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Question about VNAV descent and deceleration


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Hi there, 

I have another issue as i've been flying this aircraft a lot recently and trying to get my head around it :-)

Most of the time when I descend using VNAV, the aircraft does not seem to decelerate appropriately at the decel point, instead it seems to pitch down, causing the speed to shoot up and leaving no time to slow the aircraft down appropriately.
 

- In the attached image I am descending via the LANDU1B STAR into EDDM, for RWY26R

- At the decel point (green circle) which I am just passing, the aircraft seems to suddenly pitch forward to maintain it's rate of descent, causing a speed increase (see speed tape climbing beyond 250KTS) instead of slowing to clean config speed. See the aircraft pitch attitude on the PFD in the image.

- Sometimes when this happens the FMC calls for drag, but the speed continues to increase regardless

So my question is - am I doing something wrong here or is the VNAV a bit buggy from time to time? In most of the YouTube tutorials i've watched, people tend to manually control the speed and descent rate in this aircraft, so I am wondering is this a better approach that gives more control? I know the B737-300 is an older aircraft so perhaps VNAV does not provide the same level of automation and profile reliability as as a more modern airbus for example?

Any tips or advice would be appreciated

Thanks

83000839_B733-2021-05-0916_50_37.png

Edited by bobcat_smith
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Hi,

VNAV descents are considered to be WIP. They work somewhat, the more simple the descent path, the better. Add multiple and/or complex restrictions and things will start going south.

We fully intend to improve this in an update - for what its worth I can safely say that the VNAV PTH descent mode was rarely used in the real 737 Classics and most pilots used FL CHG or V/S to perform their descent.

The example you describe (not flying the correct speed) is a very typical thing that happens in the real airplane, too. The VNAV path is preplanned with a lot of assumptions, and sometimes those don´t work out (different wind, for example). The aircraft will try to adhere to the PATH more than to the speed it planned with - and when I used (rarely) the VNAV PTH function in the real aircraft I often had to "help" the plane by using the speedbrake. If speed becomes excessive, VNAV will disconnect and then the pilot has to pick up the shards of the shattered approach ;-)

Cheers, Jan

 

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Hey,

"Picking up the shards of the shattered approach" certainly describe the last couple of experiences I've had ;-) I'm going to do some experimentation with manual descents and see if I can learn. I've been watching a couple of your tutorials as well as the Reflected Reality ones, and they've been really useful.

Thanks again for taking the time to respond with a helpful asnwer!

Cheers

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23 minutes ago, bobcat_smith said:

I've been watching a couple of your tutorials

Have you watched this episode? I show how to use the "descent calculator" that I made for the avitab plugin - it basically does for you what airline pilots do in their head all the time when planning and executing an approach: Estimate total distance remaining to the airport - then calculating the altitude one should be at for an idle power approach:

https://www.youtube.com/watch?v=hSemWBgRNnE

Cheers, Jan

 

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I can confirm what Jan said above. During the 737-300 heyday, I talked to a lot of 737 pilots in airports while waiting for flights and I asked many of them about this specific thing, as I was struggling with this with the early PMDG. Almost all of them said they rarely used Vnav, primarily because it was not very reliable and also in the US you rarely fully fly STARS to the end, most often getting vectored off early, put in holds, late descents and other ATC related things which pretty much rendered Vnav not very useful. At least according to those pilots at that time. I have a feeling the same thing is true today.

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2 hours ago, Beachdog2001 said:

I have a feeling the same thing is true today.

I stopped flying the Classics in 2012 and transferred to the Airbus 320 family - surprisingly the use of the managed descent mode (DES) on the A320 is even less than we used VNAV PTH on the 737...

Nowadays the ATC structure changes a bit towards STARS and SIDS that have more restrictions - ATC hopes to lessen workload because the planes will move inside a "virtual tunnel", separating from each other by virtue of altitude restrictions.

