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VNAV

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For some reason I keep getting VNAV disconnect when descending with the 800 or 900. My speed is right on target, the glidepath is fine, MCP Alt is below next constraint, forecast winds are correct, yet this has happened over and over, usually just as I pass over a waypoint. I've never had a disconnect on the 600/700.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

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Sorry Dave but your discription is a little vauge, could you provide the routing with all restrictions? This way we can try it and make sure VNAV is within proper operating limits. Couple of things that come to mind are Speed constraint without an altitude constraint on the same wpt. Path angle exceeds vnav limits You are exceeding 15 knots below 10000' or couple of knots prior to VMO/MMO.. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpgAMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | WD SATA 80 GIG | 512 DDR 400 |

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Hi Randy, Iwas just on app. to KSFO flying the OAL Coaldale star to 28L. VNAV disengage flying over the MOD VOR. The correct speed was 290kts at MOD, I modified the speed on the legs pg. to 270kts at MOD. Could that have anything to do with the disconnect?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

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Yes it could IF you had only a spd restriction without the altitude on this wpt but why a spd restriction unless you are requested by ATC? I suppose if you are 270 knots you are above 10000' still, so if ATC does not require a spd on a wpt do not make it difficult for the aircraft to maintain a path and meet a spd restriction. Even the target spd down hill is only there for refernce, the aircraft makes no attempt to keep it unless you slow too much (I saw it kick in thrust at 25 knots under target). If you are going to impose restrictions simply switch to a spd descent which is a lot like LVL CH but will keep the set speeds and temporary level off at restrictions thence resume a descent without following the path. You could also just set the MCP to level off at each restriction altitude (which will also help slow) then reset the MCP and push VNAV to resume the path. Many ways but just remember that a path descent is not the best choice for a descent with many restrictions. Lately I have been using V/S manually maintaining the path sure makes for some for learning..[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpgAMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | WD SATA 80 GIG | 512 DDR 400 |

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