VMC Demonstration Explained
The VMC demonstration is a training maneuver used to teach recognition of an impending loss of directional control with asymmetric thrust. The point is not to prove a published red-line speed. It is to understand the cues that appear as airspeed decreases and rudder authority approaches its limit.
What the maneuver teaches
With one engine producing substantially more thrust than the other, the airplane wants to yaw toward the lower-power side. As airspeed decreases, the rudder becomes less effective. At some point the pilot may be unable to maintain directional control with the permitted control inputs and bank.
Why power matters
More power on the operating engine increases asymmetric thrust and therefore increases the yaw the rudder must oppose. That is why power is one of the major VMC factors.
Why bank matters
A small bank toward the operating engine can reduce sideslip and improve directional control. Excessive bank changes the maneuver and can mask the aerodynamic condition being demonstrated. The exact allowable technique comes from the current training standard and aircraft procedure.
Recognition cues
Students are taught to recognize loss of directional control before allowing the airplane to develop into a dangerous attitude. Cues can include increasing rudder requirement, inability to maintain heading, stall warning or other aircraft-specific indications. Recovery is initiated at the first applicable cue under the approved training procedure.
VMC is not a stall speed
VMC is a control speed, not an aerodynamic stall speed. A stall and a loss of directional control are different events, though both can become hazardous if mishandled during asymmetric flight.
VMC changes with conditions
The published red-line value is determined under specified certification conditions. Actual control margins vary with weight, CG, altitude, power, propeller drag, configuration and bank. Review VMC versus VYSE, zero sideslip, and the multi-engine checkride study guide.
FAA source
The FAA Airplane Flying Handbook discusses VMC demonstrations and multiengine aerodynamics. Open the current FAA handbook.