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.

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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.

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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.

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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.

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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?

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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?

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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

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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. 😁

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Ray.

I jumpseat on NG very often and I asked pilots same or similar questions. NG is a bit slippery airplane, I witnessed it myself many times. If you descend in a quite environment with no destruction from ATC and well planned descent profile in CDU  you most of the time will be OK. However in RW things go sideway very often. ATC may speed you up to 300 kts close to the airport and may change STAR and Rwy several times then your constant and smooth descent will be out of the window. In such cases you just have to improvise. NG offers you many options for that like VNAV, FL CHG, Vertical speed, speedbrake etc. With the practice you will find your way to manage off path descent and it also a lot of fun!     

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2 hours ago, Ray Proudfoot said:

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,

VNAV is both an autopilot and FMC function.  The Boeing FMC works a bit different in that it uses a performance based descent as opposed to a geometric path descent (there are exceptions, like the altitude constraints between two waypoints, and even that is optional - PMDG modeled the geometric option between two waypoint constraints). 

The FMC will compute a TOD based on an idle thrust descent profile (fuel efficiency).  For it to compute that idle descent point, the FMC needs to know the wind profile it will encounter during the descent, ISA deviations (which affect TAS), and the planned use of A/I (affect the amount of thrust output at the idle stop).  With those parameters entered into the VNAV DES page, the FMC can compute a theoretical idle descent point.  I say theoretical, because things may not be as forecast during the descent.  In those cases, the descent path may not be followed, and the autoflight system & FMC -- quietly -- transitions from VNAV PATH to VNAV SPD.  This catches real pilots as well, and one reason why it is considered "good practice" to verbalize FMA mode changes when they occur.  In VNAV SPD, the descent is a "speed on elevator" control, and path will not be followed.  

Also, when the descent starts the thrust node in the FMA indicates RETARD, and then when idle is obtained the FMA thrust mode changes to HOLD.  Depending on the PMDG option you have set, if the physical throttle is not at the IDLE stop, then the actual thrust will not be at idle.  I have B737 non-motorized throttle quad, and occasionally, I'll forget to pull the physical thrust levels back to idle.  she'll come off path rather quickly in that case.  

Just some things to consider.

Thanks,

Rich Boll

Wichita, KS 

 

Edited by richjb2
forgot a word...
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Edward,

Real world experience counts for a lot when flying sims. Although I’m a relative beginner with the 737 I have a lot more experience flying Concorde. Sadly, only in P3D.

Descents were far more dramatic. At Mach 2 at the decel point and close to FL580 you spool the engines back to 94% N2 (from 105% where they stayed for most of the flight) and when IAS drops to 350kts you engage Speed Hold and the descent starts.

Gradual to start with. Maybe 2000fpm but at Mach 1.5 a further power reduction is made to 77% but still with a speed hold of 350kts means the rate steepens dramatically, up to 4500fpm.

Then passing through Mach 1 throttles go to idle and with the speed hold still engaged descent rate can be 6000fpm. At 10,000ft 250kts is engaged and it becomes just another subsonic aircraft. FL580 down to 10,000ft in around 15 mins. Even on a simulator it is exciting.

Compared to that the Boeing is a baby and should be relatively easy to handle. But as you say, it’s slippery so appearances can be deceptive.

All I need is more flights to gather experience but sadly, the 737 does lack the thrill of a supersonic icon. 😢

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Ray. 

To confuse you even more 🙂 you may find this video interesting: 

Looks like RW pilot demonstrates how to deal with high energy approaches. I know you said you were new to NG but this video may be useful to understand how it all works. 

 

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Hats off to you, Edward. A great tutorial video. Despite having flown the PMDG 737NGX since its inception, I too learned a lot form that tutorial vid.

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13 hours ago, skysurfer said:

Ray. 

To confuse you even more 🙂 you may find this video interesting: 

Looks like RW pilot demonstrates how to deal with high energy approaches. I know you said you were new to NG but this video may be useful to understand how it all works. 

 

Just watched it. Very helpful. FL CH is your friend and increasing speed on the CDU to get a greater rate of descent is also a good tip.

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21 hours ago, Ray Proudfoot said:

Seems odd VNAV is ‘broken’ for such a good product. 

So, is it still Broken or can it be said it required more training? 😉

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3 hours ago, Adrian123 said:

So, is it still Broken or can it be said it required more training? 😉

Had a flight from EGNT to EGKK today and VNAV behaved itself after I reduced throttles to idle. Makes all the difference. 😁

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I too felt the same way until I finally got around to reading all the manuals including the massive fcoms. Then it all makes sense for the big picture. All of these systems work in unison to provide info for the pilot to get the passengers safely to their destination. The pilot of course is in control. Would you actually want an autopilot that if off descent threw on the speedbrakes and plunged into a 5000 fpm dive just to get back on track? 

The AP is no fortune teller or weather forecaster. If you give it all the information you have entered in the correct places it will do a great job but never perfect. VNAV is just one system at the command of the pilot to get to ED. The tutorial might make it look like that's the way you are supposed to do it but they are keeping it simple for your first flights. 

As soon as you take control of all your systems you will feel the power and control at your fingertips. This plane is mind blowing and I'm not easily impressed. Just finished 4 weeks flying nothing but this bird and working through every page of all the manuals and practicing every procedure. I don't even use VNAV any more as I have much more fun with ALT INTV, SPD INTV, FLCH, and VS. I love planning my own descents too so I can arrive at IAF flaps 5 and ready to intercept!

If you left it too late for descent you've already wasted the fuel so might as well be creetive in your descent. Enter a HOLD for your next way point. Or divert to a nearby NAV aid by entering into your legs. Or fly a 5 mile parallel offset between the next few legs. Or slow down manually and use VNAV SPD for descent. Or choose another star or approach that takes you circling or dme holding just to add distance. Anything but that speed brake lever. Just so lazy that is! 

The depth IS in this plane and it's all detailed in the manuals. Prepare to become one with your NGX

😎

Edited by sloppysmusic
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