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Turn Prediction

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It'd be funny if PMDG managed to simulate False Localizer/Glideslope captures. Can you imagine all the forum complaints! lol!

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In Lnav with AP engaged, the real 777 does not overshoot!

 

Never?

 

I dont know, but in 12+ years Of normal operation I cant remember having ever seen that.

 

Of course you can create situations where the airplane just cant do what you want.

And maybe that departure out of Heathrow is one of them situations (have not looked at it).

For example, create two waypoints is sequence, 1nm apart and at a 90 degrees angle!

No way it can do that at 300kt and you WILL see an overshoot!

That's why I said "normal ops"!

 

The system knows your ground speed and anticipates for strong winds!

 

In a hold for example, if the tail winds are too strong (too high ground speed), it will even tell you "unable HOLD airspace".

Because it knows exactly what the turn radius is going to be!

 

Now capturing a localizer is a different story.

Lnav can not anticipate the reception of the localizer!

Once it starts receiving it the INS/GPS system will still calculate when to turn so that you dont overshoot the loc (based on loc closure rate I think) . But if you fly to the loc with 300kt at 90degrees, than it should have turned before it even received the inbound course. Which it does not, cant, and thus will cause an overshoot.

 

 

A level D simulator, by the way, is still only a simulator!

I have seen many level D screw ups, the real 777 does not do.

 

 

And as for the PMDG777, I really does not do too bad I think.

I have not noticed those overshoots.

I am sure the logic can be improved, but for an FSX simulator....not too bad at all?


Rob Robson

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EDIT:I meant "But for an FSX simulator.....not to bad at all ! " ofcourse (?=!)

 

 

The most annoying part of AVSIM....that you can not edit your posts!!!


Rob Robson

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In Lnav with AP engaged, the real 777 does not overshoot!

 

Never?

 

I dont know, but in 12+ years Of normal operation I cant remember having ever seen that.

 

 

That's nice to know what is the T7 doing in the real world thanks.

 

 

And as for the PMDG777, I really does not do too bad I think.

I have not noticed those overshoots.

 

 

Well I can show you how the PMDG T7 flies the famous 78 degree turn north of Beijing China (ZBAA) via the following screenshots. This is a common real world route from Europe to Shanghai/Korea/Japan via north China, airways W80 connecting to B339. You can reproduce it yourself easily:

 

 

1. approaching VOR HUR: tracking correctly with a 25 kt tailwind which is FSX default and commonly encountered in the real world:

 

https://db.tt/LrngoWYY

 

2. T7 will start the turn at about 5.3 nm to HUR, it will overshoot:

 

 

https://db.tt/kYInXVFB

 

3. at about half the turn, already overshot 0.4nm:

 

 

https://db.tt/gpMF0hnG

 

4. maximum overshoot 1.8 nm to the left of track, note the distance to the next waypoint LADIX:

 

 

https://db.tt/N4kUyZZ6

 

5. finally it took 29 nm from HUR to fully correct the track:

 

 

https://db.tt/pVPdY4bI

 

Again, don't get me wrong I'm critisizing PMDG. I'm just stating the facts. The T7 is great, if it can track like the real one, magnificant: a wish list.

 

 

Francis Leung

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Yeah...I must say that is a pretty extreme turn with 500kt ground speed.

I have not been in that area, but I am going to stick to my earlier statement that it should not overshoot.

 

It knows your GS, it knows that it uses 25degrees of bank. Those two factors results in a calculatable turn radius!

 

The only thing that would cause overshoot would be a "fly over" waypoint versus "fly by" waypoint.

But your pics clearly show HUR as fly by.....thus it should not overshoot.

 

There is an oh....dont know....., something like 45 degrees turn, after Afghanistan into Pakistan (Pavlo to Jhang).....that causes absolutely no overshoot even at 600+ ground speed!

 

 

EDIT: Say.....stupid question.....you guys are not flying in accelerated mode right?


Rob Robson

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In Lnav with AP engaged, the real 777 does not overshoot!

 

Never?

 

I dont know, but in 12+ years Of normal operation I cant remember having ever seen that.

