Flying the Boeing 737 NG 600/700 with the autopilot the VNAV setting is disengaged by the program when flying at cruise altitude and cruisespeed although the FMC had been prepared thoroughly(i.e a good flight plan with a speed of 250 and a cruise altitude of 10000 feet). An announcement appears in the FMC display: unable CRZ alt, although the plane is actually flying at 10000 feet with speed 250.VNAV can not be engaged again.??!!Is the program (FMC within PMDG B 737 NG ) unstable?Am i doing something wrong?Wim J.Jongejan

>Flying the Boeing 737 NG 600/700 with the autopilot the VNAV>setting is disengaged by the program when flying at cruise>altitude and cruisespeed although the FMC had been prepared>thoroughly(i.e a good flight plan with a speed of 250 and a>cruise altitude of 10000 feet). An announcement appears in the>FMC display: unable CRZ alt, although the plane is actually>flying at 10000 feet with speed 250.>VNAV can not be engaged again.??!!>Is the program (FMC within PMDG B 737 NG ) unstable?>Am i doing something wrong?>Wim J.JongejanWim,If you get an "Unable CRZ ALT" message, then it means the FMC predicts that the airplane cannot reach the new CRZ ALT due to performance limitations. Clear the message and review the CRZ ALT selection.Or it means the FMC predicts that no cruise time is possible at the entered CRZ ALT. Clear the message and select a lower CRZ ALT. Floyd

Did you RESET the MCP to the CRZ ALT? Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/southparkcartmad.gif[h3]PMDG 747![/h3]Caution! Not a real pilot, but do play one on TV ;-)ASUS KV8 DLX | AMD 3200 64 | 1 GIG PC 3200 DDR | GIGABYTE 5700 ULTRA | ViewSonic VP192b 19" |

It doesn't have to be either of the above reasons, although they are certainly valid.The NG is equipped with powerfull engines so when flying the NG manually according to the Flight Director only (no autopilot LNAV/VNAV engaged) the aircraft may out-perform the climb profile in the FMC if this has any altitude AT constraints. For example, the FMC may have a constraint at BALOX saying cross AT 4500 ft. which, when flown manually, is actually crossed at 7340 ft. This means that the rest of the waypoints are "falling behind" and the autopilot will respond with an error message when VNAV is engaged. The solution is to manually remove the AT contraints in the FMC legs page and allow it to update the remaining waypoints (e.g. type 250/) or changing the AT constraint to an AT OR ABOVE constraint i.e.(/8000A).