metabolix; “If lower V-speeds (V1 and Vr) are acceptable with a tailwind, then why would those speeds not have been used in the 0 wind column to start with?”
Improved Climb is used when we are climb weight limited and we have a long runway. this allows us to increase the Climb weight limit by a certain amount.
This is done by adding an increment to the normal takeoff V speeds.
we basically use up more runway than we otherwise would if we used the normal speeds this gives us better climb capabilities.
in Zero wind conditions we would need less runway to get to v1 than we would in a tailwind. so it is for that reason that we reduce the Climb weight Limit increment..
eg Normal Climb Limit = 54.6t v1:137 vr:138 v2:145 (Zero wind)
if we use imporved climb we get a new climb limit of 55.8t v1:150 vr:154 v2:159 (Zero wind) (Climb weight limit is now 1.2t more than with normal speeds)
if we had 10kt tailwind we would have
a normal Climb Limit of 52.5t v1:132 vr:135 v2:142
if we used improved climb we would have
a new climb limit of 53.3t v1:141 vr:146 v2:153 (we can take 800kg more)
So therefore we can see that because of the tailwind we would need more runway to acheive the same speed as in nil wind. So they reduce the improved climb limit from 55.8t in nil wind to 53.3t in -10kts and hence use the lower speeds.
as for the V2 not changing. i disagree. if u look at the boeing improved climb performance charts u can see that there is an increment for v1 and another increment for Vr & V2 (ie the same increment for Vr as for v2) so i dont see how mr boeing could have gotten it wrong.
I hope that helps a bit