Yes, SNS3Guppy, they are used interchangeabley, but they shouldn't be.
Of course they should. Vmc is the point at which directional control is lost with the critical engine suddenly made inoperative; Vmca is that point in the air, and Vmcg is that point on the ground.
(It is very important, when talking about any speed, that we are very clear which speed it is that we are talking about. We don't use Vne, Vno and Va interchangeably because they both happen to be related to the high-speed limits of the aircraft. If we did, it would cause confusion. And so we also shouldn't use Vmc and Vmca interchangeably either, because that too will cause confusion.)
Apples and oranges, when you speak of Vmc vs. Vmca, and confusing speeds with no relation to one another such as Vno, Va, and Vne.
Vmc and Vmca are the same. Vno, Va, and Vne are not.
Vmc means minimum control speed with the critical engine inoperative. Period.
The certification standard for most light twins, or at least those produced in the United States, is 14 CFR Part 23. The definitions for Part 23 are found in Part 1, and the definition provided therein for Vmc is quite simply "
VMC means minimum control speed with the critical engine inoperative."
Provision of this information in the aircraft flight manual is a requirement of the certification regulation 14 CFR 23.1583, as noted:
§ 23.1583 Operating limitations.
The Airplane Flight Manual must contain operating limitations determined under this part 23, including the following -
(a) Airspeed limitations. The following information must be furnished:
(1) Information necessary for the marking of the airspeed limits on the indicator as required in § 23.1545, and the significance of each of those limits and of the color coding used on the indicator.
(2) The speeds VMC, VO, VLE, and VLO, if established, and their significance.
Pick up a dozen aircraft flight manuals for various light twins, and you'll find the terminology used interchangeably; some manufacturers will refer to the red line as Vmc, others to it as Vmca, and in some cases, you'll find it used interchangeably in the same aircraft flight manual or pilot operating handbook.
The FAA uses it interchangeably throughout it's literature, because Vmca is Vmc. Vmc is defined as an airborne speed by the use of Vmca, and as a ground speed by Vmcg.
The certification regulation spelling out the establishment of Vmc is 14 CFR 23.149:
§ 23.149 Minimum control speed.
(a) 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 thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.
(b) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for takeoff;
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff position throughout.
(c) For all airplanes except reciprocating engine powered airplanes of 6,000 pounds or less maximum weight, the conditions of paragraph (a) of this section must also be met for the landing configuration with -
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for an approach, with all engines operating, at VREF, at an approach gradient equal to the steepest used in the landing distance demonstration of § 23.75;
(3) Flaps in the landing position;
(4) Landing gear extended; and
(5) All propeller controls in the position recommended for approach with all engines operating.
(d) A minimum speed to intentionally render the critical engine inoperative must be established and designated as the safe, intentional, one engine inoperative speed, VSSE.
(e) At VMC, the rudder pedal force required to maintain control must not exceed 150 pounds and it must not be necessary to reduce power of the operative engine(s). During the maneuver, the airplane must not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20 degrees.
(f) At the option of the applicant, to comply with the requirements of § 23.51(c)(1), VMCG may be determined. VMCG is the minimum control speed on the ground, and is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and using the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued. In the determination of VMCG, assuming that the path of the airplane accelerating with all engines operating is along the centerline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centerline is completed may not deviate more than 30 feet laterally from the centerline at any point. VMCG must be established with -
(1) The airplane in each takeoff configuration or, at the option of the applicant, in the most critical takeoff configuration;
(2) Maximum available takeoff power on the operating engines;
(3) The most unfavorable center of gravity;
4) The airplane trimmed for takeoff; and
(5) The most unfavorable weight in the range of takeoff weights.
Let's not forget Vmcl, which is the minimum control speed in landing configuration. Then there's Vmcl-2, which is applicable to aircraft with three or more engines (before anyone gets uppity about this being a light airplane forum, let's nor forget that there are indeed light airplanes with three or more engines; the Trilander springs to mind). Clearly it's not quite as simple as you might think.
Vmc is the basic definition, and simply means the speed at which directional control can no longer be maintained. It's specified, or clarified to the phase of operation by the addition of letters. In the case of the topic under discussion Vmca, to signify the minimum control speed while airborne.
