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Guest tomahawk_pa38

Descents - Green Arc

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Guest tomahawk_pa38

I thought I understood the significance of the green arc but it always seems to move in unexpected ways. Could somebody tell me exactly what it means and what influences it the most when descending please ?Indicentally, I have taken the advice somewhere else in this forum and seem to have mastered the art of descents now I have virtually abandoned VNAV and use LVL Change instead. Is that how its done irl ?

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>I thought I understood the significance of the green arc but>it always seems to move in unexpected ways. Could somebody>tell me exactly what it means and what influences it the most>when descending please ?>>Indicentally, I have taken the advice somewhere else in this>forum and seem to have mastered the art of descents now I have>virtually abandoned VNAV and use LVL Change instead. Is that>how its done irl ?The moving green arc, sometimes referred to as the "banana" is called an Altitude Range Arc. What influences it most is the airplanes vertical speed and ground speed. It indicates the approximate map position where the MCP Altitude will be reached.Vnav Path descents are the most economical descents. Generally, they work well until ATC begins radar vectors. If you do not fly the 737NG with LNAV and VNAV engaged, then you are not flying the airplane at it's highest level of automation.Vnav Path descent does work well in the PMDG airplane but is probably the most difficult mode to master.Floyd


John Floyd

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Guest Hawkeyeted

Dave,Try a search for "VNAV Descent". I've written a few explinations on how to use VNAV. Trust me, it's easy to master, just diffcult to understand.If you still need help email/IM me and I'll talk you through it.

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Vnav Path descent does work well in the PMDG airplane but is probably the most difficult mode to master. Excuse me Floyd but there is only one area that has issues, this VNAV works better than any other out there plain facts..[h4]Randy J. Smith[/h4]


Randy J Smith

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Guest teeloo

Randy, can you be less agressive from time to time...ThanksTeeloo

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Randy,Didn't Floyd just say that VNAV PATH does work well???And you have to agree that this is the difficult mode to master.Chill out and have a beer, :-beerchug


Mats Johansson
PMDG Flight Test Dept
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Guest teeloo

Cheers!:-beerchugTeeloo

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Guest tomahawk_pa38

Ted (and others)Thanks for your help again. I still can't get the hang of VNAV. I'm flying from Glasgow to EGLL via New Galloway SID, DCS, MCT, HON,BNN Star / BNN Transition for runway 27R. I plan to fly at FL310 and the first alt constraint is 7000 feet at BNN. So my question is - if the FMC knows my speed/descent speed/current height/estimate of the winds aloft and how far the away the first alt constraint is - why doesn't it position the TOD point at a location realistic for optimum descent path ? Now I think i may be going to anser my own question (partly) I've tried TM's tutorial many times and then tried the 'Geezer guide' tutorial - both very educational ! Now call me slow or what - I was surprised with the latter tutorial to have to input data into the fmc that I previously thought had already been calculated by the fmc(descent speed, ZFW etc). So I think I may not be loading the aircraft properly therefore my weights and fuel loadings are different to what I am putting into the FMC. Is this a possibility ?If so, what's the best way of loading aircraft and then putting the correct figures inot the FMC please ?

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Guest tomahawk_pa38

Ted (and others)I think I posted this in the wrong place before and didn't get a response so i'll try again and hope you guys can help. I still can't get the hang of VNAV. I'm flying from Glasgow to EGLL via New Galloway SID, DCS, MCT, HON,BNN Star / BNN Transition for runway 27R. I plan to fly at FL310 and the first alt constraint is 7000 feet at BNN. So my question is - if the FMC knows my speed/descent speed/current height/estimate of the winds aloft and how far the away the first alt constraint is - why doesn't it position the TOD point at a location realistic for optimum descent path ? Now I think I may be going to answer my own question (partly) I've tried TM's tutorial many times and then tried the 'Geezer guide' tutorial - both very educational ! Now call me slow or what - I was surprised with the latter tutorial to have to input data into the fmc that I previously thought had already been calculated by the fmc(descent speed, ZFW etc). So I think I may not be loading the aircraft properly therefore my weights and fuel loadings are different to what I am putting into the FMC. Is this a possibility ? and would this affect the descent profile - I suspect it might !!If so, what's the best way of loading aircraft and then putting the correct figures inot the FMC please ?

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Guest tmetzinger

A VNAV descent works well if the FMC can correctly predict the effect of wind on the aircraft descent profile... With the 800/900 add-on (I think, maybe it's in SU1.3 too), the FORECAST page is active, and if you can put in the winds aloft for several flight levels on your way down, you'll find that the airplane does a much better job of staying on the path while remaining within the speed constraints.Essentially, when doing a VNAV PATH descent, the airplane predicts a point (TOD) to pull the throttles to idle and begin a descent to reach your next altitude constraint at the proper altitude. In a no wind situation this is an easy job, as the path is a linear descent profile at the chosen descent speed.when you bring in wind changes, the airplane has to alter the pitch attitude (and thus the airspeed changes) to maintain the computed path. If the nose has to be lowered, the airspeed increases and it can exceed limits and force a VNAV disconnect.Paying close attention to the airspeed during a VNAV descent and be ready to deploy spoilers. The other thing that can go "wrong" during a VNAV descent is when the descent profile between waypoint constraints is thought to be unflyable by a clean airplane in idle - this causes the DES PATH UNACHIEVABLE MESSAGE - when this happens you can use the VS mode and look at the DES path to find the needed VS to meet constraints.

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