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Ray Proudfoot

Aircraft too high on a VNAV descent

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Although I’ve only had the PMDG737 a couple of weeks I am familiar with how VNAV works. If I enter an altitude restriction for a waypoint 3 steps ahead I would have expected the aircraft to reach that altitude on reaching that waypoint.

But it doesn’t. Descent is consistently too shallow and the small display on the right of the ND shows me how much higher I am than I should be.

I pull the throttles back close to idle but not all the way. Is that the problem? We’re talking several hundred feet difference so something is amiss.

Any help appreciated.


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, as I have been using this particular PMDG model from the outset, do what I do.

Select your restricted mew altitude, dial it in, then go to FLCH rather than using VNAV. See if that does the trick. I have never had issues with descending with that particular plane.

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Thanks Rick, I’ll try that tomorrow. Seems odd VNAV is ‘broken’ for such a good product.


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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1 hour ago, Ray Proudfoot said:

Thanks Rick, I’ll try that tomorrow. Seems odd VNAV is ‘broken’ for such a good product.

VNAV isn't broken at all. Just sometimes a bit high above the path but that happens when winds change, just like IRL. Make sure you have put in the correct descent winds and, for best results, fill out the rest of the descent forecast page as well. If you enter a new restriction yourself chances are the speed/altitude is too low for the aircraft to make it based on the path it's been flying. Also don't be afraid to use the speedbrake - that's what it's there for.


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Pop up the VSD during your descent, and use it to monitor decent performance.  If the constraint ahead is an AT OR ABOVE value then it is entirely possible that the calculated VNAV path will cross that constraint above the value.  Sometimes, the path will cross at the value. Depends on what's ahead.

VNAV works most of the time exactly as it should.  There is an old problem with the VNAV path being discontinuous, for example just before reaching a waypoint the aircraft is exactly on the vertical path and then passing the waypoint the aircraft is suddenly very high above the path.  The fix for this is new FMS code and PMDG has that on their to do list.


Dan Downs KCRP

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1 hour ago, threegreen said:

VNAV isn't broken at all. Just sometimes a bit high above the path but that happens when winds change, just like IRL. Make sure you have put in the correct descent winds and, for best results, fill out the rest of the descent forecast page as well. If you enter a new restriction yourself chances are the speed/altitude is too low for the aircraft to make it based on the path it's been flying. Also don't be afraid to use the speedbrake - that's what it's there for.

I don't call hundreds of feet a bit high. The descent rate is a paltry 800fpm when it should be around 1400. Why would the aircraft not descend more quickly especially when the small display on the ND shows it's higher than it should be? I can understand if you put a mandatory altitude against a waypoint and got "path unachievable" - that's understandable.

I'm not putting winds in on the descent page. Does that make a significant difference to how the autopilot functions? And what winds am I putting in? For a particular waypoint or the destination? I have Active Sky.

Was it necessary to use the speedbrake n every descent? Seems odd. Maybe when conditions are unusual. It suggests you were probably too high to start with.


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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31 minutes ago, downscc said:

Pop up the VSD during your descent, and use it to monitor decent performance.  If the constraint ahead is an AT OR ABOVE value then it is entirely possible that the calculated VNAV path will cross that constraint above the value.  Sometimes, the path will cross at the value. Depends on what's ahead.

VNAV works most of the time exactly as it should.  There is an old problem with the VNAV path being discontinuous, for example just before reaching a waypoint the aircraft is exactly on the vertical path and then passing the waypoint the aircraft is suddenly very high above the path.  The fix for this is new FMS code and PMDG has that on their to do list.

Sorry, where can I find VSD? I assume it stands for Vertical Speed Display? I remove at or above for altitudes as I know that can cause the aircraft to stay too high.

Throttles all the way back to idle at ToD I assume?


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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5 minutes ago, Ray Proudfoot said:

I have Active Sky.

That's what I use to input my descent winds, Ray.

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5 minutes ago, vc10man said:

That's what I use to input my descent winds, Ray.

Is there a specific average for the duration of the descent?

5 minutes ago, ph-cxz said:

Push the ctr button on the efis panel.

Sorry, can you put some context into that?


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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16 minutes ago, Ray Proudfoot said:

I'm not putting winds in on the descent page. Does that make a significant difference to how the autopilot functions? And what winds am I putting in? For a particular waypoint or the destination? I have Active Sky.

With strong winds it does make a significant difference. If you don't put in any winds for descent so that the FMC doesn't know there is a strong tailwind it will obviously not take that into account with the T/D and descent path calculation - you'll find yourself way too high and not able to make the (wrong) descent path. On the descent forecast page you can put in winds for three altitudes. Just pick something like FL310, FL210, 11000 with the respective wind data from ActiveSky. You can also put in the altimeter setting and ISA deviation for your destination airport as well as the altitudes where you are planning to use engine anti-ice. Winds are more important though. I forgot to put the winds into the FMC the other day and found myself at 8000 rather than 6000 at a waypoint and had to fly a hold. Also make sure it's VNAV PTH and not VNAV SPD on the FMA. In VNAV PTH it should descent with thrust at idle. If it doesn't go down fast but only at 800 FPM perhaps you're in the wrong A/P mode.

 

16 minutes ago, Ray Proudfoot said:

Sorry, can you put some context into that?

What he's saying is you can open the vertical situation display (VSD) by pressing the CTR button twice on the mode switch for the ND on the EFIS.

 

22 minutes ago, Ray Proudfoot said:

Was it necessary to use the speedbrake n every descent? Seems odd. Maybe when conditions are unusual. It suggests you were probably too high to start with.

Normally you shouldn't use the speedbrake - that is when the aircraft descends along the VNAV path with winds put into the FMC. Winds change however and sometimes you just have a steep descent segment where you'll have to make use of the speedbrake. Definitely use it when you get a 'Drag required' message.

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Thanks Niklas, appreciate your advice. Sounds like wind input is vital.

I’ll fly tutorial 1 again tomorrow and do what has been recommended. Sounds like I may have been in the wrong AP mode.

Edited by Ray Proudfoot

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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38 minutes ago, Ray Proudfoot said:

I'm not putting winds in on the descent page. Does that make a significant difference to how the autopilot functions? And what winds am I putting in? For a particular waypoint or the destination? I have Active Sky.

Ray

You enter winds in DES Forecast page in CDU

46941661911_31286e2f89.jpg

You can use Active Sky or PFPX. I believe descent winds in AS is in briefing page. In PFPX flight plan page there is wind breakdown. In PFPX is shown as Descent winds. Just enter winds in CDU as it shown in the image. You don't have to add any fixes bc there are no fixes in DES forecast page. Make sure you enter Trans LVL as well. 

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Ray, Niklas has more or less nailed it. I was watching Capt Alan Dix, this morning, flying an IEA(later taken over by Airtours) A320-200 BHX to Zakynthos. And I saw him use that speedbrake to bring her into the descent parameters. Not all the time, but once or twice.

And Edward is spot-on with his image attached.

I now feel you will fly a perfect descent. I'm on my way from Katowice to Glasgow in an easyJet 737-700.

Edited by vc10man
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Thanks Edward, Wilco.

Rick, I trust he had entered his descent winds. 😁


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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