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Ray Proudfoot

VNav problem when descending

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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,


Ray (Cheshire, England).
System: P3D v5.3HF2, Intel i9-13900K, MSI 4090 GAMING X TRIO 24G, Crucial T700 4Tb M.2 SSD, Asus ROG Maximus Z790 Hero, 32Gb Corsair Vengeance DDR5 6000Mhz RAM, Win 11 Pro 64-bit, BenQ PD3200U 32” UHD monitor, Fulcrum One yoke.
Cheadle Hulme Weather

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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,


Ray (Cheshire, England).
System: P3D v5.3HF2, Intel i9-13900K, MSI 4090 GAMING X TRIO 24G, Crucial T700 4Tb M.2 SSD, Asus ROG Maximus Z790 Hero, 32Gb Corsair Vengeance DDR5 6000Mhz RAM, Win 11 Pro 64-bit, BenQ PD3200U 32” UHD monitor, Fulcrum One yoke.
Cheadle Hulme Weather

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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,


Ray (Cheshire, England).
System: P3D v5.3HF2, Intel i9-13900K, MSI 4090 GAMING X TRIO 24G, Crucial T700 4Tb M.2 SSD, Asus ROG Maximus Z790 Hero, 32Gb Corsair Vengeance DDR5 6000Mhz RAM, Win 11 Pro 64-bit, BenQ PD3200U 32” UHD monitor, Fulcrum One yoke.
Cheadle Hulme Weather

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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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Guest Ian_Riddell

"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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Guest Ian_Riddell

"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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Randy,What we need is for some NG pilot to confirm the correct way.I disagree. What we need is an explanation from PMDG on how it has been programmed. Whether that agrees with formal procedures is only relevant to those who want to know the inner workings of the 737NG. All I require is the correct procedure to prevent VNav disconnections.Cheers,


Ray (Cheshire, England).
System: P3D v5.3HF2, Intel i9-13900K, MSI 4090 GAMING X TRIO 24G, Crucial T700 4Tb M.2 SSD, Asus ROG Maximus Z790 Hero, 32Gb Corsair Vengeance DDR5 6000Mhz RAM, Win 11 Pro 64-bit, BenQ PD3200U 32” UHD monitor, Fulcrum One yoke.
Cheadle Hulme Weather

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