Minimum Control Speed Of a Multi Engine Aircraft

The minimum control speed (V_MC) of a multi-engine aircraft is a speed that specifies the calibrated airspeed below which directional or lateral control of the aircraft can no longer be maintained, after the failure of one or more engines. The V_MC only applies if at least one engine is still operative and will depend on the stage of flight.

Part 23 regulations mandate that manufacturers conduct (Minimum Control Speed) V_MC testing under specific conditions where controllability and performance will be evaluated.

Or as described in the Canadian flight test guide for multi engine class rating.

Minimum Control Speed V_MC – the minimum flight speed at which it is possible to retain control of the aeroplane and maintain straight flight, with maximum rudder deflection and not more than 5 degrees of bank, following sudden failure of the critical engine.

V_MC – for an aeroplane type is generally determined under the following conditions:

• All engines developing maximum rated power at the time of critical engine failure.
• The aeroplane is at a minimum practical test weight with a rearmost center of gravity.
• Landing gear retracted, flaps in take-off position and the propeller of the failed critical engine windmilling.

At speeds below V_MC, the aeroplane will yaw and roll towards the failed engine. Control will be regained only by a reduction in power of the good engine or by increasing airspeed through a change in pitch attitude, or both.

V_MC, or Minimum Control Speed, essentially assures that if your aircraft maintains a speed equal to or above it, it will maintain directional control in the event of an engine failure. However, it doesn’t guarantee overall performance. For example, consider a small multi engine aircraft like the Piper Seneca PA34 with a V_MC of 80 mph. This means that if you experience an engine failure while at maximum weight, the good engine is at full power, and the critical engine is windmilling, if you maintain or exceed 80 mph, your aircraft will continue in a straight path. You will have directional control, and the aircraft won’t yaw uncontrollably but it doesn’t ensure you’ll be able to maintain altitude. This is the fundamental concept behind V_MC.

As weight increases, V_MC decreases. In the case of the Seneca, V_MC assumes a bank angle of not more than 5 degrees toward the good engine. When you bank toward the good engine, you use less rudder because the horizontal component of lift assists in countering yaw. Lift can be divided into vertical and horizontal components, with the vertical component balancing the aircraft’s weight. Heavier weight requires more vertical lift, but with a constant 5-degree bank, more total lift is generated, resulting in more horizontal lift as well. This reduces the need for rudder input, leaving you with reserve rudder authority.

A heavy aircraft will resist the yawing force due to a failed engine more than a light aircraft, a heavy object is harder to displace than a lighter object. A heavier object is more ‘stable’ due to its higher inertia. This will tend to decrease V_MC.

Below V_MC, the rudder becomes ineffective in preventing a loss of directional control, leading to uncontrolled yawing. If you have maximum weight, the increased horizontal lift allows you to use partial rudder before resorting to full rudder. Therefore, you need to slow down even further before completely losing directional control. Hence, more weight leads to a lower V_MC.

Weight has a performance penalty because increased weight means less available thrust, leading to a decrease in climb and overall performance characteristics. Thus, you’ll maintain directional control if you hold V_MC or exceed it. If you find it challenging to hold your altitude at Vyse (the best single-engine rate of climb speed), you may experience a descent, but you will still have the ability to control the aircraft’s direction.