12.vmc-demonstration. Vmc Demonstration
The Vmc demonstration is a required maneuver during multiengine training and on the multiengine practical test. Its purpose is to develop awareness of the flight conditions that lead to loss of directional control following a sudden engine failure, and to teach the proper recovery before control is lost. The maneuver is not a demonstration of flight at Vmc — it is a demonstration of the onset of loss of control, terminated at the first indication of impending loss of directional control, the onset of a stall, or no later than the published red-radial Vmc on the airspeed indicator, whichever occurs first.
Definition. Per 14 CFR 23, Vmc is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5° toward the operative engine. Vmc is depicted on the airspeed indicator as a red radial line.
Certification conditions for Vmc. The published Vmc is determined under a specific set of conditions that yield the most adverse (lowest controllable) speed:
- Maximum available takeoff power on the operating engine
- Most unfavorable weight (typically aft CG, max gross)
- Landing gear retracted
- Flaps in takeoff position
- Critical engine's propeller windmilling (not feathered)
- Up to 5° bank toward the operating engine
- Out of ground effect
In flight, factors that reduce Vmc (improve controllability) include feathering the inoperative propeller, banking 2°–5° into the good engine, aft CG that is actually forward of the test condition, lower density altitude on the operating engine side (more thrust asymmetry actually raises Vmc — note: higher density altitude generally lowers Vmc because the operating engine produces less thrust). Forward CG also lowers Vmc by giving the rudder a longer moment arm.
Setup procedure.
- Conduct clearing turns and complete pre-maneuver checks. The maneuver should be performed at a safe altitude — recovery must be complete no lower than 3,000 ft AGL (5,000 ft AGL recommended in many training programs).
- Landing gear up, flaps in takeoff position, cowl flaps as required, mixtures and props full forward.
- Establish a heading, reduce power on the simulated critical engine (left engine on conventional twins) to idle to simulate failure. Do not feather — propeller windmilling is part of the demonstration.
- Set the operating engine to takeoff power.
- Pitch up to slow at approximately 1 knot per second while maintaining heading with rudder.
- Establish and hold a bank of approximately 5° toward the operating engine.
Recovery. Recover at the first indication of any of the following:
- Loss of directional control (rudder at the stop, heading begins to swing)
- Aerodynamic stall buffet or stall warning
- Reaching the published Vmc red line
Recovery procedure:
- Simultaneously reduce power on the operating engine and pitch down to lower the angle of attack and regain airspeed.
- Once directional control is regained and airspeed is above Vmc/Vsse, smoothly restore power on the operating engine and return to level flight at Vyse (blue line).
Critical relationship between Vmc and stall speed. If Vmc occurs at or below the single-engine stall speed (Vs), the airplane will stall before loss of directional control. Stalling in a Vmc condition with asymmetric thrust will almost certainly produce a violent roll and yaw toward the dead engine — the entry to a spin from which recovery in a light twin may be impossible. For this reason, recovery is initiated at the first indication of either condition.
Common errors.
- Failing to maintain the 2°–5° bank into the operating engine, which raises the speed at which control is lost.
- Allowing yaw to develop unchecked instead of using full rudder before recovering.
- Recovering with pitch-up rather than pitch-down — this aggravates the loss of airspeed and control.
- Reducing power too aggressively, causing altitude loss but not addressing angle of attack.
- Performing the demonstration with the propeller feathered, which invalidates the maneuver.
- Conducting the maneuver below safe altitude or at high density altitude, where the actual loss-of-control speed may be at or below stall speed.
Why this maneuver matters. Most fatal multiengine accidents following an engine failure occur not from the failure itself but from loss of control during the response — typically at low altitude after takeoff, with the gear and flaps still extended, an unfeathered prop, and a pilot who allows airspeed to decay below Vmc while attempting to climb. The Vmc demonstration trains the pilot to recognize the onset of loss of control by feel — heading swing, rudder force, sluggish controls — and to react with the correct combination of reduced asymmetric thrust and reduced angle of attack.