Expert: David Bidwell - 8/15/2007

Question
Could you explain the Vmc and the critical engine and why there is a critical engine what effect is has on the plane?  Thanks

Answer
Hello Joe,
The official definition of the "minimum control speed which we call Vmc is as follows:
Airworthiness manual 523.149 states:
Vmc is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative and then maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The ability to maintain straight and level flight at Vmc in a static condition with bank angle of not more than 5 degrees must also be demonstrated.

The Airworthiness manual also goes on the specify the following conditions for Vmc.

Vmc may not exceed 1.2 Vs1 determined at the maximum take-off weight  
Maximum available take-off power or thrust on the engines  
The most unfavorable center of gravity  
The aeroplane trimmed for takeoff  
The minimum sea level takeoff weight or any lesser weight necessary to show Vmc  
The most critical takeoff configuration with landing gear retracted  
The aeroplane airborne but not in ground effect  
The inoperative engine windmilling unless an auto feather system is installed  
I also recommend that you go to the following site for pictures and explanation: http://selair.selkirk.bc.ca/aerodynamics1/Multi/Page3.html
Hope this helps
Dave