I believe this number, VMCA, has more to do with aircraft certification than aircraft aviation. The regulation is not specific on how the number is determined and most aircraft manufacturers do little more than present the pilot with a single number.
— James Albright
Updated:
2012-09-12
Of far more used to you, as a pilot, is V2 Takeoff Safety Speed and VREF Reference Speed. Keep your speed above those and you won't have any problems.
1
Regulatory
[] VMC means minimum control speed with the critical engine inoperative.
(b) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative and maintain straight flight with an angle of bank of not more than 5 degrees.
(c) VMC may not exceed 1.13 VSR with—
(1) Maximum available takeoff power or thrust on the engines;
(2) The most unfavorable center of gravity;
(3) The airplane trimmed for takeoff;
(4) The maximum sea level takeoff weight (or any lesser weight necessary to show VMC);
(5) The airplane in the most critical takeoff configuration existing along the flight path after the airplane becomes airborne, except with the landing gear retracted;
(6) The airplane airborne and the ground effect negligible; and
(7) If applicable, the propeller of the inoperative engine—
(i) Windmilling;
(ii) In the most probable position for the specific design of the propeller control; or
(iii) Feathered, if the airplane has an automatic feathering device acceptable for showing compliance with the climb requirements of § 25.121.
(d) The rudder forces required to maintain control at VMC may not exceed 150 pounds nor may it be necessary to reduce power or thrust of the operative engines. During recovery, the airplane may not assume any dangerous attitude or require exceptional piloting skill, alertness, or strength to prevent a heading change of more than 20 degrees.
Source: 14 CFR 1.2
2
What it really means
If you lose the critical engine you should be able to maintain directional control with no more than a 20° excursion from your desired heading using no more than 5° of bank. You should be able to do this with the operating engine at takeoff power, the flaps set at their takeoff position, the gear retracted, and outside of ground effect.
All that being said, note the regulation allows the manufacturer to set this speed at sea level and at a weight of their choosing ("or any lesser weight necessary to show VMC."). In other words, VMC is a made up number that may or may not deliver what you are expecting. My advice: the number is advisory, keep it in mind, but realize it might not be enough to keep you in control.
3
Gulfstream example
VMCA, MINIMUM CONTROL SPEED, AIR (106 KCAS) - in the takeoff configuration (Flaps 10 and 20), VMCA is the lowest speed at which the airplane can have a sudden engine failure and maintain directional control in the air with maximum available takeoff thrust on the operative engine.
Source: Gulfstream G450 Airplane Flight Manual § 5.1-2
Gulfstream does not provide any charts for this number but does provide a number with the statement it is "the lowest speed..." Your slowest V2 (Takeoff Safety Speed) is significantly higher, 130 knots for a very light G450. Fortunately the aircraft comes with many slow speed cues that are far more valuable to you that this artificial number.
References
(Source material)
14 CFR 1, Title 14: Aeronautics and Space, Definitions and Abbreviations, Federal Aviation Administration, Department of Transportation
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
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