Hi All, Have a question regarding the proper usage of VNAV. I understand that it is not supposed to be switched on before reaching 4000 ft. Why is that, actually? I was flying a short trip from EGLL and taking the BPK5J departure. This SID has a final altitude of 6000 ft, with 3000 feet on one of the first waypoints and 4000 ft on the second to last waypoint before the final 6000 ft. Because of this, I set the MCP alt to 6000. After take off, I engaged the MCP at roughly 500 ft and LNAV close to 1000 ft (the MCP speed was set to 250). Shortly thereafter I engaged VNAV and all went well until I passed the first waypoint with the 3000 ft alt constraint. Just after passing the waypoint, the FMC had the error message UNABLE NEXT ALT. This baffles me at the MCP alt was set to 6000 so there should be ample room to climb higher. Did I do something wrong? I know that in real life, you would almost always get vectors but suppose that you were to fly the SID as published how would that be accomplished.With thanks in advanceBoaz

4000'? No that is wrong. First you never want to engage VNAV before you retract the flaps so this in itself limts you to at least 1000'. You can select a ROLL mode at 400' AGL and it is possible (although not wise) to engage VNAV at 400'AGL. UNABLE NEXT ALT is telling you to REVIEW the planned restrictions and modify because it's not possible for whatever reason. There can be many reasons this is the case here and you need to provide the plan with the restrictions and all other data for us to determine what's going on on this particular flight. [h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] AMD XP 2200 |MUNCHKIN 512 DDR RAM |ECS[/b ][i] K7S5A MB[/i] |GF2 MX 32 MEG and still runs GOOD!|WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"[/h3]