G'Day everyoneFor the last two days, I havent been able to engage VNAV on the autopilot. I have no idea why as nothing has changed from the way I used to program the flights.Once airborne and about 1000ft, I engage autopilot, LNAV and VNAV. I get the bell ringing thing and a message on the FMC saying "UNABLE NEXT ALT", "VNAV Disconnect". Im I dont know whats causing this.ZFW: 114.5Fuel: 6.3Crusing Altitude: FL355Distance: 640nmCI: 16 (as per the VA I fly for)I have FL355 set in the MCP so I dont know what could be stopping it from climbing to that level.Also, usually when I program the flight into the FMC.. it creates the FLs and Speeds on the LEGS page. Now.. it deletes them all once the UNABLE NEXT ALT comes up!Any help would be great :)

The fmc is telling you it cannot reach the nezt alt. constraint at the current config. Try imputing a lower alt in the LEGS PG. Just a thought, if I'm wrong someone will correct me.Dave FisherCYYZP4 2.8 GB 478P 533 MHZ CPUP4P800S Asus Motherboard512 DDRGeforce Ti 4200/128Maxtor 80 Gig ATA 133 HDhttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

Please sign your posts with your real name, as per forum rules.What happens here is that you probably set the next wpt constraint at an altitude that's not reachable by the aircraft. When the VNAV disconnect message is issued, check your CLB page and see if there is something displayed at the line ERR AT xxx, where xxx is the wpt id. This is the amount of undershoot predicted by the FMC.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

Ryanjust an idea but are both your weights in the same weight measure?If you are in pounds then I would have thought 6.3 thousand pounds of fuel is a bit light on for 640Nm and if you are in kgs then you have a very heavy aircraft. I don't know how it works but the FMC may not think you can get to FL355 at 114Thope it helps, Nick

Trust Anthony since he WROTE the code for the FMC! If you are loading a departure procedure it most certainly contains some wpt restriction that's causing an issue. Always check the route on the legs page and review what is restricted where. Sometimes one must use max climb to achieve a restriction etc. It's all about proper planning ahead of time so that nothing takes you by suprise. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpgAMD 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 |

Who didn't sign their name? I see a Ryan and a Dave Fisher.If these guys are doing it wrong, then I must be too.Looking forward to being educated in the error of my ways..TimNZWN

I believe that the 6.3 is 6300kg? I use a fuel planner (I think its the one on the PMDG download page) and you can press a button that automatically refuels the aircraft...With the ZFW, I have to convert it to lbs as the ZFW on the planner is it kg. (I manage to confuse myself all the time regarding this part).What I dont understand is that I fly this route atleast 3 times a week! and I dont usually have any problems.CheersPS.. Thanks for the help everyone! :)