Re all those discussing "reaction times". You emphatically do
not have two seconds reaction time at V1.
25.107(a)(2) requires that V1 be not less than Vef (engine failure) plus pilot reaction time.
Therefore the reaction is to the engine failure - not to the point at which V1 is attained. And the time is not necessarily two seconds either - it's however long the test pilots took during certification testing. (AC25-7A directs that the time used for calcs be not less than one second).
The notion of a two second reaction time is likely coming from a reading of 25.109, which states ...
§ 25.109 Accelerate-stop distance.
(a) The accelerate-stop distance on a dry runway is the greater of the following distances:
(1) The sum of the distances necessary to—
(i) Accelerate the airplane from a standing start with all engines operating to VEFfor takeoff from a dry runway;
(ii) Allow the airplane to accelerate from VEF to the highest speed reached during the rejected takeoff, assuming the critical engine fails at VEFand the pilot takes the first action to reject the takeoff at the V1for takeoff from a dry runway; and
(iii) Come to a full stop on a dry runway from the speed reached as prescribed in paragraph (a)(1)(ii) of this section; plus
(iv) A distance equivalent to 2 seconds at the V1for takeoff from a dry runway.
Yet a review of AC25-7A reveals that it states quite categorically that the 2 second distance allowance is not a "pilot reaction" time and must not be considered as extending the decision-making and reacting process past V1 ...
.... Thus it can be seen that V1 is not only intended to be at the end of the decision process, but it also includes the time it takes for the pilot to perform the first action to stop the airplane. The purpose of the time delays is to allow sufficient time (and distance) for a pilot, in actual operations, to accomplish the procedures for stopping the airplane. The time delays are not intended to allow extra time for making a decision to stop as the airplane passes through V1. ...
(Section 11(c)(3) of AC25-7A chg1, page 80-6, for those who wish to see it in its full context)
So AT V1 you MUST have taken the first action to stop the plane (if you're going to abort) and you have two further seconds to complete ALL the actions required to stop the plane if you are to achieve the OEM's calculated stopping performance. Any slower timings and you are not guaranteed to remain on pavement.
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Given all that, it makes perfect sense for the V1 call-out to begin before V1 - for it to be useful, the fact that V1 has been passed must be processed by the PF at the point of V1, if not before. If the PNF starts to call "Vee-one" as V1 is attained then by the time he has finished, and the PF has processed the info - perhaps 0.5 seconds for reasonable human reaction times - it's already too late.