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Guest capherc

What is wrong with my IAPs?

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Hello. I seem to be having difficulties with my CYLW airport IAPs. The FMC does not report any errors with the sidstar file, but for some reason the aircraft commences descent as soon as I reset the MCP altitude. The aircraft only descends about 1500 feet, then levels off. It does not maintain the 283kt descent as commanded by the FMC. Instead, it holds about 260kts. I don't get any DRAG REQUIRED, and the VNAV and LNAV are lit up on the MCP. On the FD, it shows VNAV PATH DESCENT. What is wrong with my file?Some of the altitude restraints are a tad aggressive, but not too bad. Has anyone had this happen?Kenny Rogers

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"Some of the altitude restraints are a tad aggressive" It is impossible to tell unless you have your DES page from the FMC on a screenshot (moved to the right so that the FMC is not blocking the MCP or PFD if you please ;) then it will be easy to tell you what is going on. I will say that it sounds like you entered a temporary level off at MCP ALTitude. In this case VNAV will stay engaged and the FMA will show FMC SPD/ALT HOLD. You would need to reset the MCP ALT and hit the VNAV button (if this was in fact the case here)[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

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Thanks for the reply. See the attached TIF image below, including my SIDSTAR file. I have just reset my MCP altitude from FL250 to 5,600 feet (glide path intercept altitude). Notice that the aircraft has started descending prior to T/D. Also note that the aircraft is disregarding the bugged descent speed of 283kts. Seconds after this photo was taken, the aircraft leveled at FL235 and maintained an indicated airspeed of 262kts. The LNAV maintained a flawless track, but the VNAV gave up. Also note that I have reproduced this occurrence three times in no-wind conditions.Please help! Thanks. Merry Christmas, and Happy Festivus to all!Kenny Rogers

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By the way,The second magenta line is the MISSED APPROACH procedure, which terminates at the YDC VOR.Kenny Rogers

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You have something really wrong going on there! The ND actually shows T/D not yet reached yet the deviation scale clearly shows you well below the PATH. I noticed you have the Legs page, not the DES page showing? We need to see the FMC DES page at the moment it starts DES. This will show any restriction on the route. I do not no a thing about proper Sids/Stars so maybe someone else can look it over and see what is going on there. One side note, the FMC makes no attempt to follow the DEScent target speed in a VNAV PATH just like the real NG, if you desire to keep the speed and forsake the PATH choose a SPD DES for the DES page.[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

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Here is another screenshot with the DESCENT page this time :-) . Same as before -- as soon as I reset MCP altitude, the aircraft descends about a thousand feet or so, and slows to 260kts which is well below the BUG speed (I usually RESET MCP ALT before being told to do so by the FMC). The vertical path gets ignored, and the aircraft continues along it's lateral track.I included my homemade CYLW sidstar file in a previous reply on this thread. Thanks.Kenny Rogers

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I tried removing all altitude and speed restrictions from the legs of this IAP. I also tried a higher cruise altitude of FL290 (just a thought). Unfortunately, I still have troubles. What a shame, this is such a beautiful place to fly into!As you can see in the attached image, the aircraft has started a descent prior to T/D, and set the speed bug to 240kts, and still only descending through FL280. I am ready to give up on this airport and build Vancouver International instead, but I would like to know if I am doing something wrong before tackling a more complex airport like CYVR. Kenny Rogers

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