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Sim Avionics - Autopilot Adjustments (Resolved)
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MichaelYSSY



Joined: 06 Jan 2015
Posts: 617
Location: Sydney - Australia

PostPosted: Fri Oct 12, 2018 12:51 am    Post subject: Sim Avionics - Autopilot Adjustments (Resolved) Reply with quote

Hello Everyone

I was wondering if someone with more knowledge than me would like to help me out.
I need some help with FINE tuning the autopilot with the following AP items

Thrust adjustment - when I push throttles to 40% thrust goes past and below for a while and would like to make it more stable.

Top of descent on VNAV rate of descent initially goes as far as 3000fpm then 1000fpm then slowly settles to about 1800fpm, would like to stabilise the descent rate so it descends somewhat more stable.

Autothrottle - some airspeed hold adjustment as sometimes the throttles allows the airspeed to drop >10kts below the Issue speed. (mainly on approach and finals)

Any help in identifying AP items to fine tune to allow for better and more stable AP.

Thank you in advance
Michael
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SSTD 737, SA 737 v1.94
P3D 4.5 FSUIPC 5.151c


Last edited by MichaelYSSY on Tue Oct 16, 2018 1:53 pm; edited 1 time in total
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lulu47



Joined: 17 Mar 2016
Posts: 84
Location: Brussels Belgium

PostPosted: Sat Oct 13, 2018 5:38 am    Post subject: Reply with quote

Hi Michael,

About the VNAV rate of descent, the VNAV Path is calculated on the ground during the preflight with the winds aloft prediction introduced but while in flight, the winds are not the same, this explain why the FMC calculate a new VNAV path with the actual winds and that the rate of descent change once reaching the TOD. You have the possibility (50nm) before the TOD to descent earlier and activate DESCENT NOW, the aircratf will then descent at a rate of 1000 fpm till he recapture the nominal descent profile, this is usefull with tailwind (with tailwind descent with a ratio 4:1, with headwind a ratio 3:1). If the workload on board is not to hard, you can manage your descent manually via the DESCENT page using V/S on the MCP. To be sure to be at a waypoint desired altitude on the LEGS page, the FPA must be egual or higher that the V/B and the V/S indicates you what rate of descent to select on the MCP, this mode need a constant attention.

A must to fly on a very professional way with more finesse is to buy the Bill Buffer 737NG FMC User Guide, more than 10 pages about the different descent modes, how to calculate and manage it and a lot of tips:

http://www.cockpitcompanion.com/pd-b737ng-fmc-user-s-guide.cfm

About Thrust adjustment and Autothrottle I think you have to see this with Mark.

Have a nice weekend
Daniel
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737NG Full DSTD MX - FDS Shell - FDSB737NG-NS6-LINER - FDS CB-Walls - IPECO CA/FO Seats
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Banananav



Joined: 08 Oct 2015
Posts: 67
Location: 5nm West of RAF Cranwell

PostPosted: Sat Oct 13, 2018 12:01 pm    Post subject: Reply with quote

Hi Michael
VNav Path does not give a steady rate of descent (ROD).
If, for eg, you are at Fl 350 and .79 IMN you will have an IAS of about 260 kts.
The ac will initially descend at about 3000 fpm as the IAS increases to 280 kts.
Once the speed transitions to IAS and the air density increases with the descent the ROD will shallow-off and will do so again as you reduce the speed at 10000 ft.
But overall you should still be close to the, still air, 1nm per 300 ft plus 1nm per 10 kts of speed reduction rule-of thumb for calculating descents.
Cheers
Chris
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MichaelYSSY



Joined: 06 Jan 2015
Posts: 617
Location: Sydney - Australia

PostPosted: Sat Oct 13, 2018 3:55 pm    Post subject: Reply with quote

Hi and thanks for comments

Maybe I didn't make it clear but it's not the path or the setup of descent that is a problem.

It's the initial capture of the descent profile. At TD (VNAV LNAV) the AP starts the descent but it goes too steep and ROD goes to nearly 4000fmp it then ends up below the profile after which it reduces ROD to about 500fpm, then it goes above the descent profile and dives again.
It does this about 4-5 times after which it captures the descent profile and all is normal.

Its this initial roller coaster ride that I am trying to smooth out.

Any ideas? what AP Setting Change would help me out here?

