My route: GCTS - LPM (VOR) - BV167/15 (NDB) - BV167/8 - GCLA, CRZ ALT 10000.BV is the NDB needed for the approach to GCLA. I inserted the two BV-waypoints (one 15NM, one 8NM on "radial" 167) for speed management: I restrict speed at the first to 200 and at the second to 160 by defining speed constraints.The aircraft correctly flies LNAV and VNAV up to the 10000 ft.Then I intend to go up to 15000: I set the ALT on the MCP to the new cruise level, cahnge the CRZ ALT on the FMC and use LVL CHG. During the LVL CHG, I switch back to VNAV (don't know if that's correct, it was more a test to see what happens), and the FMS correctly catches the new CRZ ALT, all systems nominal.Descent is pretty good as well, but speed management is not: at the BV167/15 waypoint, speed is at about 240, and suddenly the FMC reports that VNAV is disconnected.I flew the rest of the route manually with spoilers deployed and gear and flaps down as soon as possible to slow down, which was a very tough job.Why did VNAV disconnect happen at all? Why was the FMS not capable to do proper speed management?Andreas

Sure it is under normal conditions. But you must remeber the golden rule about flying a large aircraft such as this, you cannot go downhill and expect to slow at the same time, it's one or the other. If you are putting constraints for constraint's sakes then I would ask why? You only use constraints IF they are issued by ATC OR for a procedure. This plays an important part in descent planning. You either want to use the PATH or SPEED. If you use the SPD DESCENT then the aircraft will keep the speed and not the path. For tyhe PATH DESCENT it will keep the PTH and not the speed. If you desire to stay on your speed and keep the path then you have to monitor your descent using all means nessary to keep the speed under control with use of spoilers or throttle depending. The aircraft will keep soft constraints IF you do not ask VNAV to perform wonders and exteme glide angles. I leave it up to you to figure out what extreme is for what bird you are flying. But will say if you path angle exceeds 7 degrees above 10000' or 6 degrees under 10' VNAV will disconnect just like the real bird. If you are comming up to a wpt restriction and are at 240 knots and all of a sudden need ot be at 167 knots and just leave it to the aircraft to slow in time chances are you won't make it and coming down hill will not help the situation. Please keep in mind that pilots don't just program the FMC and hope for the best, it is a tool but they are in control, they must monitor every aspect so if the AFDS is not performing the way it ought to ***click "I got the aircraft". While a nice summer day with little wind can afford some nice automatics, real life ATC-Routing-Weather will make most use other modes of the aircraft like LVL CH when it's too much of a workload to reprogram the FMC and you are being vectored all over the place. So knowing what the AFDS is suppose to do is a big step in mastering the automatics of this aircraft. In most cases I believe it is only our lack of understanding such complex systems. Heck real pilots spend an enormous amount of time learning some of the things we consider basic here on this NG... Best Wishes,[h4]Randy J. Smith[/h4][h3]P M D G's 747-400[/h3][h4]coming to a runway near you[/h4][/font color]Caution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro | Windows Xp Pro 64 |