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Steve Keaton

Question for Rob Robson (777simmer)...

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Hi Rob,

 

What is your professional opinion on autobrake stopping distance with the PMDG 777 (under dry conditions of course) compared to the real 777 you regularly fly?    

 

Thanks,

 

Steven Keaton

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Hi Steven.

 

This is going to sound weird maybe......but I have not really tested it.

There are so many things about FSX that are unrealistic that I have tried to stop looking too close.

 

For example when it comes to gusty winds/cross winds/icing conditions/single engine aerodynamic behavior.....forget it.....I just try to ignore everything I see.

 

Try!....but I recently got caught in trying to recreate an experience that invloved quite violent turbulent and windy conditions around KIAD.

I loaded up the weather of that that day in OPUS FSX and.....nothing :-(

Now I am experimenting with OPUS "site effects" to try and get the behavior around KIAD that I want......but I think it just is not possible.

All very dissapointing :-(

We had extremely dangerous icing conditions where I live during the last 2 days (clear ice covering trees to the point of them snapping like matches) but I bet you would not have noticed any of that with FSX.

 

I assume braking distance falls under the same categorie (categorie=unrealistic in FSX) so I never even bothered checking it.

 

Hopefully one day Xplane gets good enough at that kind of stuff and then I will move there.

 

 

Now, on the bright side:

I most of the times land the PMDG777-200 with Auto brakes 2 and idle reverse (same as in real life).

And with that, and some manual braking towards the end of my landing roll (just like in real life), I can exit the runway at the same place as in real life.

So I am happy with that and never looked into it more.

 

But if you want I can give you some actual stopping distances from my EFB performance calculator. Just give me weights and conditions and I will give you numbers.

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Try!....but I recently got caught in trying to recreate an experience that invloved quite violent turbulent and windy conditions around KIAD.

 

What? We don't have those here...  :ph34r:

 

Best fun for a windy day from the west? Go watch people try to fly the 1R circle to 30 approach from the UHC and be glad you're watching from the ground...

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I most of the times land the PMDG777-200 with Auto brakes 2 and idle reverse (same as in real life).

 

Rob, how can I replicate idle reverse in FSX once that I only uses F2?

 

Thanks

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What is your professional opinion on autobrake stopping distance with the PMDG 777 (under dry conditions of course) compared to the real 777 you regularly fly?

Further to Rob's answer, autobrake ought to work correctly, regardless of FSX rolling friction issues (which PMDG have said they have addressed in their model). Autobrake commands a deceleration rate, if friction is unrealistic the brakes will modulate to compensate.

 

Manual braking (if that isn't an oxymoron) will be affected of course.

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What? We don't have those here...  :ph34r:

 

Best fun for a windy day from the west? .

Exactly....wind at 330 with 20G30kt.

Unbelievable!

I dont think I have ever seen only moderate winds cause such a rough ride from 4000ft all the way down to touchdown. Must be due to the mountains to the west.

 

Not to hack this thread but any suggestions are appriciated!

.....I had someone with ASN and FSGRW try the same approach with historical weather of that day and it was not as violent as real life.

Not even close.

Which is why I want to try with OPUS FSX site effects. (just have not had the time yet).

Rob, how can I replicate idle reverse in FSX once that I only uses F2?

 

Thanks

I have posted what works for me here (post#66):

 

http://forum.avsim.net/topic/453929-777-thrust-reverse-bug-still-persistent-in-sp1c/page-5

 

I have fwd and reverse thrust on the same axis......but if you use separate axis(levers) for fwd thrust and reverse thrustthisy method can still be used with small adaptations.

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Rob, you should maybe try a different weather engine. I use FSGRW and always had close to RW weather where I live.

Don't know about turbulence though as it's a completely different story...

 

I've had opus a while ago and wasn't impressed by it too much. 

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Rob, how can I replicate idle reverse in FSX once that I only uses F2?

 

Thanks

Just don't hold down F2 as long!

 

Touch down, hold F2 until you see green REVs, then let go. The reversers sleeves are now locked in place but the engines are still idling. Press F1 again at the appropriate time to close the reversers.

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Just don't hold down F2 as long!

 

Touch down, hold F2 until you see green REVs, then let go. The reversers sleeves are now locked in place but the engines are still idling. Press F1 again at the appropriate time to close the reversers.

thanks...

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Why use "idle reverse" instead of full or even half reverse thrust? Isn't the whole idea of reverse thrust to take the pressure off your brakes?

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Why use "idle reverse" instead of full or even half reverse thrust?

 

Various airports have noise reduction procedures in effect that limit the use of reverse thrust to idle (however, the crew can use up to full reverse in an abnormal/emergency situation). Also, using idle reverse saves on maintenance costs and engine wear.

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Why use "idle reverse" instead of full or even half reverse thrust? Isn't the whole idea of reverse thrust to take the pressure off your brakes?

In addition to the above, and depending on the field/conditions, idle reverse can reduce the likelihood of objects being ingested by not tossing around as much air to kick it all up.

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In addition to the above, and depending on the field/conditions, idle reverse can reduce the likelihood of objects being ingested by not tossing around as much air to kick it all up.

On top of that......every TL cycle (full thrust to idle thrust to full thrust) heats the engine, cools it and heats it again.

I think that this thermodynamic stress factor even goes into the maintenance program.

 

Same as reduced thrust for take off.....that is factored into time till next maintenance as well.

The engines can be used for a certain amount of time at a certain thrust setting (amoungst many other factors).

So you can use the engine longer (more time between overhauls) if you use lower thrust settings.......we call it power by the hour.