Cheers
Michael
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thomas747400



Joined: 14 Feb 2016
Posts: 92
Location: Perth, Western Australia

PostPosted: Sat Oct 13, 2018 11:34 pm    Post subject: Reply with quote

Hi Michael,

I fly the T7 but I do experience the same steep rate of descent when arriving at TD. This only lasts for 10-20 seconds in my sim. I do find that initiating an early descend 10-20NM prior to TD helps and once the VNAV profile is captured the transition is smooth.

Ever since upgrading to v1.90, I have had severe altitude oscillations in other flight phases, generally when capturing the cruise altitude or after a step climb. The auto pilot is not able to recover from these oscillations, all I can do in these circumstances is to disconnect the autopilot, trim the aircraft out and re-engage the auto pilot.

Mark, any chance you could give us a small auto-pilot tuning guide, perhaps a brief description on each auto-pilot setting? I work in the control industry and I am familiar with tuning control loops but I do not understand some of the settings and their effect on the auto-pilot operation. As a community, we could test and share optimised auto-pilot settings for the benefit of all.

Thomas
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lulu47



Joined: 17 Mar 2016
Posts: 84
Location: Brussels Belgium

PostPosted: Sun Oct 14, 2018 4:16 am    Post subject: Reply with quote

Michael,

This is not a Sim-Avionics problem, VNAV works perfectly (V1.91).
The answer is in my first post but I give you an additional information here.

In reality, when the TOD calculated on the ground with the wind forcast is reached, the plane has most of the time already passed the ideal and actual TOD based on the current winds, so at that moment the plane is too high for the calculated VNAV Path on the ground, this explains that the AP calculate a rate of descent of approximatively 4000 ft/min, followed by some adjustments before to stabilize the ROD to catch again the VNAV Path. The 2 solutions are eighter to descent earlier via tha DESCENT NOW prompt wich appears only +/- 50nm before the TOD on the ND or to manage your descent via the V/S on the MCP and the DESCENT page as explained in my first post. About this you will find very detailled explanations with many exemples in the Bill Buffer 737NG FMC User Guide...

Cheers
Daniel
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Former Sobelair and Virgin Express Flight Dispatcher

737NG Full DSTD MX - FDS Shell - FDSB737NG-NS6-LINER - FDS CB-Walls - IPECO CA/FO Seats
FSC Motorized TQ
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gsumner



Joined: 06 Feb 2016
Posts: 141
Location: WSM UK

PostPosted: Sun Oct 14, 2018 6:23 am    Post subject: Reply with quote

lulu47,

You still have not understood Michaels questions. Its not a question of where the tod calculation is, winds or anything like that he is refering to. You are thinking too deeply and technically for your reply. Everyone will be experiencing Michaels problem. As we reach tod the aircraft pitches nose down to intercept decent path. This initial pitch down by the sim is too much at first so it tries to correct itself by pitching up again to which it pitches up too much so its pitches down again and so on and so on until its smoothes out to intercept its calculated path.

The same problem with auto throttle on approach i think Michael. Theres too much latency in correcting speed errors then the system tries to correct, but over corrects so we can go from 10 kts under speed to 10kts overspeed from our Issue setting. I think what youre asking for is a way to dampen and control these over corrections in the auto pilot.

I have Bill FMC guide. An excellent book but will not help here. This is a sim autopilot inaccuracy.

Regards
Graham
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lulu47



Joined: 17 Mar 2016
Posts: 84
Location: Brussels Belgium

PostPosted: Mon Oct 15, 2018 9:38 am    Post subject: Reply with quote

gsummer,

One of my best friend is a real Ryanair 737-800W pilot, he comes often here to fly on the full Flightdeck Solutions Trainer, we fly in MCC and never, we have encountered this problem at the TOD with V1.80/1.90/1.91, so this is not an autopilot inaccuracy! The autopilot works fine and respond like on the real aircraft, tested by a real 737-800 pilot...
So if you think that real pilots are idiots and that the Bill FMC guide is an excellent book we will maybe suggest you to read it a second time...

