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Home > Air Safety > Why can*t a pilot abort the takeoff after the decision speed if the runway is long?
Why can*t a pilot abort the takeoff after the decision speed if the runway is long?
Aviation - Air Safety
Monday, 12 December 2005 00:24

Dear Captain Lim,

I hope everything is fine with you.

I*ve read all your FAQ regarding V1 speed but there is still a point that needs to be clarified for me. When we talk about V1 we are talking about a speed value that is the upper limit (but not included) when the pilot could safely abort a takeoff.

When I think of this I always consider the length of the runway much more than the speed itself. Consider, for instance, that the plane reached V1 in the first third of the runway having 2/3 of it left to slow down and stop. There would be no risk in this case as there is a long runway left ahead (2/3).

On the other hand, consider, for instance, that the plane did not reach V1 until 3/4 of the runway length, in which case it would be dangerous to try to abort as there is only 1/4 left to slow down and stop, even in a speed below V1. (I think in this case it becomes a matter of prayer to reach VR before the runway end).

So, how is V1 calculated? Does it consider the runway length? Does V1 need to be reached at least before the middle point of the runway? Are there any marks in the runway indicating these points?

Sometimes I see planes rotating almost in the end of the runway barely clearing the 35 feet virtual wall. So, what is the relationship between V1 and the runway length, if any?

By the way, in FAQ 41 (last updated), someone mentioned "Embracer 135". The correct name is "Embraer 135". Embraer is our Brazilian airliner factory that produces regional jets as ERJ 135, 145, 170, 175, 190 and 195, Legacy (business jet), among some other military models.

Please visit this site: http://www.embraer.com.br/english/content/home/

Thank you so much, and congratulations for your new website!

Carlos Moreira
Rio, Brasil.

Hi Carlos,

Firstly, let us refresh what is V1 for the benefit of non-aviators. To a pilot, V1 is also known as the decision speed - meaning, a speed reached during the take off when he must make a decision to either abort or continue the take off safely in the event of an engine failure. So, if an engine should fail at the decision speed (V1) during the take off, the pilot is offered two safe courses of action: He can choose to continue the takeoff on the remaining engines or elect to stop the plane by applying full braking. Either way is safe.

For calculation and performance sake, the take off distance starts from the point the take off run is initiated to a point when the aircraft has reached 35 feet. This is also known as the balanced field length. Hence, the decision speed (V1) is used in such a way that the sum of the distance required to accelerate to V1 and then decelerate to a stop is the same as the total distance when the takeoff is continued following engine failure.

Thus, it is a normal practice that, should an engine fail before V1 is reached, the plane is usually brought to a stop on the runway, whereas, if an engine fails at a speed greater than V1, the takeoff is continued.

Why can*t one stop after V1 if the runway is long? Well, overwhelming number of accident statistics have shown against such a decision. Yes, a long runway may physically give the pilot that comfort, but legally he should follow the rules regardless of the runway length.

Even before departure, a pilot knows how much runway he requires for the take off. There are no markings on the runway where he must look out in relation to the V1 as the performance is precisely calculated. If the calculations indicate that the balanced field length is insufficient for the particular aircraft weight and surface temperature, then the payload must be reduced in order to take off safely.

There is of course a relationship between V1 and runway length. The higher the V1 (so would the VR and V2), the longer the runway is required.

I know you are trying to find out why one shouldn*t abort after V1 even if the runway is longer than the balanced field length. Well, this is against conventional wisdom. The chances of a disaster are greater than continuing with the take off after the decision speed. Continuing will enable the pilot to climb to a safe altitude, sort out the emergencies and return for a safe landing.


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