Originally Posted by
FE Hoppy
But as you can see from the data VMCA(and vmcg) is scheduled according to alt, temp and weight.
I have to agree it sure looks like that.
I'll be inquiring of our friendly local regulators, who must have approved the 170 since Air Canada have them, when they decided to allow this, because it would seem to diminish VMCA as a design requirement. At least one of our designs would stand to benefit if treated the same way. Level playing field, and so on ...
If I look at the first line, for example, and make some really simple assumptions about takeoff accel, the distance from brakes release to minVR varies as follows (if you could set Vr at VMCA):
WT 47 50 55 60 65 70 75 80
VR 97 95 92 89 86 83 80 76
D 1 1.02 1.05 1.07 1.09 1.09 1.09 1.04
So the distance run to VR is a maximum at mid weights (but only 9% above the light weight case) and drops back down to only a 4% increase at max weight.
Conversely, if you were using a single fixed VMCA of 97, the distance increase at MTOW would be more like 70%. I know I'd like a paperwork change that reduced my takeoff run by that kind of magnitude! (Recognizing that there may be other limiting factors)