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Guest Art Vandalay

How to initiate perfect VNAV descend

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Just had some quiet perfect rides with the SU1-1 updated 737NG and so far everything works fine.The following question is NOT AN ISSUE ON THE AIRCRAFT ITSELF, but a general one on how to this is been handled in realLife.OK. Here we go:I

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Art, Most NG pilots like to be in manual control during the descent because the NG's VNAV is not that good at it! The NG is known to pitch abruptly and is normal. You might try the DES NOW option a *few miles* before TOD. Also try this speed 300/.79 for descent (got that from a real NG driver on PPRUNE forums). See this is what's wonderful about this aircraft when it's working correctly, it lets you LEARN on how to work with the systems, the point still is that you the pilot are in control, the A/P is a tool to help you but not to fly for you. Never put complete trust in the FMC. A couple of things to note concerning descents: In a PATH descent the FMC controls PITCH to stay on the path and you might have to use thrust or speedbrake to get the speed you want. In a SPD descent the FMC controls PITCH to keep the target speed and you are responsible to stay on the path (thrust being a good option here). You don't want to use V/S for large altitude changes, always go LVL CHG on the NG. Boeing suggests 1000' or smaller atltiude changes for V/S.[h4]Best Wishes,Randy J. Smith [/4]P M D G 7 3 7 NG[/h4]http://img.villagephotos.com/p/2003-8/196432/mineimage.jpg [h3] Realism on the horizon AMD XP 2200 |MUNCHKIN 512 DDR RAM |ECS[/b ][i] K7S5A MB[/i] |GF2 MX 32 MEG and still runs GOOD!|WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"[/h3]

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Hi Randy,thanks for your quick response. Indeed I do not put complete trust in the FMC but it

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> In a PATH descent the FMC controls PITCH to stay on>the path and you might have to use thrust or speedbrake to>get the speed you want.>> In a SPD descent the FMC controls PITCH to keep the>target speed and you are responsible to stay on the>path (thrust being a good option here).>Are you saying that the autothrottles are disconnected during a VNAV descent, and the pilot needs to jockey them to manage airspeed or vertical speed?I guess I don't understand why the aircraft can't manage pitch AND thrust in a VNAV descent. In a VNAV Path descent, I'd expect to see the throttles retarded to idle, then the airplane uses pitch to maintain the path, and throttles to maintain airspeed as best as possible. Of course, if idle thrust is still too much, it may require speedbrake application, but since the noise is disturbing to passengers, a normal descent should be fine without them.In a VNAV SPD descent, I'd expect the aircraft to use pitch as the primary control for airspeed with throttle as a way to maintain a rate of descent (or path) as best as possible.Man, I just realized that we're back to the whole pitch/power debate again!-Tim Metzinger

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In a PATH descent, the FMA goes from FMC SPD (A/T) to ARM (commands idle thrust) and the pitch mode stays in VNAV PTH. You can use thrust to increase speed if it's falling off or speedbrakes to slow without worrying about keeping the path profile. For SPEED descent, the FMC will not be following the (path) whatsoever and if you want to stay close to the calulated path you will need to use thrust to slow the ROD, if you are high of course use speedbrakes. The FMA for A/T goes RETARD/ARM and the pitch mode goes VNAV SPD. This is how the NG works, just why is a question for Boeing =)[h4]Best Wishes,Randy J. Smith [/4]P M D G 7 3 7 NG[/h4]http://img.villagephotos.com/p/2003-8/196432/mineimage.jpg [h3] Realism on the horizon AMD XP 2200 |MUNCHKIN 512 DDR RAM |ECS[/b ][i] K7S5A MB[/i] |GF2 MX 32 MEG and still runs GOOD!|WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"[/h3]

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Hmmm. glad it seems to be working better for some! I just tried a flight with the new patch. I was at FL350 and 5nm before TD I reset the MCP to FL220. TD came and went with no decent, I checked the cruise page and it said TD was 8nm and getting bigger. At 10nm past TD I started a decent manually by pressing the FMC's Decend Now button (which never did anything for me before) and the 737 started decending, at 7400 fpm! I had to cancel because the speedbrake couldn't stop the overspeed. I disengaged VNAV and did the decent manually, I needed -3400 to all the way to be at the correct approach height.In the end, it seemed to behave the same as before except Decend Now did something.Some interesting things I noticed were that.1. The FMC route page heights were all changing as I decended, even the runway height. I set a hight manually for the start of the approach (10nm out) and found a new TD on the FMC which would have given me 20,000 feet or so to decend in 10nm (something like that)2. On climb, I would reach my ATC assigned height then get a new height from ATC so I would dial it into the MCP and VNAV disengaged, I had to press VNAV to continue climbing, the second time I reach my height and VNAV disconnected, ATC gave me my next height I dialled in the new number in the MCP and VNAV reengaged and started climbing by itself.Steven.I assume that the patch didn't update properly (my livery was updated thats for sure) or something with the FMC heights was causing the problem. I'll reinstall and do the full patch later unless someone else has some other ideas. The flight plan came from FSBuild 2 beta.

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Don't forget to sign your posts =) I will say as fact that you have a bad install, I have flown so many descents prior to release I can't count them all and have not seen anything remotely like that. Am I right to assume that you have FS9? Make sure to use the REMOVER TOOL on PMDG's web site to make sure it correctly updates... [h4]Best Wishes,Randy J. Smith [/4]P M D G 7 3 7 NG[/h4]http://img.villagephotos.com/p/2003-8/196432/mineimage.jpg [h3] Realism on the horizon AMD XP 2200 |MUNCHKIN 512 DDR RAM |ECS[/b ][i] K7S5A MB[/i] |GF2 MX 32 MEG and still runs GOOD!|WIN XP PRO |MITSUBISHI DIAMOND PLUS 91 19"[/h3]

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Roger wilco, signed my earlier message too :-)Yeah, I am on Fs2004.Steven.

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Randy,Thanks for the info on the VNAV descent issues. Again, I feel people are looking toward full automated flight and that truly is not the intent behind the FMC and MCP and Autopilot. Again, we had this issue with the PIC767 FMC, would do a wonderful job on the climb portion but on the descent it could stray from the ideal path and/or speed. When the developer was queried the answer was interesting...just like the real one, when using VNAV for descents the pilots usually ending up with the comment "what is it doing now" just prior to disconnecting it. I love to utilize some automated features down to 10000 feet and then just hand fly the approaches to touchdown. You have to learn the airport and the approaches but well worth it! I fly a lot of KSFO and KLAX its a short hop but gives you all the fun stuff in about an hour of time.Enjoy the NG it is great!Tony

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You'll get abrupt climbs too in VNAV depending on the SID routing. I.e. , if the course is an immediate turn back from rwy hdg as a NGY1H at Glasgow off of rwy5. Best here is to either hand fly, which is more fun or use LNAV with v/s as pitch mode until settled at the alt restriction. Once past the restriction fix VNAV works smoothly.I read somewhere that LVL CHG was for relatively small alltitude changes up to 4000'. V/S or VNAV used for larger changes. But I have no doubt it differs from a/c to a/c and airline to airline or individual command preference.

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>I read somewhere that LVL CHG was for relatively small>alltitude changes up to 4000'. V/S or VNAV used for larger>changes. But I have no doubt it differs from a/c to a/c and>airline to airline or individual command preference.Mixed up....LVL CHG for large changes and vs for smaller. This is what you meant, isn

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