Swings and Roundabouts
Dear Kijangnim,
We are talking here of situations requiring the lowest possible V1 due to various reasons such as the short RW, or lower brake effectivity for reasons such as contaminated RW or MEL (eg. antiskid, some brks deactivated). When the reduction of this V1 is limited by Vmc considerations the derate (TO1 TO2) option may allow take-off in such a situation because of the lower Vmc figure as compared to full TO or full TO with ATM and the smaller minimum RW length requirement.
As an example, take a DRY 5000ft RW with a brake deactivated - for the T7 that we fly, the V1MCG (at 30degC and 0 PrAlt) and minimum RW length requirements are:
TO=137kts, and a 5900ft RW (ASDA)
TO1=130kts and a 5300ft RW
TO2=122kts and a 4800ft RW.
If a V1 decrement of 15kts is to be applied to a V1 of137kts resulting in 122kts, this would have to be increased to 137 for the TO case , and increased to 130 in the TO1 case thus requiring a minimum RW of 5300ft or 5900ft respectively. On the other hand, TO2 would allow a take-off because 122kts = V1MCG and the RW is longer than the minimum 4800ft required for a TO2 accel-stop at V1MCG.
(the figure of 15kts decrement used above is for illustrative purposes only, and so also, the discussion does not comprehensively cover various scenarios.)
Upshot is - the case is one that arises due to low TOW/TO spds at VMCG and a short RW (ASDA) and controllability is the underlying issue.
Increased climb perf on the other hand depends on utilising the extra margin of available RW when NOT field length limited. It results in an increased V2 providing better CLB limited perf and TOW.
Salaams.
catpinsan