During an asymmetric situation, the main effect of the asymmetry is that it causes yaw towards the failed engine. Rudder is required to counteract that yaw.
At lower airspeeds, there is less airflow over all of the aerodynamic surfaces. This means that all of the aerodynamic controls become less effective. In the case of Vmc (which is subtley different to Vmca, but I think Vmc is what you are actually refering to), the speed has reduced sufficiently that there is not enough rudder authority to counteract all of the yaw that's being created by the asymmetry. The net effect, then, is that even with full oposite rudder applied, there is still some yaw in the direction of the failed engine.
What is the effect of this yaw? Well, what is the effect of any yaw? The end result is no different to if you applied yaw by using the rudder pedals. The yaw will cause roll, the roll will cause more yaw, and so on, and the two will combine to result in a spiral dive, in exactly the same way as applying rudder in any aircraft will result in a spiral dive.
I have demonstrated this to countless students in the 5 years I've been an MEP instructor, and it's never put the aircraft on its back yet! (Mind you, I tend to recover when it gets much beyond about 45 of degrees of bank.....)
FFF
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Last edited by FlyingForFun; 22nd March 2011 at 23:18.