The most definitive answer that I can give you is courtesy of the Federal Aviation Authority Takeoff Safety Guide.
V1 Speed Defined.
What is the proper operational meaning of the key parameter “V1 Speed” with regard to the Go/No Go criteria? This is not such an easy question since the term “V1 speed” has been redefined several times since commercial jet operations began more than 30 years ago and there is possible ambiguity in the interpretation of the words used to define V1.
Paragraph 25.107 of the FAA regulations defines the relationship of the takeoff speeds defined in the AFM, to various speeds defined in the certification testing of the airplane. Although the terms
engine failure speed, decision speed, recognizes, and reacts are all within this “official definition”, for our purposes here, the most important statement within this “official” definition is that V1 is determined from “
… the pilots application of the first retarding means during the accelerate stop test”
One common and misleading way to think of V1 is to say that “V1 is the decision speed”. This is misleading because V1 is not the point to begin making the operational Go/No Go decision.
The decision must have been made by the time the airplane reaches V1 or the pilot will not have initiated the RTO procedure at V1. Therefore by definition the airplane will be travelling at a speed higher than V1 when stopping action is initiated, and if the airplane is at a Field Length Limit Weight, an overrun is virtually assured.
Another common held misconception: “V1 is the engine failure recognition speed”, suggesting that the decision to reject the takeoff following engine failure recognition may begin as late as V1. Again, the airplane will have accelerated to a speed greater than V1 before stopping is initiated.
The certified accelerate-stop distance calculation is based on an engine failure at least 1 second (see note 1) prior to V1. This standard time allowance has been established to allow the line pilot to recognize the engine failure and begin the subsequent sequence of stopping actions.
In an operational Field Length Limited context, the correct definition of V1 consists of two separate concepts:
First with respect to the “No Go” criteria,
V1 is the maximum speed at which the rejected takeoff maneuver can be initiated and the airplane stopped within the remaining field length under the conditions and procedures defined in the FAR’s. It is the latest point in the takeoff roll where a stop can be initiated
Second, with respect to the “Go” criteria,
V1 is also the earliest point from which an engine out takeoff can be continued and the airplane achieve a height of 35 feet at the end of the runway.
The Go/No Go decision must be made before reaching V1.
A “No Go” decision after passing V1 will not leave sufficient runway remaining to stop if the takeoff weight is equal to the Field Length Limit Weight
When the airplane actual weight is less than the Field Length Limit Weight, it is possible to calculate the actual maximum speed from which the takeoff could be successfully rejected. However, few operators use such takeoff data presentations. It is therefore recommended that pilots consider V1 to be a limit speed: Do not attempt an RTO once the airplane has passed V1 unless the pilot has reason to believe that the airplane is unsafe or unable to fly.
This recommendation should prevail no matter what runway length appears to remain after V1
Note 1: the time interval between VEF and V1 is the longer of the flight test demonstrated time or 1 second.
Xxxxxxxxxxxxxxxxxxxxxxxxx
Lear23 has pointed out that aircraft are certified to different criteria depending on when they were certified, while this is true, you are better off forgetting about it. If you have reached V1 you are going. Forget about trying to second guess the transition times that were used in certification.
Just remember,
80% OF RTO ACCIDENTS WERE AVOIDABLE.
Finally, your local Boeing rep should be able to get you a copy of this takeoff safety guide and the video that goes with it.
Mutt.
