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Guest venquessa

Another VNAV issue.

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I have noticed an error occasionally saying "UNABLE NEXT ALT" the last time this happened I had time to play around and see what was wrong with it.Situ was (roughly)Descending into Gatwick, was using time acceleration and just slowed it down to x1 when it was nearing the first ALT restriction of 7000ft, followed by another 7000ft hold and then a 5000ft.The VNAV dropped under the 7000ft ALT and then discoed. No error. When I realised (at about 6600ft) I reengaged the VNAV and it popped back off, "UNABLE NEXT ALT", which was 7000ft. MCP ALT was 7000ft. To get the VNAV to reenable I had to asscend manually to just above 7000ft and it worked again.I was going to do a test, but I assume the VNAV is capable of doing more than one ascent and descent in a flight? I mean, say going down to 7000ft then back up to 10,000ft or whatever?Thanks.PS. I can't be sure but this may be STAR related. I have found some of the STARs unreliable, or due to implementation limits of the current FMC they can't actually manage to fly/plan the STAR correctly. An example is EGSS Lorel3F arrival, the multi-pointed hold with entended entry is a mess and it should be flown (as far as I can tell) not as a hold initially but as part of the STAR. Deleting the HOLD AT completely messes up the STAR. Another example is the Northern arrival Lanzorote STAR, which has WPTs back in England. LOL LONG star that one. Generally I just fly them with standard radio NAV when this happens.

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Hi Paul,Unfortunately we need to see the exact situation. For me, the disco's usually occur when the FMC is not programmed correctly for the procedure. Example: the FMC is asking the autopilot to do vertical navigation which, for the current conditions, can't be done. To follow the FMC would put the autopilot outside of its performance constraints.VNAV can have multiple points, but again they have to be within the limits of autpilot performance.See if you can give us the details for and maybe a few screen shots, we can try to sort this out.Concerning some of the STAR data, you have to review them closely. Many times, altitudes published for STARS are changed by ATC when cleared for the procedure. The JVL4 at KORD is a good example. VATSIM ATC almost always gives me instructions to cross Teddy at 10,000 when the published STAR says "... can expect 11,000". Key word here is "Expect". In the real world, ATC typically will give instructions on how to decend on the STAR.Ok? :-)

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Paul, open up the DEScent page on thew FMC and snap a screenshot BETWEEN the two WPTs. The V/B= think ANGLE cannot be more than 7 degrees on any path segemt or 6 degrees under 10000'. The FMC calculates here as it does in the RW. Each segment between wpts is a path calculation unto itself. Now others things can become an issue. While you can have an altitude on a wpt without speed restriction this cannot go the other way without an altitude and just a speed. In other words VNAV has strict criteria that must be adhered too. If you are in VNAV path and expect a bunch of restriction simply forsake VNAV path and go VNAV SPD. Vnav path should be the mode when it is smooth sailing not littlered with restrictions etc. 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 |

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