Can someone explain to me the general logic behind the reason why commercial aircraft these days fly higher yet slower? As my altitude increases in my iFly 73X I understand that my mach speed doesn't change, and why it's more EFFICIENT for my aircraft to fly at higher altitude, but what I don't quite understand is why does my ground speed go down? And wouldn't this decline in ground speed negatively impact commercial airlines scheduling vis a vis ETE and arrival times? Wouldn't this concern the airlines a bit more than they apparently do?? I mean, it seems that even for shorter routes commercial aircraft are climbing to FL390 AND FL410 more than they used to, at least in the US they are. I'm sure it has something to do with mach and ground speeds in relation to the surface of a sphere (the Earth) but I'm just not getting it into my antique noggin, and I can't really find anything on it in all my manuals or Google (don't know what to ask!). Thanks in advance for any insight you might be able to provide!

Your ground speed decreases as you fly higher once you've transitioned to holding mach because the speed of sound is decreasing as you go higher. The speed of sound is dependent on temperature. As temperature decreases, mach 1.0 decreases. At a nominal sea level, 15 celsius, mach 1 is about 660 kts. At a typical temperaure of -45 celsius at 30,000', mach 1 is about 580 kts. That is why you see a decrease in speed as you go higher. Flying higher does impact ETE and arrival times. If the airline wanted to get somewhere as fast as possible, then they would fly the flight at the altitude where the plane's Vmo is the same as the plane's Mmo. That altitude is where the plane can actually travel across the Earth fastest. Any lower and you could have been flying at a higher TAS, and any higher, Mmo begins restricting your speed. But flying higher wil also decrease fuel consumption. So the two have to be weighed against each other. Fly lower, get there faster, but burn more gas per hour. Or fly higher, get there slower, but burn less gas per hour. These days, the fly higher, get there slower, burn less gas per hour generally wins.

Thank you for such a clear and concise explanation, KevinAU. I now understand!