Originally Posted by
Speedwinner
2. Old discussion: but does the cwc have a real big effect on the vmcg? Example B 737 wet runway and v1 is equal to vmcg. Boeing says that there are enough margins calculated. Is that true? what about 25kts tailwind which is limit at out airline company?
Depends somewhat on aircraft, but Vmcg at test conditions may have a sensitivity in excess of 1knot VMCG per 1 knot Xwind. So a typical max demonstrated type Xwind of the order of 30 knots will increase the 'real' Vmcg by the same kind of magnitude.
Boeing are slightly talking out of the side of their mouth - but then so is everyone else when asked the same question. The reality is that a large enough xwind component with a critical engine failure and other factors against you (such as cg and runway state) will VERY quickly have you off the side of the runway. Pretty much true for every transport category aircraft - they all meet pretty much the same rules (VMCG demonstrated in calm conditions, NWS off, aft cg) so they all have pretty much the same margins.
The regulators (not the OEMs, it's ultimately a regulatory decision) accept this state of affairs as an acceptable risk, given the inherent unlikelihood of that bad day happening, and the massive operational impact of trying to provide absolutely safety (something that can't be attained anyway).
Any tailwind effect will be far less marked - the Vmcg is basically an airspeed, so at the same airspeed you'd have a higher groundspeed, so MIGHT get a propritionally larger deviation since the aircraft-to-runway relationship depends somewhat on speed over the ground. (This might also imply that Vmcg should be a function of altitude, all other things being equal). But I doubt it's anything like as powerful as xwind.