 

Of course you can create situations where the airplane just cant do what you want.

And maybe that departure out of Heathrow is one of them situations (have not looked at it).

For example, create two waypoints is sequence, 1nm apart and at a 90 degrees angle!

No way it can do that at 300kt and you WILL see an overshoot!

That's why I said "normal ops"!

 

The system knows your ground speed and anticipates for strong winds!

 

In a hold for example, if the tail winds are too strong (too high ground speed), it will even tell you "unable HOLD airspace".

Because it knows exactly what the turn radius is going to be!

 

Now capturing a localizer is a different story.

Lnav can not anticipate the reception of the localizer!

Once it starts receiving it the INS/GPS system will still calculate when to turn so that you dont overshoot the loc (based on loc closure rate I think) . But if you fly to the loc with 300kt at 90degrees, than it should have turned before it even received the inbound course. Which it does not, cant, and thus will cause an overshoot.

 

 

A level D simulator, by the way, is still only a simulator!

I have seen many level D screw ups, the real 777 does not do.

 

 

And as for the PMDG777, I really does not do too bad I think.

I have not noticed those overshoots.

I am sure the logic can be improved, but for an FSX simulator....not too bad at all?

 

Thanks for this post. When I was told in the support ticket that the aircraft could not compensate for winds, I was extremely skeptical.

 

The 777 FMS uses the Motorola 68010 processor, and it is a processor I know well. It is not beyond its capabilities to factor ground speed into a turn prediction algorithm, thus alowing for ground speed in turn prediction (and in fact, as it is following a ground track, it would HAVE to consider ground speed). The processor is slow, but not THAT slow. Heck - it is discussed as if a turn prediction algorithm is complex, which it is not (see my post above for the calculation of turn radius which would form the basis of turn prediction).

 

I agree that LNAV could not *detect* LOC intercept, but it does pre-empt it by starting the turn even before LOC is actually captured by the system, based upon FMS position. It helps "feed" the aircraft onto the localizer.

 

I also know that LNAV and ILS are totally seperate systems, but the ILS example (and it was ILS intercept I was talking about) was merely an example of what a real aircraft is capable of doing, which is something many simmers dismiss as "impossible" or "not accurate".

 

Best regards,

Robin.

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If this is true, and the overshoot in the sim is exaggerated, perhaps Rob should raise a ticket quoting his real world experience? It may get the issue noticed and addressed,

 

Regards.

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We all know  that Pmdg  do a good job in what they do in simulating,  but surely we cant expect what the real world t7  to do  exactly what it  does on our Sims?  If it  does 95 percent or even 98   do we have to worry about  the other 2  percent  that it  cant do exactly. Guess we are expecting  to much  on what our Sims can  do comparing  it  to the real t7


I7-800k,Corsair h1101 cooler ,Asus Strix Gaming Intel Z370 S11 motherboard, Corsair 32gb ramDD4,    2  ssd 500gb 970 drive, gtx 1080ti Card,  RM850 power supply

 

Peter kelberg

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Tks all for the reply and support. Yes of coz the sim was not running at 2x or more, you don't see that announced on the upper right corner of the screen shot right  :biggrin:

 

BTW, the next screenshot is the ASN real world weather on 7/3 last month above ZBAA. The tailwind is basically 82 knots, making my original upload of 25kt quite mild! 

 

 

Francis Leung


Screenshot missing on last post:

 

 

https://db.tt/2Ksuz2FF

 

 

Francis Leung

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We all know  that Pmdg  do a good job in what they do in simulating,  but surely we cant expect what the real world t7  to do  exactly what it  does on our Sims?  If it  does 95 percent or even 98   do we have to worry about  the other 2  percent  that it  cant do exactly. Guess we are expecting  to much  on what our Sims can  do comparing  it  to the real t7

I agree with that. For me when I see the PMDG plane do something silly, I just correct the mistake and forget about it. I am not saying this is the way to go....it is just that it all does not bother me too much. Things that do bother me and I would like to see improved (FBW system or irratic GoAround thrust behavior) I do (did) report.

