Vmc - Minimum Control Airspeed
What is Vmc? And how can we manage it?
According to FAR 23.149, Minimum Control Airspeed (Vmc) is the calibrated airspeed at which a multi-engine aircraft should fly when there is a loss of a critical engine. In an emergency scenario with an inoperative engine, Vmc allows for:
Maintained control
Maintained leveled flight (with an angle of no more than 5º)
In the light twin-engine airplanes we fly at Aviator Zone Academy, Vmc doesn’t guarantee a climb or altitude hold. The Vmc for any aircraft can be easily identified by looking for the first red radial line on the airspeed indicator.
Vmc is calculated for each aircraft under specific conditions. A helpful memory aid for pilots is C.O.M.B.A.T.S. Let’s break down what this means:
C — Critical Engine Failed and Windmilling: The critical engine is, of course, more crucial, especially when considering Vmc in the most critical condition. A failed engine with a windmilling propeller generates substantial drag, similar to holding a large piece of plywood against the airstream.
O — Operating Engine at Maximum Takeoff Power: The more power available to the operating engine, the higher the Vmc. More power generates more lift and increases the yawing tendency around the longitudinal axis.
M — Maximum Gross Weight: Although a heavier airplane is more stable, it also demands more lift to sustain flight, requiring more power. Gross weight is almost neutral but is used to standardize certification.
B — Bank of No More Than 5º: Banking into the good engine creates a horizontal lift component that counters the forces trying to turn the aircraft toward the dead engine. Each degree of bank up to 5 degrees reduces Vmc by approximately 3 knots. A 5-degree bank into the good engine can lower Vmc by about 15 knots, as demonstrated in Vmc demos.
A — Aft Center of Gravity (CG): An aft CG reduces the distance between the CG and rudder, decreasing the rudder's effectiveness.
T — Takeoff Configuration: This includes normal takeoff flaps and retracted landing gear. Gear and flaps down create drag but also provide a stabilizing keel effect.
S — Standard Day: The published Vmc (red line) on the airspeed indicator is based on standard day conditions at sea level. As density altitude increases, the red line becomes less reliable because Vmc decreases with altitude, bringing Vmc closer to stall speed.
Factors That
Affect Vmc
| Cause | Effect |
|---|---|
| Increase Power in Good Engine | 🡑 Vmc 🡑 Performance |
| Windmilling Propeller | 🡑 Vmc 🡓 Performance |
| Feathered Propeller | 🡓 Vmc 🡑 Performance |
| Flaps Down | 🡓 Vmc |
| Gear Down | 🡓 Vmc 🡓 Performance |
| Max Takeoff Weight | 🡓 Vmc 🡓 Performance |
| Aft CG | 🡑 Vmc 🡑 Performance |
| High Temperature | 🡓 Vmc 🡓 Performance |
| Low Pressure | 🡓 Vmc 🡓 Performance |
| Bank of 5° into Good Engine | 🡓 Vmc 🡑 Performance |
| Split Ball | 🡓 Vmc 🡑 Performance |
| High Altitude | 🡓 Vmc 🡓 Performance |
| Ground Effect | 🡓 Vmc 🡑 Performance |
Stalls vs VMC
In normally aspirated airplanes, stall speed might be reached before Vmc at higher altitudes due to reduced engine performance. When stall speed and Vmc align, there's a high risk of spinning. If an engine fails during a stall, immediately reduce both throttles to idle and pitch down.
At Aviator Zone Academy, a leading flight school in South Florida, we emphasize understanding these critical concepts as part of our private pilot training South Florida programs. Our comprehensive pilot training programs South Florida include in-depth sessions on Vmc management, ensuring our students are equipped to handle real-world flying scenarios. Whether you're pursuing a commercial pilot license in Florida or engaging in discovery flights South Florida, we provide the knowledge and skills necessary to master these essential aviation principles. For more details about our training, visit our aviation academy in Florida, offering hands-on flight training Miami with modern flight simulators Florida and experienced instructors.
