ความเร็วควบคุมขั้นต่ำ (VMC: Minimum Control Speed)

What Is Minimum Control Speed?

Minimum Control Speed (V_MC) is the lowest airspeed at which a multi-engine aircraft can maintain directional control following the sudden, complete failure of the most critical engine, with the remaining engine(s) developing maximum available thrust. Below V_MC, the aerodynamic authority of the rudder is insufficient to overcome the yawing moment created by asymmetric thrust, and the aircraft will yaw uncontrollably toward the failed engine.

V_MC is one of the most important V-speeds in multi-engine operations. Certification requires that V_MC must not exceed V₁ (decision speed) and must be demonstrated with the aircraft in a specific, worst-case configuration. For transport category aircraft, V_MC must be established at or below 1.13 V_S1 (stall speed in takeoff configuration).

How It Works

When one engine fails on a twin-engine aircraft, the operating engine creates an asymmetric yawing moment proportional to thrust and the distance between the engine and the aircraft centerline. The rudder must generate an equal and opposite moment to maintain heading.

• Critical engine: On conventional propeller aircraft, the left engine is usually critical due to P-factor and spiraling slipstream (the right engine's propeller blade descending on the right side generates more thrust). On jets, the engine farthest from the centerline is critical.
• 5° bank: FAA regulations (FAR 25.149) allow up to 5° of bank toward the operating engine during V_MC demonstrations to reduce rudder force required by approximately 150 N.
• Density altitude effect: V_MC decreases at higher altitudes because the operating engine produces less thrust, reducing the asymmetric yawing moment. This is why training for V_MC rolls is most dangerous at low altitude.
• V_MCA vs V_MCG: V_MCA is the air minimum control speed; V_MCG is the ground minimum control speed (only nosewheel steering & rudder), always higher than V_MCA.

Performance Specifications

The flight envelope defines the operating boundaries within which V_MC data is valid. For the Boeing 737-800, V_MC is approximately 108 KIAS at sea level, standard conditions, maximum takeoff thrust, and landing gear up. For the Airbus A320 family, V_MC is approximately 106 KIAS. Regional jets such as the Bombardier Q400 have a V_MC of approximately 84 KIAS due to their lower engine-to-centerline distance.

Aircraft Examples

• Boeing 737-800: V_MC ≈ 108 KIAS; V₁ on a standard departure must be set at or above V_MC.
• Airbus A320: V_MC ≈ 106 KIAS; fly-by-wire rudder limiter reduces available rudder deflection above 160 KIAS, requiring updated V_MC analysis.
• Boeing 747-400: With four engines, V_MC is less critical but still certified; worst case is two engines failed on the same side.
• Piper Seneca V: Piston twin with counter-rotating propellers to eliminate the critical engine — V_MC is the same regardless of which engine fails.