Minimum Control Speed - V_MC

When an engine fails or is inoperative in-flight, the rudder is used to counteract the asymmetrical thrust yawing moment. Roll effects are counteracted by the ailerons. The counteracting forces generated by the aerodynamic control surfaces are proportional to the square of the airspeed (V²), to the area of the control surfaces (S) and to the air density. For a given size of the vertical tail with rudder, there is a speed below which the generated rudder side force is not large enough to counteract the asymmetrical thrust, or below which the ailerons are not effective anymore: the heading and/or bank angle cannot be maintained below this speed. This speed is called Minimum Control speed in the Air V_MCA.

• With regards to this question,
  Cold air is more dense. Engines are able to produce more power in colder weather due to being able to achieve higher mass flow with the higher density air. As power increases, the operative engine requires more rudder authority to maintain directional stability due to the increase in thrust. As a result, the airplane requires higher indicated airspeeds to maintain directional stability. Therefore, V_MC increases.