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opherben

VNAV DES

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My observations of VNAV descent, on properly programmed FMC flights:a. During descent without altitude or speed restrictions (so the descent should be smooth and at constant angle), the rate of descent cycled between 0 and 4000 FPM, and the was aircraft unable to regain a proper descent path without pilot intervention.b. On another flight, the aircraft ignored programmed altitude restrictions and descended through them.VNAV climb worked fine.

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I had the same problem than rich09. My vnav at climb works great -- the same at descent until i reach a vor or a waypoint -- than my vnav profile shows from one second to another that i

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Yes, I have this problem in every flight...I think the FMC calculates the TOD too far from the arrival airport...about 40 mn before a normal start of the descent...another issue: Throttle Hold doesn't work...PSS team please try to answer this questionRiccardo

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I've just reinstalled FS9!! Then I installed the 757...And the 757 has the same problems!! Help me finding a solution!!Riccardo

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Confirm the issue.I used to make flight plans in FS NAvigator - export to FS. But VNAV DEC doesn't appear in a flight plan imported from FS.I guess all these FMS issues will be fixed.

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Here same problem.I think it is a bad calculation of des path by the FMC.I have notice that after always waypoints, the aircraft seem to be lower than the true path.I'm going to show what happens.Passing TEA VOR, the aircraft is lower than the true path (it was in the right path before TEA).2006sep%38%3003av7.jpgThen the 757 decrease his rate of descend from 3000ft/min to 400ft/min until establishing again on the true path after few NM. After that it starts descending again keeping the path.2006sep%38%3004ba1.th.jpgThat happens after all waypoint during VNAV DES...

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Another example:Passing PAL VOR the VNAV path became 2140 feet higher...senzatitolo4xh7.jpgRiccardo Malpica

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hi! suppurt i have the same problem can you tell our something about??

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I have noted this as well. I usually only use FLCH to do my approaches, but after reading this thread I did a couple of VNAV approaches and was able to duplicate this behavior.

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I also noticed that when using FLCHG from higher altitudes the plane is descending at up to 5000 fpm. My pax are not happy!! Once the a/c gets down to around 10,000 feet the descents become more realistic (2000 fpm or less). My way around this is to use level change with a set rate and speed, but this is not ideal because if you don't watch your speed it is very easy to overspeed.

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This particular problem is reported on several threads and PSS are fully aware of all the details - and differences with the 777 VNav performance.John Rooum

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That's exactly what I saw BUT with the 777.Thanks guysThis is clearly a bug in ToD calculation by the FMC. I hope they will find a solutionregardsYves Lefevre

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I haven't really seen this problem, only because I use a different method. Normally, I'll take the A/C out of VNAV mode, and manually set the airspeed and descent on the MCP (flight plan built with FSBuild, which imports my TOD to the FMC). This seems to work for me when I use FS Passengers. Just offering another possible work-around.....Regards,Trent

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I mentioned this bug right after the release, but seems to have gotten buried and still no reply from Norman or Steven or Graham. I don't know how any beta tester could have not caught this, as VNAV descents are the most fuel efficient and smooth for the PAX. However, real life and simulated ATC seldom seems to have the same descents in mind as the FMC's on board do. You got to love those Pilot Discretion to the IAF altitude's.But PSS, please help with this bug.Steve Park

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I dunno. I stated above I was abkle to duplicate the behavior -- yet I did some cleaning of my computer and some tweaking to my FS and the problem went away. I am now able to do smooth VNAV approaches, so the problem went away for me after cleaning my machine. So I take back what i said above about duplicating the behavior.

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Hi Chris,This is interesting. Could you confirm that in a 757 VNav descent, as you pass a waypoint, you do NOT get the VNav path bug jumping to indicate well below the required path? If this is the case, grateful if you could let us know what tweaks managed to clear it.John Rooum

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THAT I get, but she always levels off to meet the restrictions. I guess I should have been more specific . . . . . Beforwe when I was trying to reproduce this issue, my 757 would just dive and NOT stay on the path at all. Now it does. At waypoints it does seem to have an odd bit where it "pops" under at a waypoint. The god news here is that it doesn't climb to try to catch it. She just levels off and gradually re-meets the line.I think my diving issue before had to do with computer resources free, etc. On the waypint issue, I am sure that has to do with gauge coding. In this sense, I am still happy that she comes down and seems to meet the restrictions as dictated in the FMC.Note -- I didn't notice what we meant here until John replied above to my post. I just reenacted the KAVQ to KCOS cargo flight and noticed the waypoint "pops". I think I misunderstood the thread when i first replied, getting it confused with the "diving" that my 757 was doing during VNAV approaches before I cleaned my system. Apologies for the miscommunication.

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Chris,Thanks - exactly what I am seeing: the aircraft is suddenly below the path as it passes a waypoint so it reduces the rod - or levels off depending on the size of the error - until it is back on path.I tried delaying the descent point for 20 miles and this improved things when I reselected VNav descent.Tonight I will try delaying by 40 miles [from FL370 and will equate to the descent point used by the 777] and see what that gives me.John Rooum

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A couple of follow ups, which might help the fix, on what exactly appears to be happening to the 757 VNav. On 2 identical flights, I was cruising at FL370 as I hit TD:Flight 1. I delayed descent by 45 nm [to the usual PSS 777 TD point] then went back into VNav. At this point I was some 13000 ft above the programmed VNav path. The aircraft started descent at the usual 3 - 3500 fpm and gradually "caught up" at each waypoint until we were back on path at about 8000 ft.Flight 2. I initiated a VS descent at about 2300 fpm - we met every waypoint at around the correct height until 10,000 ft when I adjusted the rod to meet the lower VNav schedule.It seems that the 757 VNav is based on the 777 programming but with a revised, lower, planned rod above 10,000 ft. However, it appears that this revised rod is not passed to the AP which still uses the 777 rod when "on path".John Rooum

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I'm still a bit surprised and concerned that we've not received any formal acknowledgment of this big problem from the PSS staff. Anybody in there?

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Give them a break, they will be back to us on the issues not just this.>I'm still a bit surprised and concerned that we've not>received any formal acknowledgment of this big problem from>the PSS staff. >>Anybody in there?

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>I'm still a bit surprised and concerned that we've not>received any formal acknowledgment of this big problem from>the PSS staff. >>Anybody in there?We are yes. The beauty of a support forum is that there are some folk here with more knowledge than me. John, one of the tester was already answering elements.A lack of a PSS banner doesnt mean its ignored.

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Thank you Norman for the answer, now I hope you are working on the fixes...because of the problem in VNAV DES and THR HOLD (see the other topic) I have to disconnect auto throttle every time before descent...

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Just on an aside, why would you be disconnecting the autothrottle? One of our 757 skippers mentioned numerous times that in real life he never ever ever lets the real plane do a VNAV descent.

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I never let the plane do a VNAV descent because of the problem I explained before, I am unable to do a descent with FL CH because Throttles move themselves to IDLE and I can't move them so the plane goes down with very high descent rates (-5000 -6000ftm). So I use FL CH with Auto Throttles off. In this way I can move throttles...

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