As the multi-engine aircraft increases in altitude, Vmca decreases, but calibrated stall speed does not change (indicated stall speed will not change on any given day if we assume no compressibility) for a specific weight, configuration, and altitude. So, there exists an altitude where each of the following exists:
a) Vmca is less than Vs
b) Vmca is the same as Vs
c) Vmca is greater than Vs
The density altitude where Vmca and Vs are equal is called Critical Density Altitude. At this altitude, the aircraft slows to Vmca and Vs at the same time. This may cause a non-controllable, non-recoverable flight attitude such as a spin.
Above the critical density altitude the aircraft will reach the calibrated stall speed before Vmca.
Below the critical density altitude the aircraft will reach Vmca before the calibrated stall speed.
Found this online but maybe not a reliable sorce, I would look in some actual FAA Multi Engine publications but if all else fails there is this, Sorry I dont have my books in front of me to check this one out. Again no promises but hope this helps.
