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Ray Proudfoot

VNav problem when descending

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Anthony / PMDG,I'm having a lot of difficulty trying to understand how the FMC works compared to the one on the 767PIC. Sorry to keep mentioning that product but I use that as the benchmark for all others. Whether that is correct I'll leave up to you to decide.My latest problem is at ToD on a flight back to Manchester (EGCC) from London Gatwick (EGKK). On the attached screen shots you can see the state of play just after ToD when the FMC had disconnected VNav and warned of an overspeed. Quite what this warning means I don't really know. The engines hadn't spooled back to idle as I would expect when starting a descent. Instead they were running around 65 N1.The MCP altitude is set to 10,000ft which I have entered in the FMC as the altitude for TNT. The rest is for info. In short, why is the FMC disconnecting VNav when it should spool back the engines and start the descent keeping a speed of 290kts? It seems incapable of this operation.http://forums.avsim.net/user_files/34823.jpghttp://forums.avsim.net/user_files/34824.jpghttp://forums.avsim.net/user_files/34825.jpgAny help or an explanation as to why this is happening would be appreciated.Thanks,

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Ray,Did you say you got a OVERSPEED, VNAV DISCONNECT directly after TOD? If so the probable cause it that you have a higher CRZ TGT SPD than DES TGT SPD. The target descend speed should equal or be greater than the target cruize speed. VNAV PATH will not ccontrol the speed by pitch. Controlling the speed is up to the pilot. If actual speed is 10 knots over TGT SPD you will get a DRAG REQUIRED message. When actual speed exceeds TGT SPD + 20 knots VNAV will disconnect to CWS Pitch. Actually it disconnects at TGT + 15 in real life. But the PMDG guys gave us some slack here! ;-)Hope it helps,

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"Actually it disconnects at TGT + 15 in real life. But the PMDG guys gave us some slack here! ;-)" Yeah I really wish they would change that back! I mean we are going for realism and if it says +15 let's make it +15...[h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] 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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Much simpler solution, put the Altitude constraint at 10,500 and don't worry about it at all. You'll come down at rocket speeds and not have to deal with the slowdown, THEN you can loose the speed, then ontinue your descent through 10,000 at the proper speeds.Ray

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Ray, one more thing here I see, you entered /10000 @ TNT, when you do this make SURE you also put a SPEED, what matters that you input both ALT and SPEED...[h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] 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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>> Ray, one more thing here I see, you entered /10000 @ TNT,>when you do this make SURE you also put a SPEED, what matters>that you input both ALT and SPEED...I thought that didn't matter Randy?

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Why would it not matter? It might not be his reason here but will be one disconnect problem when he gets to the waypoint and wonders why Vnav went on vacation.. EDIT: Mummm here is what Anthony said, but he does say SPD without ALT, I thought this also applies to ALT without SPD but now you got me wondering hehe"Hi Ray, others,VNAV will disconnect if it encounters a constraint with speed only (no constrained alt). This is stated in the manuals, and is modelled.So if you remove your speed constraints (by the way VNAV will limit speed according to flaps retraction schedule) it will be fine.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com"[h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] 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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> If so the probable cause it that you have a higher>CRZ TGT SPD than DES TGT SPD. The target descend speed should>equal or be greater than the target cruize speed. I don't see how this could be the case. With normal ECON speeds, the descent speeds are nearly always less than the cruise speed. With the big Boeings, I believe that the FMC inserts a DECEL leg just prior to TOD if necessary.Don S.

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With normal ECON speeds but notice Ray's speed of 321 knots, that's not a ECON speed this FMC would give for either CRZ or DES, so it tells me that he entered a different speed. It's all in the details and without knowing everything he did we are all just guessing.I know if he slowed the bird down to within VNAV limits (there are limits for engagement, his being within 20 knots of 295) he could have resumed VNAV des..[h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] 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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>With normal ECON speeds but notice Ray's speed of 321>knots, that's not a ECON speed this FMC would give for either>CRZ or DES, so it tells me that he entered a different speed.>It's all in the details and without knowing everything he did>we are all just guessing.>>That's true Randy, but what I was responding to was the statement that the VNAV descent speed must be equal to or greater than the cruise speed.I don't know why he had 321 knots in the MCP speed window in the screenshot, but I suspect that that is immaterial to the issue he was describing. He states that he expected VNAV to maintain a speed of 290 knots. It would be informative to know whether he had been using the FMC computed target speeds, or had manually overridden them. (Basically, I agree with you that we need more information.)Don S.

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Ray,The decceleration at FL100 (European you know ;-)) is beautifully handled by the FMC. Even if you set MCP at a lower altitude.In VNAV PATH you won't come down in rocket speed. You have to choose VNAV SPD for that or V/S. In VNAV PATH the FMC will keep you on the path no matter if it means it slows down or speeds up. YOU as PIC are responsible for speed management.Cheers,

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According to mr Bulfer VNAV will only disconnect on SPD without ALT.Cheers,

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Hi Don et al,>I don't know why he had 321 knots in the MCP speed window in>the screenshot, but I suspect that that is immaterial to the>issue he was describing. He states that he expected VNAV to>maintain a speed of 290 knots. It would be informative to>know whether he had been using the FMC computed target speeds,>or had manually overridden them. (Basically, I agree with you>that we need more information.)Yes, we need more information for sure. I just made a statement based on a written assumption. See my first reply.>That's true Randy, but what I was responding to was the>statement that the VNAV descent speed must be equal to or>greater than the cruise speed.The speed I referred to was the target speeds. And this relation with descent target speed being equal or greater than the cruise target speed is pretty well covered in the FMC guide.Cheers,

