For reciprocating, non-turbocharged engines, how does Vmc change with altitude?

For reciprocating, non-turbocharged engines, Vmc, which stands for "minimum control speed," decreases with altitude. This phenomenon occurs because as altitude increases, air density decreases, leading to a reduction in the overall performance of the aircraft, including engine power and aerodynamic efficiency.

At higher altitudes, the engine produces less thrust due to the thinner air, which affects the maximum amount of yawing moment that can be countered by the ailerons with full rudder deflection. As a result, the speed at which the aircraft can maintain controlled flight while in a one-engine-inoperative condition (achieving a zero sideslip condition) is lower compared to sea level.

In summary, an increase in altitude leads to a decrease in Vmc due to the reduction in engine performance and aerodynamic effectiveness, making it the correct choice.