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curt1

737NG, No more VNAV descents for me

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I've come to the conclusion that the top of descent computed by the flight management computer is the absolute last point a descent should start. Otherwise, starting down 10 miles earlier makes life a whole lot easier. I've decided to quit fighting the battle of VNAV descents, always descending too fast, grabbing for speed brakes etc.. The descent is so much better managed when I use VS or FLT CHG and use my head to calculate the appropriate descent angle. Anyone know what the real airline pilots do?Curt

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I'm don't fly'em, but I've read in this forum that VNAV is used for climbs but the descents are managed by the crew (and ATC). Makes sense... the VNAV path is still there as a reference and we know the difference between a crossing restriction and a hard constraint (makes it easier on the pax and fans).

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VNAV descent calculates for an IDLE thrust path. If you don't have the proper information entered in like winds etc, it doesn't work as well. The thing is not perfect even in real life, and like Dan said many pilots don't even use in descent and will instead just rely on FLCH and VS.The NG has older code too and as such the VNAV isn't as good as what you'll see with the 747. I think we updated it with the 800/900 expansion, but it's not the newer code that's in the 744.

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Curt-Dan hit it right on the head... It is EXCEEDINGLY rare that you get the manage the descent from TOD on your own.ATC has a complex system of interoperations and arrival "gates" that they run traffic through in order to keep things going smoothly- and that means they start managing your descent path pretty early in the process.I think the UPS/FEDEX guys doing latenight runs might be hit with the descent problem more often than anyone flying during "normal" hours...

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Being an Ex-ATC I know that pilot seldom use VNAV for descent mainly because of crossing restricions imposed by ATC. A good rule of thumb for descent is to start it at 3 times your altitude from the airport. ie: If you are at FL300 you should start your descent at the most at 90 miles from the airport.Bat

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For a different perspective, I use VNAV descents all the time, including when I've got crossing restriction imposed by ATC. It's a matter of planning. It is imperative that you provide the FMC with forecast descent winds on the DES page. With that information and the crossing restriction(s) entered on the LEGS page, my experience is that even this "old" code nails the descent almost every time. I used to ignore the forcast winds and I would get "drag required" or VNAV disconnects all the time. I'll never forget the first time I put everything in and, with no messages, watched the altitude and speed roll right to the correct numbers just as the waypoint turned white on the nav display.This is made easier by having Active Sky and FS Flightkeeper running during my flights. Active Sky provides a fantastic weather system and FS Flightkeeper, which interfaces with Active Sky, has an ACARS guage that can provide me with updated weather along my route of flight, making it easy to find and plugin my forcast descent winds.The PMDG 737NG FMC does have an issue with multiple crossing restrictions along a descent path. Upon reaching the first restriction altitude it will not continue the descent but will level off at that restriction altitude. The solution is just a little baby sitting. If you just deselect and reselect VNAV just after you pass the first restriction waypoint, it will continue the descent to the next, and so on.

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Thanks for the responses everyone. The technology available today is incredible, but the descent mode became so unpredictable for me. I guess it's pilot discretion or company policy that determines the proper mode to descend, and with crossing restrictions FL CHG is what I'll rely on. It's an interesting discussion, and I'm glad my descent style is not considered unusual with real world flying.Curt

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I had the same problem in the 747. The thing would take a sudden nose dive when it reached the TOD (equating to screaming passengers to those of us running FSPassengers). Fortunately, a real pilot who flew the 747 for thousands of hours gave me this tip (this works in the 747 and I'm sure it's modeled in the 737): On your FMC, while in cruise, hit your VNAV button and go to the thrid page, or ECON DES page. You should see a line that says - ECON SPD. Change that value to .800/250 and execute. Your VSI will max out at around -2400; your descent profile will be much smoother and not force you to deploy the speedbrakes.

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I used to have problems with VNAV descent in the NG but not any more. All I have to remember to do is click VNAV twice after crossing a waypoint with an altitude restriction, and it flies the descent perfectly. That said, I rarely get the chance to descend in VNAV when flying online with ATC. LVL CHG and V/S modes make it easier to comply with ATC clearances, I find.Rob

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For me V/S is the best way.Nebojsa

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Tried today for the 737ng setting the ECON DES to.800/250.Worked out nice.Thanks for the tip.Nebojsa

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>Being an Ex-ATC I know that pilot seldom use VNAV for descent>mainly because of crossing restricions imposed by ATC. A good>rule of thumb for descent is to start it at 3 times your>altitude from the airport. ie: If you are at FL300 you should>start your descent at the most at 90 miles from the airport.>>Bat>Check out this article- all the way down- right on the moneyhttp://www.fsvisions.nl/site/content/view/61/7/I love the tip about drawing a circle around your destination airport to allow for visual planning of the VS descent. Good for other scenarios as well- closest diversion airport etc.Best-Carl F. Avari-Cooper BAW0225http://www.precisionmanuals.com/images/forum/supporter.jpg

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Same here, Dave. VNav does work if you do it like you described.I've also flown the newer style RNP/GPS Approaches all the way down to minimums. You really have to do some pre-planning and FMC whizzing ahead of time though. You have to hit your entry gates on the STAR's at the right altitude and on speed. If you're flying overseas, some charts are hard to figure out what those are, so you'll have to do some research as to the 'normal airport ops'.I wonder if the 37NG's are capable of receiving GPS glideslope info (legally in real life)? LOL I can't remember the equipment codes and requirements.:-)

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I've fianlly had many successful VNAVs now mainly as said before, since I started putting in wind speeds and direction in the forecast page. Not sure how critical these are but I start by using the speed/direction shown at the bottom of the Progress Page which i presume is 'actuals'.In general I find VNAV works best when there is either a headwind or crosswinds with a good headwind component. My most recent VNAV failures always involved tail winds which I can understand (I think !)

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