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Guest Beatle1967

T/D VNAV Calculation always seems to be wrong (738)

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Guest Beatle1967

On nearly every occasion I've tried to use VNAV to plan a descent I find that the FMC shows the "drag required" and VNAV disconnects unless I add speedbrakes and/or take the gear down. My question: Why doesn't the descent algorithm start the descent earlier if drag is required? In particular I've noticed this happens a lot when getting close to 10,000' and the 250 speed restriction. It's almost as if the VNAV algorithm doesn't factor in time & distance to decelerate to 250.I usually turn off VNAV when it starts giving me trouble and control the descent myself using V/S.What's going on here, does the real thing behave this way or did I do something wrong?

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Guest balt

Are you using STAR's in your flightplan? Are the altitude constraints set correctly? I found that on some of the included STAR's the alt constraints on arriving waypoints are too high (they're often upper limits, in reality you might be cleared lower)You can manually enter speed and altitude constraints in the legs page. If you just want altitude and no speed change, enter /8000 and press the righthand LSK next to the waypoint to set the altitude at that point to 8000 feet.Cheers- Balt

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Guest Beatle1967

>Are you using STAR's in your flightplan? Are the altitude>constraints set correctly? I found that on some of the>included STAR's the alt constraints on arriving waypoints are>too high (they're often upper limits, in reality you might be>cleared lower)>>You can manually enter speed and altitude constraints in the>legs page. If you just want altitude and no speed change,>enter /8000 and press the righthand LSK next to the waypoint>to set the altitude at that point to 8000 feet.>>Cheers>>- BaltYep, and I'm adding my own constraints where appropriate.Let's say I'm at FL330 and I add a constraint of 240/9000 for the Foo VOR which is 200NM away. I've found that VNAV starts the dsscent too late, the aircraft arrives at 10500 at ~300kts and then requires speedbrakes to slow down or VNAV will overspeed at 10,000.

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You can also try this:Open the FORECAST sub-page in the DESCENT page of the FMC, you can enter up to three "altitude/wind-direction/wind-speed" forecasts (I don't know why, but FS wind at altitude is almost always 270


signed: José Luis

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To add to Jose, are you using any active weather? If so, and you have a tail wind, and the FMC doesn't know about it, then chances are VNAV isn't going to have a correct descent profile. I have flown the JVL4 with a 108knt tail wind and the descent profile was correct. FYI: The real plane works the same way. :-)


George Morris

 

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. . . and that is also the place to add your descent altitude TA as well as I recall which also affects performance adjustments.


Ron Ginsberg
KMSP Minnesota, Land of 10,000 Puddles
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>On nearly every occasion I've tried to use VNAV to plan a>descent I find that the FMC shows the "drag required" and VNAV>disconnects unless I add speedbrakes and/or take the gear>down. My question: Why doesn't the descent algorithm start the>descent earlier if drag is required? In particular I've>noticed this happens a lot when getting close to 10,000' and>the 250 speed restriction. It's almost as if the VNAV>algorithm doesn't factor in time & distance to decelerate to>250.>>I usually turn off VNAV when it starts giving me trouble and>control the descent myself using V/S.>>What's going on here, does the real thing behave this way or>did I do something wrong? It's pretty basic, VNAV does not keep the speed on descent as a target but the path. There are always variations in the profile unaccounted for in terms of the predicted path. Think of it like an atom. If you desire to know it's exact position then you must forsake any information about it's momentum. This is how the real one works. VNAV SPD on the other hand will keep the decent profile's speed with neglect to the path as the target. So, when on descent you have a range in speed away from target that is acceptable, if you get too fast or slow the AFDS will alert you. If you do not correct it -the AFDS will switch to LVL CH for over speed and add thrust if too slow. So use spoilers when your speed starts to bleed away from target and add thrust when slow. This does not mean you need to keep the speed exactly on target, just watch it for excessiveness. Best,Randy J. Smith<<>>


Randy J Smith

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What I see in a lot of cockpit videos is that they use the fix page to set up a range ring around destination and then engage V/S on descent to the descent ring trends just reach those range rings.For purposes of fuel efficiency using V/S pitch control at near idle thrust is the way many operate once ATC descent clearance is given. I generally shoot for 10,000 AGL at 35 nm out unless terrain and ATC dictate otherwise.Sometimes steep descents are needed though requiring slow down at 10,000 or wherever speed restriction takes affect and drag is then required. It does not happen often.


Ron Ginsberg
KMSP Minnesota, Land of 10,000 Puddles
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" I generally shoot for 10,000 AGL at 35 nm out "Most approach airspace ceilings are 10,000 - 12,000 ft about 40 nm from the airport. Note that most STARS have an arrival fix in this vincinity with an altitude constraint consistent in this range. The handoff between center and approach is usually at this fix.


Dan Downs KCRP

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Guest adampihavek

Can someone explain me,how to set correct wind data to the forecast page in FMC when I use real online weather(ASV6.5)? Where can I find,which wind intensity and direction will be exactly at xxx altitude?And how is it made on the real plane?Thanks,Adam

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>Can someone explain me,how to set correct wind data to the>forecast page in FMC when I use real online weather(ASV6.5)?>Where can I find,which wind intensity and direction will be>exactly at xxx altitude?Adam,If you are using ASv6 then you can import your flightplan and let it work out the wind direction/speed for verious flight levels. From the ASv6 front page select "New Route" and then import your flightplan. I can't remember the exact way it's done (am at work now so cant check) but once the flightpaln winds are worked out ASv6 will provide you with a data sheet with winds aloft.I am afraid that I can only supply a way for you to get the data, as for how to place the information in the FMC, I have not done that yet.Hope this helps.Justin


Justin Paull

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Personnaly, I think the 737ng, being PMDG's first full VNAV simulation for MS FS, was greatly exceded in the follow-on 744 version. Who knows, maybe they modeled it exactly the way it operates (descent calcs and all) as in the real 737. But, using crossings, ASv6, STARS, rang-rings, etc, I never, ever miss with the 744. The 737ng however, is a bit, ok, a lot more picky. I bet the FSX version of the 737 and the MD-11 will excede the 744 VNAV descent calculations. Isn't the core 737ng FMC code alomst 3-1/2 or 4 years old now? I like how they improve with every release. Going backwards and using older models from time-to-time, I think I just get a bit spoiled. :)


Regards,
Al Jordan
------------
KMLB

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