Hey, just wondering if someone could help me on this. I am quite new to the 737NG which will most likely explain my ignorance but I cant figure out why I keep getting this FMC message. Flew from EHAM to EDDM and tried to descend via LNAV/VNAV. VNAV kept disengaging and LV CHG kept activating over and over again although I was on the correct descent path and as far as I could tell my speed was close to the calculated FMC value. Ended up high on the approach onto 26L and had to abort, grrrrrrrrrrr!!!Thanks for any info Regards: Zsolt

The error message indicates that although you believe you were close to the FMC value, in fact you speed was higher than the descent speed (as shown on the descent page) by more than a few knots. It will usually ask for spoilers before switching to LVL CHG, so I suspect something is going on besides what can be discerned from your brief explanation. Some things to check: Give the FMC forecast winds for the descent, use wind smoothing available (FSUIPC or Active Sky if using that), keep an eye on your indicated airspeed and control it before it exceeds the bug.Also, I generaly do not use VNAV for descent and prefer instead to use either LVL CHG or V/S as appropriate. The 737NG programming has some known bugs in the VNAV, which everyone expects to be resolved when 737NGX comes out (whenever that is). This is not unrealistic, since most the time in the real world your descent is controlled by ATC and not the computer, at least below 23000 in the US.

VNAV reverts to LVL CH when you reach a few knots prior to VMO/MMO (Maximum Operating Limit Speed). VNAV works exactly like the real 737 in this regard. VNAV also does not keep target descent speed on descents but the path profile and will sacrifice speed for path. To fix your issue you need to actively monitor the speed on descent and intervene when required. This is accomplished by adding thrust when more than 10 knots below target (Pushing the F4 button for a few seconds will do fine) or by use of spoilers when speed becomes excessive (more than 10 knots over target.) Of course more than 10 knots on descent is not normally an issue unless you are coming up to the 250/10000 airport restriction speed/altitude when the VNAV commanded speed target becomes 240 knots. So by monitoring your speed on descents you will no longer have this issue to worry about..Best,Randy J. Smith<<>>

Thanks guys for the help, will try these hints next flight.Regards: Zsolt

>Also, I generaly do not use VNAV for descent and prefer>instead to use either LVL CHG or V/S as appropriate. Say I need to be by a certain altitude by a set distance, for example when flying the STAR I try to keep to the altitudes the FMC gives me. VNAV does this almost perfectly, but how do I work out what V/S I need to reach the altitude if I wanted to stop using VNAV?Dave