I having some problems while in flight, while in cruise i seem to lose airspeed (or wont hold airspeed that set in FMC for that desire flight level example 84.3 at FL340 ) the Vnav switches off and FL CH comes on why does this happen i seem to program the FMC correctly and i use FS Build for planning routes and active sky for my weather any help or advice would be gratefulMichael Troy

Hi Michael,I'm not sure. I might suggest also posting this in the PMDG 747 forum; this is more their area.Cheers,Bryan