Vmca
Interesting forum and one in which there will always be a confliction of ideas and opinions. My own experience/comments are are follows:-
1. I have always simulated an engine failure close to the ground by closing a throttle, not the mixture. One reason is that by closing the mixture it could be argued that the instructor or examiner has deliberately shut the engine down which is at variance with the CAA and I believe FAA ruling about not practicing engine shut downs below 3000 feet agl.
2. As a CAA/JAR multi engine instructor I have never taught a student to demonstrate Vmca. He/she is, after all, a student not a test pilot and neither the CPL Skill Test nor the Multi Engine Class Rating require the student to demonstrate it. My understanding of the exercise is to demonstrate to the student those factors that affect the airspeed at which directional control is lost. There is a very long list of factors but on any one day and altitude and on any one aircraft type, only a few can be demonstrated. In practice these are
a. Critical engine (if applicable)
b. The use of bank towards the live engine
c. Whether or not the propeller of the failed engine has been feathered
The worst case scenario would be the loss of the critical engine, windmilling propeller and wings level. For the very reason that Naughty S described in the third paragraph of his last posting, I only give control of the rudder to the student for this and all the other demonstrations. Another reason for retaining control of everything but the rudder is that the rate of speed reduction is under my control and there is therefore less risk of the student mishandling the aircraft and spoiling the demonstration, plus the student does not have to concentrate on maintaining an accurate angle of bank where required.
In the worst case scenario mentioned above this inevitably results in full rudder being reached at an airspeed comfortably above stalling speed so there is absolutely no risk of a spin. The exercise then progresses through the other combinations. Since I teach on a Seneca I cannot demonstrate critical engine, but for those aircraft that do have one the next demonstration would be with the non-critical engine failed and and wings level This should result in a slightly lower airspeed at which full rudder is reached. Then back to the critical engine, this time with zero thrust set and wings level followed by the critical engine again with a windmilling propeller and bank towards the live engine. The latter two demonstrations could result in a critical speed below the stall speed. In this case the demonstration stops when the stall warner sounds and the student is asked to see if he or she can apply any more rudder. Inevitably the answer is that they can so it is not difficult to elicit friom them that they could have maintained directional control down to a lower airspeed. Since the airspeed is above stalling speed there is no risk of a spin but the student has been shown that bank and/or feathering the propeller will result in the ability to fly slower before directional control is lost.
3. The airspeed at which directional control is lost is invariably at a speed well below that at which the aircraft will normally be flown unless deliberately carrying out stalling. Therefore whenever teaching and practicing engine failures after take off the airspeed should always be comfortably above Vmca and the stalling speed.
4. I agree with those who have said that the rejected take-off does not have to be as a result of an engine failure. There are many reasons why a pilot would want to abandon take off and these can be introduced and practiced with no danger of departing the runway even if the student is slow in reacting.
This posting is not intended as a lesson in how to teach asymmetric flying and I am sure that there are many other equally good ways of getting the message across. However, there seems to be some confusion between Vmca and what I was taught to call Critical Speed, ie the airspeed at which directional control is lost. They are not the same thing.