Brgds Very Happy
Daniel
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Former Sobelair and Virgin Express Flight Dispatcher

737NG Full DSTD MX - FDS Shell - FDSB737NG-NS6-LINER - FDS CB-Walls - IPECO CA/FO Seats
FSC Motorized TQ
Dual Opencockpit Yokes
Sim-Avionics V1.931
P3d V4.3
FSUIPC 5.15
Dual PC Windows 8.1
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gsumner



Joined: 06 Feb 2016
Posts: 141
Location: WSM UK

PostPosted: Mon Oct 15, 2018 11:46 am    Post subject: Reply with quote

Hey im really glad your sim doesnt porpoise. Thats cool.But this is not all about you and your sim, this is about Michaels. Ive spent 35 years in aerospace, one of my old workmates flys for Thomas Cook another that taught me to fly Easyjet and a third Tiger Airways in Austrailia let alone a friend who flies for DHL in Barhrain. If you want to name drop I can start on military jets next. Pilots arent idiots cos i know a lot of them.

Just because you have friends in the job doesnt mean i'm wrong or you understood Michaels post really does it. Lets not squabble about this though its not helping.

Graham
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MarkHastings



Joined: 18 Jan 2006
Posts: 897
Location: UK

PostPosted: Mon Oct 15, 2018 12:20 pm    Post subject: Reply with quote

ok - going back to the original question.... Wink
Michael - Can you confirm that you recalled the "Default Autopilot Values" ?

The parameter that would stop that aggressive pitch to the target V/S would be "Accel Damping" within the Altitude Hold section on the A/P Tuning Tab.

I have the value set to -700
Decreasing that value to -900, -1200 etc would cause the pitch to be slower to respond....
But be careful, as if this number get's too low it will start to have the opposite affect and introduce bouncing.

Remember to SAVE the aircraft details before closing the Server.
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MichaelYSSY



Joined: 06 Jan 2015
Posts: 617
Location: Sydney - Australia

PostPosted: Mon Oct 15, 2018 1:35 pm    Post subject: Reply with quote

Graham

Yes that is exactly what happens, you understand what I mean.

Daniel
My problem is in the first 10-15 seconds of initial descent from TD to stable capture of the calculated descent path and that is all. After capture there is no further problem with the descent, the descent settles down to about 2500fpm and reduces as altitude gets lower. The AP follows the profile descent and all is good. ITS JUST IN THE FIRST 10-15 SECONDS OF INITIAL CAPTURE. I dont have Bill Buffers Guide I do how ever have Boeing FCTM and FCOM which I have read.
I also understand the importance of flight planning and in relation to above, I load winds aloft and the forecast descent winds into the FMC on every flight along with relevant pre-flight pages input. This problem that I have with initial capture has not always been there, it is something that has shown up in the last few avionics updates.
If Bill's guide mentions something about this first 15 seconds of capture I would really like to know.

Thank you all for your help

Cheers
Michael
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P3D 4.5 FSUIPC 5.151c
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MichaelYSSY



Joined: 06 Jan 2015
Posts: 617
Location: Sydney - Australia

PostPosted: Mon Oct 15, 2018 1:40 pm    Post subject: Reply with quote

Mark

Thank you for information.

1. YES I can confirm that I recalled "Default Autopilot Values"

2. I will check what my Accel Damping value is and will try your suggestion with regard to Accel Damping tonight, will advise.

Thanks again, much appreciated

Cheers
Michael
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gsumner



Joined: 06 Feb 2016
Posts: 141
Location: WSM UK

PostPosted: Mon Oct 15, 2018 4:47 pm    Post subject: Reply with quote

Michael,

We should compare notes. I just tried - 800 and got a near perfect capture. Not much wind where i was though.

Graham
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MichaelYSSY



Joined: 06 Jan 2015
Posts: 617
Location: Sydney - Australia

PostPosted: Mon Oct 15, 2018 11:06 pm    Post subject: Reply with quote

Graham

Ill be home soon (Its 3pm here in YSSY Smile ) definitely will try it out tonight.
Will let you know my results.

Cheers
Michael
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MichaelYSSY



Joined: 06 Jan 2015
Posts: 617
Location: Sydney - Australia

PostPosted: Tue Oct 16, 2018 1:52 pm    Post subject: Reply with quote

Hello All

Tried adjustment in Altitude Hold section and changed Accel Damping from existing -700 to -800 (as Graham did) and pleased to report that capture of descent profile was much better. Thank you everyone, much appriciated.

Michael
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