Jump to content
Sign in to follow this  
Ray Proudfoot

VNav problem when descending

Recommended Posts

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,


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

Share this post


Link to post

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,

Share this post


Link to post

"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]

Share this post


Link to post
Guest Ray CYYZ

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

Share this post


Link to post

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]

Share this post


Link to post
Guest ba747heavy

>> 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?

Share this post


Link to post

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]

Share this post


Link to post

> 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.

Share this post


Link to post

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]

Share this post


Link to post

>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.

Share this post


Link to post

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,

Share this post


Link to post

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

Share this post


Link to post

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,

Share this post


Link to post

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,


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

Share this post


Link to post

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,


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

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
×
×
  • Create New...