1515Blue,
Bit confusing, isn't it ? ... Vmcg, like Vmca, depends mainly on thrust levels as it is a matter of balancing the moment due to asymmetric thrust against what can be achieved with the rudder.
The rudder moment is limited by airspeed and cg ... with the certification Vmcg being taken at max aft cg. .. if the moment due to increased thrust goes up, the only way to increase the rudder moment is to increase the airspeed ... hence Vmcg goes up.
Mutt is talking about different certificated levels of engine max thrust (hence derated rather than reduced thrust takeoffs .. quite different animals). The sorts of crossovers he refers to are a bit like looking at different flap settings and optimising the takeoff for the particular conditions on the day.
Where the weight confusion comes in probably is to do with the observation than Vmcg normally is limiting at lower weights where the desired V1 is lower than what would be the case for normal commercial weight takeoffs. In the same sort of way Vmca is mainly a problem at low weights where the Vmca requirement can become more critical than the Vs considerations in scheduling V2.
Last edited by john_tullamarine; 24th Jun 2002 at 03:48.