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Let's say you were planning to ferry some passengers and cargo in a jet but after some calculations found out the ASDR (Accelerate-Stop Distance Required) was greater than ASDA (Accelerate-Stop Distance Available) of the only runway available in the airport. Can you address this situation by lowering V1 speed for the flight and thus shortening the ASDR?

This question has a context. People discuss the effect of a runway length on V1 on this post and seem to agree a runway length has no bearing on V1. In other words, however long runway you may have for your takeoff roll, if an engine fails after V1, you should continue the roll and take off, not the least because you never know you might be able to make a full stop without a runway excursion.

Granted what they say is true, is it as well not permitted to plan a reduced V1 speed for the runway whose ASDA is shorter than the ASDA for a maximum takeoff weight flight of the day?

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  • $\begingroup$ You can't just arbitrarily lower V1. If an engine fails after V1 you still have to have enough rwy to reach Vr then V2 on one engine. $\endgroup$ – TomMcW Sep 16 '17 at 2:10
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Yes it's a viable option, but it must be approved by the manufacturer.

Lowering V1 requires lowering the thrust as well. Because high thrust at a slow V1 means the rudder won't have enough control to counter the thrust asymmetry when an engine fails. Low thrust equals low Vmcg, which can be the lower-limiting factor for the V1.

This reduced thrust setting (derate) can't be overridden in case an engine quits.

It works for short runways and contaminated runways and may allow higher MTOW, see here:


Another trick is the opposite: to increase the V-speeds (again, must be approved). It's called "improved climb". This only works if the limiting weight is the climb limit (not the field limit).

By increasing the V-speeds you get a better climb gradient for the same flap and thrust setting, i.e., there can be two sets of V-speeds for one flap setting, e.g., Flap 15, and Flap 15 Improved.

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(Boeing)


RE: Is it really the case that a runway length does not affect V1?

Runway length affects V1 only indirectly and not always. In the takeoff performance calculation there isn't a graph or table to lookup that links length to V1. Headwind, cold weather, and high ambient pressure are some of the factors that make a runway longer than it actually is (performance wise).

The first step in performance planning is calculating the corrected length. From there there are three limiting weights: field, climb, and obstacle clearing. The lowest of the three will be the limiting weight of the day.

If the takeoff weight will be lighter than the lowest, then reduced thrust can be used to save on engine wear, for example.

Once the weight is known, then the V-speeds are checked. If the weight is between the climb and field limits, then the aforementioned improved climb calculation can be used (if approved). Likewise for the field limit and using derate.

In other words, the corrected length only affects one of the three weights: the field limit weight. Which may not be the limiting factor. If it is the limiting weight, then yes length affects V1, but it usually isn't for the big transport planes.

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  • $\begingroup$ If you really can adjust V1 depending on the runway length available, like you have explained above, is it really the case that a runway length does not affect V1? Am I mistaken about something here? $\endgroup$ – lemonincider Sep 16 '17 at 11:51
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    $\begingroup$ @lemonincider - yes and no, check update, let me know if it's not clear. $\endgroup$ – ymb1 Sep 16 '17 at 12:09
  • $\begingroup$ @lemonincider Well, it's that thrust setting and stuff influences V1 and this influences your minimum runway length. So you may adjust things to accomodate for shorter runway, but the runway length is not directly involved in the calculations. $\endgroup$ – yo' Sep 16 '17 at 12:22
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    $\begingroup$ @lemonincider Well, I'm not a pilot nor anything close, just a mathematician :-) I suppose that you simply first try the calculations with standard setup, and if it goes through (i.e., the runway is long enough), you are done. Only if you can't make it, you look for other ways how to fit in the runway with your takeoff. $\endgroup$ – yo' Sep 16 '17 at 12:40
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    $\begingroup$ @lemonincider - it's because acceleration and stopping depends on the weight $\endgroup$ – ymb1 Sep 16 '17 at 12:50

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