If this is true, and the overshoot in the sim is exaggerated, perhaps Rob should raise a ticket quoting his real world experience? It may get the issue noticed and addressed,

 .

Ofcourse I also agree that if things CAN be improved....then you would normally expect from PMDG that they will do so.

 

I will have a look at Lnav turn predication when I have some time....

I have no problem reporting my findings to PMDG.

 

As it is, I have some problems with my system due to Windows Defender that was running during GEX/UTX installation (yes stupid....I thought it was of but it was not).

So I cant test untill maybe in two days or so.


Rob Robson

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I am trying to post some pics here.

Been at it for 45min....but no luck.

 

I dont post pics a lot, and everytime I do, I have to re-learn how to do that.

It is incredibly difficult to post pics at avsim!

 

Help would be appriciated.

 

I have tried:

compatibility option - but then nothing works right

more reply options - this has an "image" button, but it only opens a field with an URL request???

(and after that URL field shows, I cant get rid of it anymore. I must exit IE and reopen it....drives me crazy)

 

I remeber in the past I was able to attach a file, but I cant find any attach button anymore :-(


Rob Robson

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I am trying to post some pics here.

Been at it for 45min....but no luck.

 

I dont post pics a lot, and everytime I do, I have to re-learn how to do that.

It is incredibly difficult to post pics at avsim!

 

Help would be appriciated.

 

I have tried:

compatibility option - but then nothing works right

more reply options - this has an "image" button, but it only opens a field with an URL request???

 

I remeber in the past I was able to attach a file, but I cant find any attach button anymore :-(

 

You will have to use an external image hosting service such as Photobucket to upload your pictures. Then you can link them in your Avsim posts using the image button, by pasting the URL.

 

Hope this helps.

 

Regards.

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I could swear I have posted pics before without any image hosting...

 

You sure I can not just insert a picture here?


I mean, it is not a setting in my Internet explorer that prevents me from seeing an "attach" button?


Rob Robson

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I use  this  works on other forums  and  done  a few on here before  but  it might have  changed

http://fsfiles.org/flightsimshotsv2/


I7-800k,Corsair h1101 cooler ,Asus Strix Gaming Intel Z370 S11 motherboard, Corsair 32gb ramDD4,    2  ssd 500gb 970 drive, gtx 1080ti Card,  RM850 power supply

 

Peter kelberg

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Thx, but no idea how to use that.

 

i can upload and image with that link.....but then it is there....and not on AVSIM.

No idea how to get it to AVSIM :-(

 

Never mind.

 

 

For what it is worth:

 

The real world 777 allways pre calculates a smooth magenta curved line (radius) that the aircraft will follow during the turn.

This pre calculated magenta curved line passes INSIDE the waypoint you are tuning at. (to avoid overshoot)

 

The PMDG777 does not not allway pre calculate this radius.

Or at least not on my PC.

Instead, I have seen the magenta line go all the way TO the waypoint you are tuning at, and then the magenta line leaves the waypoint at a sharp angle (more like a knick than a radius).

 

Not untill just before getting to the turning waypoint, when the PMDG777 has just initiated the turn, is the magenta knick re calculated and displayed as a smooth magenta radius. But then it is too late. The aircraft can often not stay on track even with 25 defrees of bank. This has resulted in up to 2nm off track during some of my PMDG flights. Not good ofcourse!

 

 

I finally had a real world leg that allowed me to take a closer look at turn prediction. FL370 and GS537kt

About an 80 degrees course change (left turn) in Chineese airspace.

The ND displayed a beautifull pre calculated magenta radius for the 80 degrees left turn.

It started to turn about 5.7nm before reaching the waypoint.

With 25 degrees of bank we ended up 0.28 right of track after the waypoint.

Yep, that´s right....slightly off track.

 

Conclusion:

PMDG is correct in saying that it is normal for the aircraft to sometimes get slightly oftrack in extrem turns.

 

However, it is not normal for the the magenta track not being pre calculated as a smooth radius.

And it is also not normal for the airplane to get off track by 2nm or more.

 

 

 

Too bad it is not possible to upload pictures on AVSIM!

 

I will send my pics to PMDG support.

 

cheers.


Rob Robson

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