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windycloud

FMC descend

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I made a recent flight from La Guardia to Toronto on a 700. I let the FMC and AP handle as much as possible. The plane entered descend automatically at T/D as I monitor. During the decend, V/S is really unstable. Top V/S I noticed was more than 8,000 feet/min. Is this normal or does this mean I didn't do everything the right way? As long as someone can confirm that it's not how the NG should fly, I'll go back and dig the tutorial again.Thank you.

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Mr Thank you.First. May I suggest you read up on the forum guidelines and adhere to the rule of using your real name when signing your posts. ;-)Second. You might find a search here useful. Try to do searches on "VNAV PATH". Don't be ashamed though. It seems this topic is one of the most discussed amongst real world pilots as well. ;-)A V/S of 8000fpm is not what I would expect without some revertion from the normal VNAV PTH mode i.e. CWS P or V/S. What was the modes in reported in the FMA when this was happening? That said, you should expect some variations due to outside influences like winds, pressure and temperature as the aircraft is trying to descend on path. I can recommend monitor the descent from the DES page and particularly the bottom part of this page. Hope it helps,

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Guest bunchie

I' ve noticed recently that VNAV is predicting the TOD around 8 to 12 miles late resulting in the 700 almost going into a dive before vnav disconnects with the message vertical path unachievable.I have tried entering the cruise winds an temp in the perf. page and also the des. page but to no avail.I use AS2004.5 but have only recently started to experience these probs.Any advice would be most welcome.Thanks, Stu (bunchie)

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Stu - >I' ve noticed recently that VNAV is predicting the TOD around 8 to 12 miles lateLate compared to what? This is the point where your TOD is supposed to be according to the FMS with regards to the input you've made to it.>resulting in the 700 almost going into a dive before vnav disconnects with the message vertical path unachievable.A descent is a dive so nothing strange with that. ;-) J/K If the path, and consequently the dive is too steep though, you will get a VNAV DISCONNECT. Go VNAV SPD. Try to catch up with the path. Use the path deviation bars on the ND and the data on the DES page for awareness.You can't expect the FMS to do the descent planning for you. It's your responsibility to check all calculations and see they are feasible and reasonable. Use the FORECAST page as unexpected head- or tailwinds will toss the FMS calculations over end. Hope it helps,

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Hi Mats, thank you for your kind reply. First of all, my name is Jason Zhang. :)I guess I need to study the FMC A/P system more. I've been flying the 737NG since it came out but I tend to hand fly it. I use A/P when cruise. So shame to say but I don't have a very clear understanding of the different modes and VNAV. So I can't even clearly answer your questions. Sorry for throwing another stupid question but what tutorial or document would be the best for me to get a good understanding of this system?Thanks again.Jason ZhangPrivate pilot, New York, USA

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Jason - First. There are no stupid questions. There are only stupid answers! The AFDS (Automatic Flight Director System) do need some studying to get the grip on.I would suggest you start with reading the manuals that came with the package. Especially the chapters about the FMC and Automatic flight. I think it's Chapter 08 and 09 if you have the 800/900. A good tutorial to start with is Fred Clausen's one. It can be found here in the Avsim library. Be sure to pick up version 2. Tim Metzinger had a very good tutorial made up as check flights. He is working on a new revised, and revamped one. Maybe you can get hold of an old one...Key when doing these tutorials is to have the appropriate manuals handy as well.Good luck,

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Guest bunchie

Mat,I meant that the TOD is calculated 8 to 12 miles too late according to the destination airfield distance, (using the rule of thumb 3x altitude and drop the zero's).I have used this aircraft (600/700) pretty much since its release and I am not doing anything different with the FMC programming than I have before. This TOD prob is very recent and I have been using ActiveSky in its previous and current versions with out probs.(regarding this issue, at least.)The TOD calculations have never been problematical before, so why now?Your input and help is most welcome.Thank you, Stu.

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Guest mangold

Hello all,in 08_FMC_73702_06DEC03.pdf (PMDG FMC Users Manual)concerning upper wind conditions I found this:------During descent, a vertical path is computed based on the flight plan entered into the FMC/CDU. The FMS will evaluate expected wind conditions, aircraft speed, altitude, position relative to the planned end-of-descent point and any intermediate altitude or speed constraints between the aircraft and the end-of-descent point. This information will be passed to the AFDS for pitch based speed and vertical speed control and the autothrottles for vertical speed and thrust management. In ideal conditions, an idle thrust optimum descent profile is flown, however in many cases thrust and pitch will be varied to account for wind conditions or to ensure proper tracking of the vertical descent profile. Important Note Regarding MSFS: It should be noted that the PMDG FMC uses realistic algorithms to compute the effect of wind conditions and reported wind conditions on the planned descent profile. Some weaknesses in the modeling of weather transitions within MSFS may cause rapid shifts in reported wind-speed and direction, which may have the effect of changing the predicted descent path. For this reason, PMDG does not recommend using this airplane with any third party weather generation software, as we have found large variances in manner in which these software packages attempt to manage sudden wind shifts within MSFS, thus causing unreliable descent angle and wind effect prediction.------ G

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