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 Aircraft Manufacturers Determine 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. The following configurations are generally the most critical.

Standard Day Conditions at Sea Level

Baseline Measurement: 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.

Density Altitude: Increasing density altitude reduces engine performance and propeller efficiency. Therefore, V_MC decreases as altitude or temperature increases with normally aspirated engines. For turbocharged engines, V_MC remains constant up to the critical altitude.

Most Unfavorable Weight (Light)

Bank Angle: Increased weight increases the horizontal component of lift in a given bank angle, helping to offset the yawing moment caused by asymmetric thrust. As a result, less rudder pressure is needed to maintain control, and V_MC decreases.

Inertia: A heavily loaded airplane has more inertia than a lightly loaded one, giving it greater resistance to yawing, which also reduces V_MC.

Most Unfavorable CG Location (Aft)

Rudder Authority: As the CG moves aft, the moment arm of the rudder is shortened. This reduces the rudder’s leverage, giving it less authority to counteract yawing forces. Therefore, V_MC increases as the CG is moved aft.

Critical Engine Windmilling (Propeller Controls in the Takeoff Position)

V_MC increases as drag on the inoperative engine increases.

Windmilling: A windmilling propeller (low pitch–high RPM) creates more drag than a stationary or feathered propeller, resulting in the highest V_MC.

Unfeathered: When the propeller is stationary, an unfeathered position creates more drag than a feathered propeller.

Feathered: When the propeller is feathered, the blades align with the relative wind, minimizing drag. This reduction in drag lowers V_MC compared to an unfeathered or windmilling propeller.

Flaps, Gear, and Trim in the Takeoff Position

Wing Flaps: For most light twins, the takeoff flap setting is 0° of flaps. Extending the flaps increases drag behind the operative engine. This can have a stabilizing effect that may reduce V_MC.

Landing Gear: Extending the landing gear can create a keel effect that aids directional stability and decreases V_MC. V_MC increases when the landing gear is retracted.

Trim: Since the airplane is usually trimmed to a neutral position on takeoff, having the aircraft pre-trimmed as an aid to an engine failure would be “cheating.”

Up to 5° of Bank Toward the Operating Engine

Banking Toward the Operative Engine: In a bank, the horizontal component of lift helps the rudder counteract the operative engine’s thrust. 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 Operative Engine: Banking away from the operating engine increases V_MC.

Maximum Available Takeoff Power on Each Engine

Power Imbalance: A higher power imbalance between the operating and inoperative engines results in more asymmetrical thrust, increasing V_MC.

Power Effects: Any condition that increases power on the operating engine raises V_MC, while factors like power reduction, higher altitude, increased temperature, or lower air density decrease V_MC.

Static Versus Dynamic V_MC Certification

Dynamic: In dynamic V_MC certification, test pilots cut the critical engine at gradually reduced speeds to find the minimum speed where directional control can be maintained within 20° of the original heading. This involves high power settings and quick pitch adjustments, making it too risky for typical flight training.

Static: Static V_MC certification involves maintaining straight flight with up to 5° of bank toward the operating engine. This method is safer and more closely reflects V_MC demonstrations in flight training.

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, due to asymmetrical lift, the aircraft tends to roll toward the inoperative engine. 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 be verified by comparing the climb performance charts for one 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 and Configuration

• Clear the area
• Heading established and noted
• Altitude established:
• No lower than 3,000′ AGL or the manufacturer’s recommended altitude (whichever is higher)
• Position near a suitable emergency landing area
• Set power and aircraft configuration:

• Perform the clean (cruise) configuration flow, leaving the landing gear retracted
• 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, or an airspeed below V_FE if appropriate

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 the entry 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.

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

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

Recovery

1. Reduce the operating engine’s power sufficiently promptly and simultaneously and decrease the AOA as necessary to regain airspeed and directional control. Rudder pressure needs to be reduced to maintain directional control as the throttle is reduced.

2. Once above V_MC and control is regained, smoothly advance the throttle on the operative engine to full power. Apply rudder pressure as needed to maintain the entry heading.
3. Accelerate to V_XSE or V_YSE as appropriate. Do not use the failed engine (simulated).
4. 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 V_MC Demonstrations

Risk Examples for Flight Maneuver and Stall Training

Distractions, Task Prioritization, Loss of Situational Awareness, or Disorientation

• Distractions, task prioritization, loss of situational awareness, and disorientation increase the likelihood of errors, delayed or missed actions, and the inability to process information accurately and timely; minimize non-essential activities, follow the “Aviate, Navigate, Communicate” prioritization, and stay focused.

Configuring the Airplane During Flight Training Maneuvers

• Exceeding aircraft limitations during configuration changes or maneuvers increases stress or damage to the airframe, flaps, or landing gear; verify the proper configuration and make changes by following AFM/POH procedures.

Risk Examples for Multi-Engine Operations

Maneuvering with One Engine Inoperative During Training Maneuvers

• Lack of airspeed awareness during single-engine operations can result in an inadvertent stall, spin, or loss of directional control (V_MC); increase focus and awareness during slow-speed maneuvers.
• Aft CG during single-engine operations can decrease stall speed but increase V_MC, potentially leading to a loss of control; avoid an aft CG and stay focused during single-engine operations.