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I got asked two questions on my checkride:

  1. Why is it important to lose 1 knot per second (approximately) in a Vmc Demo?
  2. How do we know we're losing 1 knot per second?

My answer to question 1 would be be: because you are flying with one engine simulated inoperative and creating asymmetrical yaw, if you lose airspeed too fast you might blow past the stall indicator and actually stall the aircraft with a large yawing effect throwing you into a spin. Does that make sense?

For question 2, other than counting 5-10 seconds while losing 5-10 knots of airspeed is there any other way?

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Why do we want to lose 1 knot per second in a VMC demo?

  1. To more accurately determine where VMC is: As Carlo Felicione's answer says, VMC changes day by day, and you want to determine it precisely. You can't do that if you're slowing too rapidly.

  2. To permit smooth control of the aircraft: During a VMC demo, rudder required is determined by aircraft speed. The slower you go, the more rudder you need. The more abruptly your speed changes, the more abruptly your rudder needs to adjust to compensate. Since the point of a VMC demo is to determine when your rudder fails to control your heading, abrupt speed changes should be avoided.

  3. To allow consistent control techniques: Have you noticed that it's really hard to hold a particular vertical speed using the vertical speed indicator? Usually, you select a pitch, hold it, and then reference the VSI to determine whether a pitch adjustment is necessary. The same is true of airspeed loss. During a VMC demo, usually, the pilot will configure the airplane for engine-out flight and then pitch to an attitude that will create a 1 knot per second loss of airspeed. As pitch settings are fairly unchanging from flight to flight, this allows you to do the maneuver with more consistency. You can put the rate of airspeed loss more or less in the back of your mind and concentrate on your rudder control, which is the point.

How do we know we're losing 1 knot per second?

  1. Counting: As you say, looking at the airspeed indicator and counting seconds is a simple and reliable way to do it.

  2. Known pitch settings: As mentioned above, pitch to a known setting and you'll get pretty close to 1 knot per second without the fuss of counting. As you gain more experience this will become the way you'll do it.

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Because in a Vmc demo, you are attempting to determine exactly where Vmc is for your airplane in a particular configuration and the way that’s determined is to maintain straight and level (or coordinated, if your light twin cannot maintain altitude on a single engine) flight with the critical engine out and determine the exact airspeed in which you can no longer maintain directional control of the vehicle. Remember Vmc (redline) is an aircraft design, not a operating, requirement and varies depending on a number of factors, so it may or may not occur at the indicated red radial line on your airspeed indicator, depending on circumstances. Therefore, we creep toward it, slowly approaching actual Vmc by slowing 1 knot or so of airspeed a second until we can no longer maintain directional control with the rudder.

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