Bacalau,
Checked a few of your numbers -
SL ---> 15.000' - @SL, 227kts then increases linearly up to 230 at 15.000.
230 KIAS at 15000 feet = 227 Knots EAS, the same as the CAS at Sea Level, where EAS=CAS. Obviously the Aerodynamis structural limit which is dependant upon EAS.
22.000'--->25.000 (max) - decreases linearly from 221 at 22.000' to 208 at 25.000'
221 KIAS at 22000 ft = M 0.505 : 208 KIAS at 25000 ft = M 0.506.
For practical purposes both Mach Numbers are the same, only M 0.001 difference due to rounding Up/Down of the relevant CAS. Obviously a Mach No. limit, and PROBABLY limited by propeller tip speed.
15.000'--->22.000' - at/above 15.000 it has suddenly reduced to 220 then it increases to 221 @ 22.000'
A bit of a mystery that one, although the other 2 are very obvious. More than likely a simplified speed in the transition area between EAS limitation and Mach No. limitation. I suspect that the constant 220 KIAS is a 'rounded off' version of the 221 KIAS at 22000 feet.
Some numbers for your thought processes -
227 EAS = 231 CAS = M 0.506 at 19988 feet Pressure Height.
I hope that that solves 2 out of 3 mysterys.
LGB,
At last a dream come true, I hope that you're correct in stating that the newer B777 versions will have direct EAS readout. Great for piloting, but the problem remains for ATC in speed separation for the EAS aircraft with all of the other CAS aircraft.
Regards,
Old Smokey