hi quid,
when TO-derate is selected, the selected derate is deem to be the max. thrust limit for that takeoff. due to the lower thrust, therefore, Vmca is lower (less yaw in an eng fail scenario). on a contaminated runway, as less traction is avail from the nosewheel, a lower Vmca is always desirable. you are not allow to reduce TO thrust further by Assumed Temp though. i believe boeing do have a chart showing the benefit of using TO-derate on a contaminated runway, just can't remember where i put it. anyone else seen it?
SR