Yes, SNS3Guppy, they are used interchangeabley, but they shouldn't be.
You said:
how accurate is the little red line?
For you, on any given day? It's not. It reflects a demonstrated certification number obtained by the manufacturer. Any change in weight, center of gravity, technique, configuration, air density, propeller efficiency, power setting, aerodynamics, vertical gust (turbulence), etc, will have an impact on that figure.
Vmc - the value which changes changes depending on weight, centre of gravity, etc etc etc, is generally measured and demonstrated by first failing an engine at a safe speed, then gradually slowing down.
In contrast, red line speed, Vmca, is measure by first flying at the given speed, then seeing whether control can be maintained when a sudden engine failure occurs.
By using the two interchangeably, this important difference is being overlooked (or at least might well be overlooked by someone reading this thread who doesn't know better), which might lead someone to expect an aeroplane to be capable of doing something which it can't.
(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.)
FFF
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