Vmca...ME instruction
Gentlemen,
It's been many years since I instructed in light twins...even longer since my initial flight instructor check ride. In turbine equipment, we don't do Vmca demonstrations. (Never saw it in any airline's syllabus.)
However, in the U.S. it used to be required for the checkride for the multi-engine applicant to demonstrate Vmca, as it was also required for the instructor applicant to demonstrate the ability to teach Vmca (both in the classroom and in the airplane). Additionally, for the instuctor applicant, it was also required to demonstrate the knowledge of how not to you kill yourself and the student during such instruction.
The problem is stall speed and Vmca. If Vmca is well above the stall speed, the aircraft will roll before it stalls...and recovery is intuitive and conventional. If the stall speed is well above Vmca, the aircraft will stall before it rolls, and (assuming you have coordinated flight), recovery is, again, a 'normal' stall recovery exercise.
If the two speeds (for the conditons you have at hand) are close to or equal, you will surely spin. In most light twins, this means death to all on board, as spin recovery in light twins is difficult...as John says...better left to the test pilots.
We all know what affects stall speed...what increases it, what decreases it. And, we know what affects Vmca. It's up to the instructor to 'adjust' these variables to ensure the two speeds are not close or equal to eachother. And, all this should be briefed with the student prior to the flight.
In my humble opinion, it is important for the student to know how to handle a Vmca situation...what causes it, what to do to recover...how the aircraft 'feels', etc. This goes along with the demonstration of how Vyse and Vxse vary with weight altitude, bank angle, etc...how Vmca varies with engine power output, CG...et. al. All this is necessary for the student to be 'the master of the aircraft' (as, ultimately, the FAA wants to see on check rides).
Tom