I'll endeavour to explain by a step process (YMML - WSSS using FS2Crew).
1. Open Simbrief and select/create flight but do not enter Freight, Pax or ZFW values
2. Crank up the 777-200 (FSX) cold and dark - open FMS
3. In FMS go Payload/Random and scroll until I get what I want (e.g. Pax 195, Freight 60.3t, ZFW 442.6)
4. Transfer these values to Simbrief and 'generate Op'. This provides me with Cruise (FL340), step climbs (FL360), fuel (136506 & 16.9 res) and cost index (15)
5. Fuel is then added to FMS before starting up FS2Crew
6. Once power is established in 777, I load up FMS. When I open Perf Init, the cruise (FL360), CI (85) and reserves (16.0)
7. I reject the page and enter the Simbrief values and execute
8. Approaching cruise (FL340) all appears normal as aircraft begins to level off. Within seconds, it will then begin to climb to the initial pre-determined PMDG value (FL360). MCP has automatically changed to FL360 and VNAV values in FMS VNAV has changed also. CI has reverted back to 85
9. If I attempt to change these values back to the Simbrief values it removes TOD calculations (ie there is no TOD on ND or in Prog page in FMS..... no big deal, but not expected)
cheers
Steve