[B]Dariuszw,
You asked - Can you tell me one airline or commercial aircraft which flies above Mcrit as normal operation ?
All of them, as long as we're talking about Jet high altitude operations. At lower than normal levels, EAS will be the controlling speed.
rudderrudderrat,
Seasons Greetings to you too rudderrudderrat.
I hope that a thousand words is better than a picture, as I cannot post the requisite diagrams from this computer. There was considerable discussion on this topic 2 to 3 years ago, and you may find diagrams with a little digging. The word version goes like this -
As an example, if we are talking about Maximum Range Cruise (MRC), it is developed (for still air) by taking the point of tangency from the 0/0 origin to the Total Drag curve. At lower altitudes where only the Low Speed Drag Polars come into play (typically not used by jets excepting short sectors), for a given weight, this results in a constant EAS for all levels. This means a slowly increasing CAS with increasing Altitude. With increasing Level (typically about 30,000 feet for aircraft such as the A320/B737) the High Speed Drag Polars resulting from Wave Drag formed above Mcrit are added to the Low Speed Polars to ascertain the total drag. If the typical drag curve is envisaged, it appears first at the high speed end of the curve, above Vmo and of no concern. As altitude increases further, the higher end of the normal drag curve is 'lopped off' as the High Speed polar slides down the curve, and replaced by a more steeply rising curve.
The nature of the now-modified curve is that for approximately the first M0.04 or so after Mcrit, we have only gently increasing drag, with a steeper increase after that. When the tangent is taken for MRC, the point of tangency is still somewhat ABOVE Mcrit. MRC is about the lowest of the commonly used en-route speeds, and higher speeds (such as LRC or Cost Index above 0) will be further above Mcrit. Earlier crude fatter and unswept wings did not enjoy such an increase above Mcrit, but still had a point of tangency .02 to .03 above Mcrit.
LRC is out of fashion now, and there is indeed a penalty in flying at this speed, in fact the LRC speed is derived from an arbitary penalty of 1.0%, i.e. a penalty of 1% fuel burn increase for flying above MRC. An unusual figure - derived from a penalty.
With respect to your comment - "I thought engine efficiency improved with higher temperature and pressure, so up to MAX Cruise power was more efficient"., I think that we agree, Max Cruise Thrust is typically ABOUT 91 to 92% N, which is within the Optimum TSFC band. We have to be careful, as individual engine characteristics vary.
Best Regards,
Old Smokey