Whilst we are on the topic of VNAV. I would like to ask a question of my own that I get puzzled by.
I fly 95% of the time on the VATSIM network and the PMDG 737 has really helped me so much in understanding systems of flight and everything in between. I cannot express enough how much I have learnt from this aircraft.
However one thing puzzles me. I reach top of decent with VNAV & LNAV armed. I am given a decent altitude by ATC and down we go as per normal. For instance I am flying FL340 and get given FL250 everything is normal. I then get a further decent from FL250 to FL120 for instance and when I push ALT INTV to confirm my chance of Altitude sometimes the VNAV will drop out? I rearm VNAV and It will activate however it changes the Altitudes in the FMC. I have attached a before and after as it happened last night again.
I am assuming that it is because I am “Intervening” with the planned decent compared to what I am actually given by ATC but what stumps me is how come this only happens sometimes and how can I avoid it?
Upon rearming VNAV the aircraft behaves normally and will decent correctly so there is no issue there only it dropping out on the initial ALT INTV push.
Many Thanks