Velocity - minimum controllable airspeed

  Vmca is the minimum controllable airspeed
during which it is possible to maintain directional
control of an airplane with one inoperative critical
engine.  At this speed or greater, the pilot can 
maintain straight flight by either the zero yaw 
technique or zero sideslip condition using an
angle of bank of not more than 5 degrees into 
the operating engine, i.e. "raise the dead (engine)."

  Vmca may not exceed 1.2 times Vs1 (based on max 
gross weight) and the following conditions are assumed:

1)  Max takeoff power on operating engine
2)  Most unfavorable (rearward) C.G.
3)  Airplane trimmed for takeoff
4)  Maximum sea level takeoff weight - lowers Vmca*
5)  Flaps in takeoff position 
6)  Landing gear retracted
7)  Cowl flaps in takeoff position
8)  Propeller windmilling or most likely position
9)  Airborne in negligible ground effect
10) Yaw and/or roll for directional control

* max gross weight is the only condition which helps Vmca

  Yaw/roll is caused by asymmetric:

P - p-factor of the remaining engine 
A - accelerated slipstream/loss of lift on failed side
S - spiralling slipstream yawing the tail
T - torque of the remaining engine

  In many twins all of these factors combine to make the 
left  engine the "critical" engine because each factor 
is more pronounced when the left engine is inoperative 
compared to the right engine.