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V1 is the speed that an aircraft may abort a takeoff so long as the airspeed is below it, or is committed to flight if it is over that speed.

V1 calculations are based only on the brakes (not sure about spoilers) being available to stop the airplane (regardless of the number of engines), why?

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  • $\begingroup$ Doesn’t that depend on certification basis and runway state? In EASA world, V1 for virtually all commercial jets flying these days, on a wet runway, is calculated taking credit for reverse thrust on operating engines. Spoilers are considered on dry and wet runways. All of these devices can be considered inoperable, but then V1 will change (except of course if V1 wasn’t determined by the stopping case to begin with). $\endgroup$ – Cpt Reynolds Apr 11 '18 at 8:50
  • $\begingroup$ Nope, thrust reverse is not allowed to be considered for V1... interesting, I am not sure about spoilers, (I hadn't thought of them) but they probably are allowed. $\endgroup$ – jwzumwalt Apr 11 '18 at 8:54
  • $\begingroup$ I’ve always thought it was a conservative distance in that if you forgot to deploy spoilers or thrust reversers you would still be able to stop within the calculated distance. Airplanes with auto spoilers include data for takeoff distance with and without spoilers. $\endgroup$ – wbeard52 May 1 '18 at 4:36
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Before V1, abort. After V1, continue. Simple, isn't it?

Brakes only during an abort, no thrust reversers. Why abort? Loss of an engine means the thrust reverser is lost on one side and using the thrust reversor on the other side would cause asymmetrical thrust and might cause loss of directional control, especially on a contaminated runway.

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  • $\begingroup$ I have to disagree as Boeing calculates the ASDR (accelerate-stop distance required) with half the reversers on wet runways. $\endgroup$ – ymb1 May 24 '18 at 0:15
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I don't have a proof for the thinking behind the rule making. But my understanding behind it is to prove that the brakes alone can do it in case the reversers are inop. But usually there are exceptions to rules:

dry runways

Dry runway takeoff data does not (with very few exceptions, such as the 737-300/-400/-500 certified to CAA rules) include credit for thrust reverse in the accelerate-stop calculations.

non-dry runways

Historically, Boeing has taken credit for reverse thrust when producing data for non-dry runways. This has been accepted by the FAA. The UKCAA rules, and now the JAA and European certification rules, also allow credit for reverse thrust under these conditions.

For the non-dry rule, Boeing uses half the engines, so for example a 747 would be based on two reversers, and a 737 on one.

Spoilers on the other hand are required so that the landing gears are compressed, if they weren't the tires will slip more. It is also true the heavier the aircraft, the better the brakes work up to their limit.

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Quotation and image source: Jet Transport Performance Methods.

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