Higher weight does two things which affect single engine ops...

Reduces climb performance. This is not really a Vmca issue, and light twin certification does not require SE climb capability anyway.


As far as Vmca certification, higher weight actually improves (lowers) Vmca. This is because in proper (banked) SE flight, all of the total lift generated by the wings is NOT directed vertically...some of it goes horizontally in the direction of bank (towards the good engine). This horizontal lift component helps to offset some of the yaw tendency caused by the good engine...therefore requiring less available rudder, since some of the rudder's work is already done by the horizontal lift component. Since the horizontal and vertical lift components are both part of the total lift, and a heavier airplane requires more total lift to generate the required vertical lift, the horizontal lift component will also be greater for a more haevily loaded airplane.

Note: Some airplane designs will stall prior to reaching Vmca at max gross...in these cases, Vmca is determined for a lower weight where a Vmc effect will be present (without stalling first).