort the takeoff for whatever the reasons.

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    $\begingroup$ An explanation of each of the terms in the graphic would probably help the people that don't already understand this a lot. $\endgroup$
    – Lnafziger
    Oct 25, 2015 at 13:07
  • $\begingroup$ @aeroalias I would agree that the takeoff distance can be greater than the runway for unbalanced field length calculations. If that is the case which declared distance must you be lower than? $\endgroup$
    – wbeard52
    Oct 25, 2015 at 20:58
  • $\begingroup$ I wouldn't say this is really an answer... $\endgroup$
    – Caleb Woodman
    Oct 26, 2015 at 22:58
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Balanced field length calculations by definition are the greater of three factors. As Lnafziger noted in his answer to the linked questions above these three factors are:

  1. 115% All engine takeoff distance
  2. Accelerate-stop distance
  3. Accelerate-go distance

The manufacturer will choose a $V_{1}$ speed that will all the accelerate-stop and accelerate-go distances to be as close as possible. This reduced the takeoff distance to the lowest possible since the takeoff distance is the greater of those three factors.

This means that our takeoff distance must be less than certain declared distances. See the linked questions for a discussion on declared distances.

  • The TORA value may or may not equal the runway length but it will never be greater than that length.
  • The TODA is the TORA plus a clearway.
  • The ASDA is the TORA plus a stopway (it can be lower than the TORA value).

Using common sense, our accelerate-go distance cannot be greater than the TODA value and our accelerate-stop distance cannot be greater than the ASDA value.

If we take this one step further, since we have one number for takeoff and we don't know if that number is limited by the accelerate-stop or accelerate-go performance we cannot use the TODA value for the accelerate-go distance. We must use the TORA value.

Hence, for multi-engine airplanes with balanced field length calculations we are limited to the lower of the TORA or ASDA values for our takeoff distance.

As Aeroalias and Lnafziger pointed out, for unbalanced field length calculations we can indeed use the TODA value for the accelerate-go distance and the ASDA value for the accelerate-stop distance.

References:

The NBAA has some great videos on the subject. Here is the video on YouTube.

AOPA also has information on the subject

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