Vmc Demonstration

What is a V_MC Demonstration?

V_MC demonstrations closely resemble how manufacturers determine V_MC during the airplane's certification process:

1. Power on the critical engine is reduced to idle.
2. The airplane is configured according to the V_MC certification criteria.
3. The pitch attitude is increased to reduce airspeed gradually until a loss of control occurs.
4. Proper recovery procedures are performed to reestablish controllable, single-engine flight.

Definition of V_MC

V_MC is the calibrated airspeed at which, following the sudden critical loss of thrust, it is possible to maintain control of the airplane. For multi-engine airplanes, V_MC must be determined, if applicable, for the most critical configurations used in takeoff and landing operations.

Directional control has been lost when full rudder deflection is applied towards the operating engine, and the aircraft begins to yaw toward the inoperative engine.

How the Manufacturer Determines V_MC

14 CFR Part 23 requires airplane manufacturers to determine V_MC, if applicable, for the most critical configurations used in takeoff and landing operations. Specific configurations are not addressed; however, the following are typically the most critical.

Standard Day Conditions at Sea Level

An airplane must meet performance requirements for certification in still air and standard atmospheric conditions at sea level. This is necessary to establish a standard of measurement.

Most Unfavorable Weight (Light)

A heavier airplane has a lower V_MC due to the horizontal component of lift and inertia.

For a given bank angle, an increase in weight will require more lift to maintain altitude. The increase in the horizontal component of lift will require less rudder pressure to keep the aircraft from yawing.

A heavily loaded airplane has more inertia than a lightly loaded one; thus, a heavier airplane will have a higher resistance to yawing.

Most Unfavorable CG Location (Aft)

The aft-most CG limit is the most unfavorable CG position. As the CG moves aft, the rudder's moment arm is shortened, resulting in less leverage. The rudder will have less authority in overcoming yawing forces, causing V_MC to increase. At the same time, the moment arm of the propeller blade is increased, aggravating asymmetrical thrust.

Critical Engine Windmilling (Propeller Controls in the Takeoff Position)

V_MC increases as drag increases on the inoperative engine. A windmilling propeller creates more drag than a stationary propeller. When the propeller is stationary, an unfeathered position creates more drag than a feathered propeller. Therefore, V_MC is highest when the critical engine's propeller is windmilling at a low pitch, high RPM blade angle.

Flaps, Gear, and Trim in the Takeoff Position with the Airplane out of Ground Effect

• Extending the flaps increases the drag behind the operative engine. This can have a stabilizing effect that may reduce V_MC.
• V_MC increases when the landing gear is retracted. Extended landing gear creates a keel effect that aids directional stability and decreases V_MC.

• Since the airplane is usually trimmed to a neutral position on takeoff, having the airplane pre-trimmed as an aid to an engine failure would be "cheating."
• Ground effect decreases drag and increases V_MC.

Up to 5° (Not More) of Bank Toward the Operating Engine

V_MC is highly dependent on bank angle. In a bank, the horizontal component of lift helps the rudder counteract the operative engine's thrust. Historically, 14 CFR Part 23 prevented airplane manufacturers from using more than 5° of bank toward the operative engine when determining V_MC.

When banking toward the operating engine, V_MC decreases by approximately 3 knots for every degree of bank less than 5°. Banking away from the operating engine increases V_MC.

Maximum Available Takeoff Power Initially on Each Engine (Engine Failure Should Happen Suddenly)

A higher power imbalance results in more asymmetrical thrust. V_MC increases as power is increased on the operating engine.

Control and Performance After an Engine Failure

Directional Control

When a multi-engine aircraft experiences an engine failure and the engines are not mounted on the longitudinal axis, there are unbalanced forces and turning moments about the CG.

Pitch Down: The loss of induced airflow over the horizontal stabilizer results in less negative lift produced by the tail. Additional back pressure is required to maintain level flight.

Roll Toward the Inoperative Engine: The loss of the accelerated slipstream air over the wing of the inoperative engine results in a reduction of lift on that wing. Therefore, the aircraft tends to roll toward the inoperative engine due to asymmetrical lift. This requires additional aileron pressure into the operative engine.

Yaw: Asymmetrical thrust requires rudder pressure toward the operating engine.

Climb Performance

The loss of one engine results in a 50% loss of power but an approximate 80% loss of climb performance. This can verified by comparing the climb performance charts for one engine operating and two engines operating under the same atmospheric conditions.

Four Factors that Make the Left Engine Critical

Asymmetrical Thrust (Yaw)

Each engine's descending propeller blade will produce greater thrust than the ascending blade when the airplane is operated under power and at positive angles of attack.

Even though both propellers produce the same thrust, the descending blade on the right engine has a longer moment arm (greater leverage) than the left engine's descending blade. As a result, the left engine's failure will result in the most asymmetrical thrust.

Accelerated Slipstream (Roll and Pitch)

Asymmetrical thrust (P-Factor) results in a longer moment arm to the center of thrust of the right engine than the left engine. Therefore, the center of lift is farther out on the right wing, resulting in a greater rolling tendency with a loss of the left engine. The left engine's failure also results in a more significant downward pitching moment due to the greater loss of negative lift produced by the tail.

