POSSIBLE VNAV problem cause (resolution)

[Guest mulletman]

Hey all.I am unsure if this has been baught up before, and if it has, i am sorry. feel free to delete this post.However, today i found a possible cause for the VNAV climb issues that some users have been experiencing. This is there the VNAV starts to descend whild you are supposed to be in the climb phase of the flight.I have only had this happen once, but i saw a cause. Altitude limits in the SID on the LEGS page. As soon as VNAV engaged, it was following these, or as far as i know. That ment it stopped climbing at the first restriction.ATC (default) had assigned me 12000ft as my first altitude, so that was set on the MCP. Unknown to me, the SID i was flying, to ELB VOR from LIRF 34L, had a restriction altitude of 4000A. I took off, with LNAV and VNAV armed which engaged at 50' and 400' respectively. VNAV began the climb. Near 4000ft the aircraft started doing strange things. it flew up through 400, but then came back down. So i engaged FL CH, still naware of the restrictions. After initiating the climb, throught around 6000, i re-engaged VNAV. The aircraft started to descend. Some users have experienced 'DRAG REQUIRED' messages at this point, and as the VNAV page of the CDU was now showing ACT ECON DES, i presumed the CDU thought i was in sescent mode. Thus, this message could be expectted. I did not fly as far as the first restricted waypoint, but exited, as as far as i know there is no way to go back to CRZ page form the DES page, and with a 10 hour flight ahead of me, i figured best to restart.On the next attempt i paid close attention to the LEGS page, and sure enough, when i deleted the restrictions, all showed well. I beleve this to be the solution.Of course this leads to three anomalies.1 - Why did VNAV let me climb through 4000 in the first place? Unless i have figured this worng, and i wasnt watching things properly.2 - the Veritical indicator appeared when i re-=engaged VNAV, due to CLB mode. But it was showing my vertical path to be ABOVE me (by about 2000ft), when according to my FMC,it should have been below. Unless it was thinking of the STAR at the end of the route.3 - When did the cruise phase initiate. there has to be one, to move to DES. I presume it was in the time that i spent at around 4000'.So in conclusion, my basic hypothosis to the descending-climb is that the VNAV does not follow the sid accordingly. Perhaps for the next patch, (when ever it may come) some coding could be written, that tells the CDU and autopilot, when it is flying a SID, and therefor is not in the cruise, and when it is actually flying a STAR.I currently have only two other issues. But judging my the length of this tread, i deem them to be allowed to wait.I hope someone can test this theory (by flying a SID with restrictions, and then the same, but delete the restrictions) and that it works.Happy flying.Fred 'mulletman' Clark.