Lest we simply read Part 23 and let it drop, let's keep in mind that not only do various manufacturers interchangeably use Vmc and Vmca, but so does the FAA throughout it's various publications. In fact, in some handbooks, such as the Pilots Handbook of Aeronautical Knowledge, Vmca is used, rather than Vmc. In Appendix A of Part 60, dealing with certification of simulators, for example, the FAA only refers to Vmca. In Appendix F, providing definitions, we find the following:
Vmc Minimum Control Speed.
Vmca Minimum Control Speed in the air.
Vmcg Minimum Control Speed on the ground.
Vmcl Minimum Control Speed--Landing.
In FAA H-8083-3A, the Airplane Flying Handbook, Vr is defined as:
• VR — Speed at which the rotation of the airplane is initiated to takeoff attitude. This speed cannot be less than V1 or less than 1.05 x VMCA (minimum control speed in the air). On a single-engine takeoff, it must also allow for the acceleration to V2 at the 35-foot height at the end of the runway.
FAA H-8083-25a, the Pilots Handbook of Aeronautical Knowledge (under Chapter 10, Aircraft Performance), we find the terms defined as:
• VMCG—minimum control speed on the ground, with one engine inoperative, (critical engine on two-engine airplanes) takeoff power on other engine(s), using aerodynamic controls only for directional control (must be less than V1).
• VMCA—minimum control speed in the air, with one engine inoperative, (critical engine on two-engine aircraft) operating engine(s) at takeoff power, maximum of 5° bank into the good engine(s).
Advisory Circular AC25-7A, a flight test guide for certification, states:
(1) General.
(i) Prior to beginning the minimum control speed tests, an evaluation should be conducted to determine which engine's failure will result in the largest asymmetric yawing moment (i.e., the "critical" engine). This is typically done by setting one outboard engine to maximum thrust, setting the corresponding opposite engine at idle, and decelerating with wings level until full rudder is required. By alternating power on/power off from left to right, the critical engine can be defined as the idle engine that requires the highest minimum speed to maintain a constant heading with full rudder deflection.
(ii) For propeller-driven airplanes. VMCA, VMCG, and VMCL (and VMCL-2, as applicable) should be determined by rendering the critical engine(s) inoperative and allowing the propeller to attain the position it automatically assumes. However, for some engine/propeller installations, a more critical drag condition could be produced as the result of a failure mode that results in a partial power condition that does not actuate the automatic propeller drag reduction system (e.g., autofeather system). One example is a turbopropeller installation that can have a fuel control failure, which causes the engine to go to flight idle, resulting in a higher asymmetric yawing moment than would result from an inoperative engine. In such cases, the minimum control speed tests must be conducted using the most critical failure mode. For propeller-driven airplanes where VMCA is based on operation of a propeller drag reduction system, VMCA should also be defined with the critical engine at idle to address the training situation where engine failure is simulated by retarding the critical engine to idle. If VMCA at idle is more than one knot greater than for the engine failure with an operating drag reduction system, the idle engine VMCA should be included in the Normal Procedures section of the Airplane Flight Manual (AFM) as advisory information to maintain the level of safety in the aforementioned training situation.
(iii) Airplane Flight Manual values of VMCA, VMCG, and VMCL (and VMCL-2, as applicable) should be based on the maximum net thrust reasonably expected for a production engine. These speeds should not be based on specification thrust, since this value represents the minimum thrust guaranteed by the engine manufacturer, and the resulting minimum control speeds will not be representative of what could be achieved in operation. The maximum thrust used for scheduled AFM minimum control speeds should represent the high side of the tolerance band, but may be determined by analysis instead of tests.
(iv) When determining VMCA, VMCL, and VMCL-2, consideration should be given to the adverse effect of maximum approved lateral fuel imbalance on lateral control availability. This is especially of concern if tests or analysis show that the lateral control available is the determining factor of a particular VMC.
Note this last sentence, "of a particular Vmc." There is more than one Vmc; various terms are used to cllarify Vmc. Vmc means only the speed at which directional control is lost. It doesn't define which speed is to be used, hence the application of Vmca, Vmcg, and Vmcl (and of course, Vmcl-2).
Vmca is Vmc. Vmcg is Vmc. Vmcl is Vmc. Vmcl-2 is Vmc.
The terms may be used interchangeably, except that when the terminology "Vmc" is given, it doesn't specifically state whether in flight or on the ground, or reference the configuration or circumstances under which control is expected to be lost. It is,however, used frequently by the governing bodies, certification authorities, manufacturers, and test facilities and personnel, in an interchangeable basis with Vmca.