Mad (Flt) Scientist
Many years ago I came across this definition of Vmca & Vmc in an American aviation textbook. A definition that I have also seen in military text books as well.
VMCA
The minimum speed at which directional control can be maintained with full rudder deflection is designated Vmca. This speed is established by a manufacturer under criteria specified by the Certifying Authority, taking account of the factors below. For American aircraft Vmca must not be more than 1.2 x the Stall Speed (Clean) at maximum Take-off
Weight.
VMCA CRITERIA
The Vmca as determined by the manufacturer is marked on the airspeed indicator (ASI) of twin-engine aircraft by a red radial line. For aircraft certificated by the American FAA, the criteria used to determine Vmca are as follows:
1. Failure of the "critical engine",
2. Engine controls set for take-off or maximum available power,
3. The most unfavourable (most rearward) centre of gravity (within limits),
4. The aircraft trimmed for take-off,
5. The maximum allowable sea-level take-off weight which permits 3, in ISA conditions,
6. Flaps in the take-off position,
7. Landing gear retracted,
8. Cowl flaps in the take-off position (open),
9. The propeller of the inoperative engine windmilling,
10. The aircraft airborne and out of ground effect,
11. No more than 150 lbs. of force required to maintain full rudder deflection, and
12. A maximum of five degrees of bank towards the operating engine.
These criteria reflect a "worst case" situation, ie, a complete engine failure after take-off.
VMC
Unlike Vmca, Vmc is a variable minimum control speed and must be determined by the pilot. Usually it is very close to Vmca and in most circumstances is less than it. It should be therefore stated that even thought Vmca and Vmc are different, they should be treated as the same and not experimented with.
VMC CRITERIA
Vmc as mentioned above is determined by the pilot. It is not marked on the airspeed indicator and in general there isn't enough information in the pilots operating handbook for the pilot to work it out. The criteria used for determining Vmc by the pilot are:
1. Which engine has failed,
2. The configuration of the inoperative propeller,
3. Power setting of live engine,
4. The altitude of the aircraft,
5. The level of turbulence the aircraft is experiencing,
6. The position of the centre of gravity,
7. The aircraft configuration, ie, Flaps, Landing Gear, and Cowl Flaps up or down, open or closed,
8. Bank angle of the aircraft,
9. The weight of the aircraft.