VMC Demonstration

The minimum airspeed at which directional control can be maintained after a sudden failure of the critical engine, with the remaining engine at takeoff power and not more than 5° of bank toward the operating engine. It's marked as the red radial line, determined under specific certification conditions — real-world VMC changes with conditions (AFH ch 13, 14 CFR 23.149 heritage).

• Higher density altitude: the good engine makes less asymmetric thrust, so VMC decreases.
• Aft CG: shorter arm for the rudder — VMC increases.
• Windmilling prop: more asymmetry — VMC increases (feathering lowers it).
• Bank toward the good engine: each degree up to 5° lowers VMC substantially; wings level or banked toward the dead engine raises it sharply.
• Gear down: tends to lower it (keel effect) (AFH ch 13).

VMC decreases with altitude but indicated stall speed stays essentially constant — so as I climb, the two speeds converge, and up high the airplane can stall before losing directional control. A stall with full asymmetric power is a spin entry. That's why the demo is recovered at the first sign of either, and why it's flown at a safe altitude (AFH ch 13).

The engine whose failure hurts most — on conventional twins (both props clockwise from the cockpit), the left. The PAST factors:

• P-factor — right engine's descending blade has a longer arm.
• Accelerated slipstream — asymmetric lift from prop wash.
• Spiraling slipstream — left engine's slipstream helps the rudder; the right one's doesn't.
• Torque — left-rolling tendency adds to the left-engine-out problem (AFH ch 13).

Configure per the manufacturer (or gear up, takeoff flaps/trim/cowl flaps, props high rpm): critical engine to idle and windmilling, operating engine at takeoff power. Then from about 10 knots above VSSE:

1. Establish a single-engine climb attitude and up to 5° bank into the good engine.
2. Raise the pitch slowly to decelerate about 1 knot per second.
3. Feed in rudder to hold heading (ACS X.B S1–S4).

Recovery is triggered at the first indication of any of:

• Loss of directional control — heading starts walking with full rudder in.
• Stall warning.
• Buffet.

Recover by simultaneously reducing power on the operating engine and lowering the angle of attack to regain airspeed and control — never add power on the simulated-failed engine. Standards: recover within 20° of entry heading, then smoothly power up and accelerate to VSSE/VYSE +10/−5 kt (ACS X.B S5–S8).

Because asymmetric thrust is the whole problem. Cutting the operating engine's power removes the yawing moment instantly, while lowering the nose restores rudder effectiveness. Adding power on the dead side isn't available in a real failure, and pulling harder on the yoke only deepens the loss of control (AFH ch 13).