A famous arrival here in Germany was the EMPAX3W STAR to EDDF, but it did not go beyond the trial phase because pilots failed to meet the restrictions too many times (unfamiliar with how to properly fly managed descents, erroneous behaviour of the FMS,...).

Cheers, Jan

 

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My acquaintance, the captain of the A380 at Emirates, also says that even on such a modern and technological aircraft, Vnav is almost never used.  Air traffic control always interferes with any calculated descent.

I always fly online and also noticed that if there is air traffic control, Vnav becomes useless, since the ATC commands are executed in the first place.  Only if you arrive and there is no control, then you can try to use Vnav, but it becomes a little boring to sit and just look at the monitor screen. 

Here is such a strange thing this Vnav. It seems to be there, but no one uses it

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4 minutes ago, disidd said:

but no one uses it

It gets used in climbs and cruise - and it can be of limited use in descents, especially calculating the optimum T/D for the first restriction (Cross XXX at or below xxxx feet) - but it is very rare that you can really fly the planned path all the way to the intermediate approach fix (it can happen in "progressive" countries with little air traffic like Sweden or Norway).

Edited by Litjan
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Happy to hear that the descent calculator works out for you! And once you used it a few times you will have memorized the altitude/distance relation and work the problem in your head...like most line pilots do, too. (We cheat and use the metric altimeter - 33.000 feet = 10.000m => 100NM)

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2 hours ago, Litjan said:

Happy to hear that the descent calculator works out for you! And once you used it a few times you will have memorized the altitude/distance relation and work the problem in your head...like most line pilots do, too. (We cheat and use the metric altimeter - 33.000 feet = 10.000m => 100NM)

Earlier I used a coarser calculation FL / 3 + 15 (margin to smoothly reduce the speed).  And your calculator is just an immersive effect.  So thank you very much and thank you for the plane

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By the way, my friend is a former А380 captain at Emirates.  Now he is undergoing training on a Boeing 747 in a large cargo company (ABC) and is currently in Frankfurt, on a Lufthansa 747 simulator. Jan, I got it, you fly to LH

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On 5/11/2021 at 5:00 PM, Litjan said:

Have you watched this episode? I show how to use the "descent calculator" that I made for the avitab plugin - it basically does for you what airline pilots do in their head all the time when planning and executing an approach: Estimate total distance remaining to the airport - then calculating the altitude one should be at for an idle power approach:

https://www.youtube.com/watch?v=hSemWBgRNnE

Cheers, Jan

 

Thanks Jan, I've watched that video. Very useful.

I've practiced a few manual descents yesterday using lvl change, V/S and the speedbrakes to try and meet the different waypoint restrictions on a STAR without VNAV. It worked better without VNAV on the final approach too.

Do you have any thoughts on what distance from the airport is it best to start slowing for flap deployment? I've heard some people say 25NM, some 20NM and some 15NM. In the video you talked about being at 180kts, 10NM from the airfield. 

Cheers for all your help

 

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8 hours ago, bobcat_smith said:

Do you have any thoughts on what distance from the airport is it best to start slowing for flap deployment?

In general, airline pilots like to fly as fast as possible and as high as possible. They former saves time, the latter saves fuel - both is money ;).

So speed wise we like to fly at ECON speed as long as possible (or close to VMO if late on schedule). However there is often a speed limit below 10.000 feet (250kts) and also collision avoidance and birdstrike hazard is an issue, so I refrain from flying faster than 250 when lower than 8000 feet AGL, even if the airspace structure allows it.

Most of the time ATC will slow you down as you get closer to the airfield, often to 220.

If unencumbered by any of the above I will slow down "just in time" before descending on the glideslope (10NM, 3000AGL, 180kts with Flaps 5). My rule of thumb is "almost 1 NM per 10 kts deceleration" - so if I come in at 250kts I will start the deceleration at "almost" 7 NM prior to that. The lighter you are, the faster you will decelerate...so with a light aircraft 5NM will be enough, with a heavy one 6.5. Also consider head/tailwind component.

Cheers, Jan

 

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