Originally Posted by
Uplinker
I have always "struggled" with V1, since it isn't a fixed point on the runway. It's physical location depends on many variable factors. So how can we be sure that we have sufficient distance left to emergency stop from just before V1 ? - that depends on factors such as acceleration, which is not measured.
In theory, it should be fairly simple with modern GPS equipped aircraft to write a routine in the FMS to use GPS data, the take-off distance calculation and aircraft airspeed, to determine a physical location on the runway for a true, dynamic V1. This could be relied on and used to provide a green GO indication on the PFD, or whatever sort of alert was thought to be appropriate.
This would take into account slow acceleration, incorrect ZFM, incorrect thrust setting, incorrect thrust achieved, etc.
Some flight planning programs do spit out the TORR and TODR. While this doesn't tell you about V1, it does give you some idea of when you should be airborne or when you should start to be concerned that you might not become airborne. Regardless, I agree with you that having this displayed to us in the flight deck as some form of indication on the PFD is a no-brainer. Surely it has to be more simple than the programing required in a ROPS.