Cessna 340 - lookin' for advice...

[Lifeisgood]

Vmc

Regarding “if Vmc speed reduces as altitude increases”. I pondered this question for a quite a while, discussed it with knowledgeable CFI’s and commercial pilots and would appreciate input from others.

Is this true in a turbocharged aircraft or for that matter any aircraft that is flat rated to a given altitude? I believe this generally held belief is due to DA decreasing with altitude and consequently less air molecules available for combustion, and therefore less power produced, and therefore less torque, and therefore a lower Vmc speed in non-turbocharged aircraft.

Regarding the wings; Because DA reduces with altitude there are less molecules of air hitting the front of the Pitot tube, so therefore we are trained to understand and use indicated air speed, IAS. Or to paraphrase, IAS is always used by pilots because IAS automatically compensates for DA. Therefore as pilots we understand that IAS is not TAS but to use IAS for all maneuvering decisions and understanding that true airspeed is higher due to effects caused by DA.


As per WIKI.
“Unless an aircraft is at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) and no wind, the IAS bears little relation to how fast an aircraft is moving in reference to the ground; however, because the air pressure and density affect IAS/CAS and an aircraft's flight characteristics in exactly the same way, IAS and CAS are extremely useful for controlling an aircraft, and the critical V speeds are usually given as IAS. In aneroid instruments the indicated airspeed drops-off with increasing altitude as air pressure decreases, and this leads to an apparent falling-off of airspeed at higher altitudes.”

So the question is;

Does the IAS Vmc speed reduce in an aircraft that has flat rated engines as DA increases?

Opinions appreciated.