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Mats,Did you say you got a OVERSPEED, VNAV DISCONNECT directly after TOD?Yes.If so the probable cause it that you have a higher CRZ TGT SPD than DES TGT SPD.Yes, I did. I had increased the cruise speed to 325 kts (possibly the equivalent Mach speed - sorry can't remember which). If the VNav disconnect is proper procedure in the real B737NG then I suppose I'm going to have to readjust my procedures. I would have thought that the FMC was intelligent enough in the real aircraft to spool back the engines and then start the descent.If actual speed is 10 knots over TGT SPD you will get a DRAG REQUIRED message. When actual speed exceeds TGT SPD + 20 knots VNAV will disconnect to CWS Pitch.Very useful information - thanks!But the PMDG guys gave us some slack here! ;-):-lol I need extra slack due to ignorance on 737 procedures.Many thanks,

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Hi Randy,but notice Ray's speed of 321 knots, that's not a ECON speed this FMC would give for either CRZ or DES, so it tells me that he entered a different speed. Please see my reply to Mats. I had increased the cruise speed but I expected the FMC to be able to cope with this. Clearly I'm going to have to readjust my procedures from those used to fly the 767PIC.I'm very impressed with the quality of the replies to this question. Thanks to everyone who chipped in. I'll try the flight again tonight and with all this new-found knowledge I'm hoping for a much better flight.Many thanks,

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Randy,you entered /10000 @ TNT, when you do this make SURE you also put a SPEED, what matters that you input both ALT and SPEED...Okey dokey. It makes a BIG difference to how the aircraft is controlled by the FMC as I have found out to my cost.Cheers,

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Hello I too am having problems with vnav and target speeds. If I try to change them (target speeds)it'll always reset to 295knots and I end up blowing through TOD, Speed disconnects and Unable alt show up. I would very much like to see a PMDG tutorial of the FMC and all of its functions in clear, pictorial presentation. It would clear up alot of the problems and discrepencies between this program and that program.regradsNoboru

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To all who've helped on this topic,I'm pleased to report that after following the excellent advice given my flight from EGKK up to Manchester was much better tonight. VNav behaved itself and using the flaps I was able to control the approach speed.I did increase the cruise speed to 315kts but 10 miles from ToD set it to 285kts - 5kts LESS than the descent speed. I didn't get the DRAG REQUIRED warning which was nice. It's easy when you know how! :7 Cheers,

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Great Ray! I suggest you can do this to avoid this disconnect VNAV problem. Increase both CRZ and DESCENT speeds on the FMC and keep the speeds within the 20 Knot range, you increased CRZ spd so you can lower it a bit prior to your TOD. The FMC is SMART, that's why it's telling you that it's disconnecting OVERSPEED when the speed goes past 20 knots. This will also happen in descent if ones does not pay close attention, sometimes you must use the spedbrakes to keep it within limits. As to my other point, I am WRONG! That applies to and inputted SPEED without inputting the ALT, the other way around is fine....[h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] 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,Roger on your first point. Good advice and I will increase the cruise and descent speeds to overcome that message appearing.On your second point I'm not quite certain what you mean. Do you mean if I enter a speed constraint but not an altitude constraint I'll get the VNav disconnect issue?I know that if I enter both or neither VNav remains active. Change one or the other and I thought VNav would disconnect.Cheers,

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Ray,Here's what our dear cpt Bulfer states in the FMC guide regarding VNAV disengage on speed only constraints. page 79 in May 98 ed:"Speed constraint entries require an altitude constraint at same wpt for VNAV to remain engaged" Seems a bit odd, and that's the only note on this subject I've foundBut alt constraint without speed constraint at same wpt works.Hope it helps,

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"Here's what our dear cpt Bulfer states in the FMC guide regarding VNAV disengage on speed only constraints.page 79 in May 98 ed:"Speed constraint entries require an altitude constraint at same wpt for VNAV to remain engaged""Mats, I think this is applicable to the Honeywell FMC only. I suspect you are using the Big Boeing FMC Users Guide?In the Bulfer FMC User's Guide for the 737NG (which uses a Smiths FMC), it says:"Speed only constraints are allowed on the LEGS page for climb and descent wps (waypoints)."It goes on to say that speed restrictions are treated by the FMC as "at or below" speed limitations. Only mandatory restrictions may be DELeted. They are replaced by predicted values"It probably confuses the issue, but that's how the real one works.... according to Bill.Cheers.Ian.

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Ian,Actually it is the "B737 FMC guide" for the Smith's U1 through U10.4 I'm quoting! This is getting confusing :-hmmmRegards,

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What we need is for some NG pilot to confirm the correct way. Maybe a post on PPRUNE will be needed? Ian ;)[h4]Best Wishes,Randy J. Smithhttp://img.villagephotos.com/p/2003-8/196432/winglets_lg.jpg [h3] 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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"Maybe a post on PPRUNE will be needed?"Not 100% sure that would help, Randy... I've seen just as many contradictory answers on that site, too :-)My book is dated July 3rd of this year, so the software may have been updated. However, I don't know enough about the U10.x numbering system to know if software updates can be made without altering the dot number. It's not exactly a minor update adding this kind of feature/ability is it?The Bluecoat forum should know, but I've been annoying the heck out of them recently (I think I've exceeded my yearly question limit) ;-)Cheeers.Ian..

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