Spiraling Slipstream (Yaw)

A spinning propeller produces a spiraling wind pattern that rotates in the same direction as the propeller's rotation. The spiraling slipstream of air from the left engine strikes the vertical stabilizer from the left. This helps counteract the yaw caused by a failure of the right engine.

The right engine's slipstream does not reach the vertical stabilizer. If the left engine fails, the right engine's slipstream will not counteract the yaw toward the inoperative engine.

Torque (Roll)

For every action, there is an equal and opposite reaction. A propeller that rotates clockwise (right) will tend to roll the aircraft counter-clockwise (left). The airplane will tend to roll toward the inoperative engine regardless of which engine has failed.

On a multi-engine airplane with clockwise rotating propellers, the effect of torque during OEI flight will:

• Increase the tendency of the airplane to roll toward the inoperative engine (left engine failed); or
• Counteract the tendency of the airplane to roll toward the inoperative engine (right engine failed).

Critical Density Altitude

With normally aspirated engines, an increase in altitude or temperature results in reduced engine performance and propeller efficiency. This results in a lower minimum control speed (V_MC) as altitude is increased. However, the calibrated stall speed (V_S) does not decrease with altitude.

There exists an altitude where each of the following exists:

• V_MC is less than V_S (stall occurs first)
• V_MC is the same as V_S (stall and yaw coincide)
• V_MC is greater than V_S (yaw occurs first)

The density altitude where V_MC and V_S are equal is called the critical density altitude. Here, the airplane will "stall and yaw" simultaneously under a condition of asymmetrical thrust. The airplane could experience an abrupt change in attitude or enter into a spin.

How to Perform a V_MC Demonstration

Pre-Maneuver Checks

• Clear the area
• Heading established and noted
• Altitude established:
• No lower than 3,000' AGL or or the manufacturer's recommended altitude (whichever is higher)

• Adjust the mixture controls as required
• Set the propeller controls full forward
• Retract the landing gear
• Set the flaps to the normal takeoff setting
• Set the trim for takeoff and do not readjust it during the maneuver
• Set the cowl flaps for takeoff

• Establish a normal cruise speed

Execution

1. Simulate a failed engine by retarding the throttle lever of the critical engine to idle.
2. Slow the airplane to 10 knots above V_SSE or V_YSE, as appropriate, or the manufacturer's recommended speed.
3. Apply full power on the operative engine while maintaining heading.
4. Bank the aircraft up to 5° towards the side of the operative engine and sustain 1/2 ball deflection toward the operative engine.

1. Increase the pitch attitude slowly to decrease airspeed by one knot per second. A fast reduction of airspeed can be hazardous and does not provide an adequate demonstration.
2. Aileron control deflection should be increased as necessary to keep the bank angle constant as the ailerons become less effective as airspeed decreases.
3. Maintain heading as long as possible by increasing rudder input.

1. Recognize, announce, and recover at the first sign of a loss of directional control, stall warning, or buffet, whichever occurs first.

Recovery

1. Promptly and simultaneously reduce power sufficiently on the operating engine and decrease the AOA as necessary to regain airspeed and directional control. To maintain directional control as the throttle is reduced, rudder pressure needs to be reduced.

1. Once above V_MC and control is regained, smoothly advance the throttle on the operative engine to full power. Simultaneously reapply rudder pressure as needed.
2. Accelerate to V_XSE or V_YSE as appropriate. Do not use the failed engine (simulated).
3. Recover to the original heading.

Exit

1. Return to cruise speed with both engines, trimming as necessary.
2. Complete the cruise checklist.

Sideslip During the V_MC Demonstration

During the V_MC demonstration, the pilot applies rudder pressure to maintain directional control. The loss of control occurs under conditions of sideslip. V_MC is not determined under conditions of zero-sideslip during aircraft certification; therefore, it is not part of a V_MC demonstration.

A zero-sideslip may be established after the initial V_MC recovery procedure is completed. Pilot certification standards require the pilot to accelerate to V_XSE or V_YSE as appropriate during the recovery, which is normally maintained in a zero-sideslip condition for best climb performance.

Safety Considerations for V_MC Demonstrations

• Airplanes with normally aspirated engines will lose power as altitude increases because of the reduced density of the air entering the engine's induction system. This loss of power will result in a V_MC lower than the stall speed at higher altitudes. Therefore, recovery should be made at the first indication of loss of directional control, stall warning, or buffet.

• The rudder limiting technique avoids the hazards of spinning as a result of stalling with high asymmetrical power, yet is effective in demonstrating the loss of directional control.

• Do not perform this maneuver by increasing the pitch attitude to a high angle with both engines operating and then reducing power on the critical engine. This technique is hazardous and may result in loss of airplane control.

• Instructors should be alert for any sign of an impending stall. The learner may be focused on the maneuver's directional control aspect to the extent that impending stall indications go unnoticed.
• Follow the airplane manufacturer's guidance regarding the CG location during slow flight maneuvers. Generally, a forward CG aids in stall recovery, spin avoidance, and spin recovery.

Common Errors for V_MC Demonstrations

• Inadequate knowledge of the causes of loss of directional control at airspeeds less than V_MC, factors affecting V_MC, and safe recovery procedures
• Improper entry procedures, including pitch attitude, bank attitude, and airspeed
• Failure to recognize an imminent loss of directional control
• Initiating recovery steps too early
• Failure to use proper recovery procedures

Airman Certification Standards for VMC Demonstrations