The outside air temperature and tropopause might not be modelled world wide or even at all but above 30,000 feet in Aerofly observing the Mach limit in the climb will appropriately result in a rapidly reducing indicated airspeed so keep an eye on it but follow Mach No.
If a Mach No. hold in the climb is not possible try a carefully monitored vertical speed mode and creep up very slowly continuously reducing the rate to maintain Mach No whilst keeping the engines from overspeeding.
‘Reverse engineering’ the high altitude performance in Aerofly generates a remarkable specimen atmosphere miles away from the nominal International Standard Atmosphere.
You see for the flight i did yesterday, i let the aeroplane climb up to about 20/23 or 24,000 feet (while still having a target of 33,000 set in the MCP) and engaged the CRZ thrust limit. it promptly dropped the nose to about 300fpm and climbed the rest of the way inching up foot by foot. Today, i did almost the same thing but got a tad impatient so manually entered about 1000fpm around 32,000’ up (still in CLB or CLB 1 thrust limit) I have gathered so far that if it is allowed to climb up slowly then it will cruise at the right speed. Activating CRZ above 30,000 is still fine as long as the vertical speed is lowered, which the aircraft did automatically last time. In terms of values in the MCP, i aimed for Mach .785 on both occasions, which at 33,000’ is around 279KIAS, and 267KIAS at 35,000’, keeping the airspeed value punched in instead of switching to the Mach reading (either .78 or .79)