 

10 years ago or so.....airlines that used only idle reverse (unless otherwise required ofcourse) had WAY less inflight engine failures than airlines that used full reverse after every landing.

 

So ever since, our SOP have us use only idle reverse if possible.

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Hi Steven.

 

This is going to sound weird maybe......but I have not really tested it.

There are so many things about FSX that are unrealistic that I have tried to stop looking too close.

 

For example when it comes to gusty winds/cross winds/icing conditions/single engine aerodynamic behavior.....forget it.....I just try to ignore everything I see.

 

Try!....but I recently got caught in trying to recreate an experience that invloved quite violent turbulent and windy conditions around KIAD.

I loaded up the weather of that that day in OPUS FSX and.....nothing :-(

Now I am experimenting with OPUS "site effects" to try and get the behavior around KIAD that I want......but I think it just is not possible.

All very dissapointing :-(

We had extremely dangerous icing conditions where I live during the last 2 days (clear ice covering trees to the point of them snapping like matches) but I bet you would not have noticed any of that with FSX.

 

I assume braking distance falls under the same categorie (categorie=unrealistic in FSX) so I never even bothered checking it.

 

Hopefully one day Xplane gets good enough at that kind of stuff and then I will move there.

 

 

Now, on the bright side:

I most of the times land the PMDG777-200 with Auto brakes 2 and idle reverse (same as in real life).

And with that, and some manual braking towards the end of my landing roll (just like in real life), I can exit the runway at the same place as in real life.

So I am happy with that and never looked into it more.

 

But if you want I can give you some actual stopping distances from my EFB performance calculator. Just give me weights and conditions and I will give you numbers.

 

Rob,  thank you for your perspective on this.    My impression is that the PMDG 777 is not so much affected by the inherent friction problem in the native FSX engine as it is overpowered autobraking from touchdown speed to about 80 knots.     It's the initial autobrake deceleration from touchdown speed that seems far too much, although the PMDG 777 autobrakes do seem to modulate better as taxi speeds are reached.     Perhaps this results in the same stopping distance as the official Boeing charts, but with a different braking effectiveness in high speed regimes.     

 

I would really be appreciative of your running some numbers through your EFB, and I will certainly get those figures to you for comparison.      This would be a definitive way to compare the PMDG sim against the performance of the real aircraft!

 

For whatever reason though, the PMDG 737NGX seems to be spot on with autobrake deceleration throughout all speed regimes; in fact, I've always thought the 737NGX was the only FSX aircraft ever developed that finally got braking and ground friction coeffecient exactly right.      The PMDG 777 by contrast seems to almost catch an arresting wire right after touchdown with autobrakes, but this is my unscientific impression of course.    :)      

 

Manual braking with the PMDG 777 seems accurate too, although I think using the DynamicFriction lua script with FSUIPC does make taxiing behavior much more realistic.     From my perspective it just seems the PMDG 777 autobrakes are just too effective in of themselves, irregardless of friction modeling.   

 

Thanks again Rob!

 

-Steven Keaton  

Further to Rob's answer, autobrake ought to work correctly, regardless of FSX rolling friction issues (which PMDG have said they have addressed in their model). Autobrake commands a deceleration rate, if friction is unrealistic the brakes will modulate to compensate.

 

Manual braking (if that isn't an oxymoron) will be affected of course.

 

Kevin,  do you know for a fact that PMDG's 777 autobrakes actually modulate to compensate for the inherent FSX friction bug?     From my experience, manual braking seems to work quite well during rollout whereas the autobraking system seems too effective at high speeds.     

 

Thanks,

 

-Steven Keaton

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Kevin, do you know for a fact that PMDG's 777 autobrakes actually modulate to compensate for the inherent FSX friction bug? From my experience, manual braking seems to work quite well during rollout whereas the autobraking system seems too effective at high speeds.

 

Thanks,

 

-Steven Keaton

I've never measured it but it feels reasonable to me. As autobrake controls deceleration rate that ought to inherently overcome the excessive friction FSX has. With higher friction the brakes would apply less to get the same deceleration. This assumes PMDG have simulated it correctly and there's no reason to think that's not the case.

 

I'll do some comparative tests and so how it turns out.

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do you know for a fact that PMDG's 777 autobrakes actually modulate to compensate for the inherent FSX friction bug? From my experience, manual braking seems to work quite well during rollout whereas the autobraking system seems too effective at high speeds.

 

Steve,

 

I did some testing on the 777-200LR autobrakes, with and without the FSUIPC friction patch. I used autoland and a fixed weight to keep everything standard, no weather.

 

Conditions: ZFW=Max, Fuel=10,000 kg (gross weight 219.1 tonnes). Vref30 = 137. Flaps 30, Autobrake 2. Time from brakes on to zero ground speed.

 

FSUIPC patch OFF:

 

Case 1: without speedbrake, time to stop 35 seconds

 

Case 2: with speedbrakes, time to stop 35 seconds

 

FSUIPC patch ON:

 

Case 1: without speedbrake, time to stop 32 seconds

 

Case 2: with speedbrakes, time to stop 31 seconds

 

Finally, I tried a comparison test with manual braking by landing with parking brake on. No speedbrake, stopping timed from ROLLOUT green to groundspeed zero

 

FSUIPC patch OFF, time to stop 17 seconds

 

FSUIPC patch ON, time to stop 15 seconds

 

So it appears the patch does affect braking, both with and without autobrake. It shouldn't affect it in autobrake but it does. Stopping times are reduced by a couple of seconds. However as would be expected, for a given friction state autobrake stopping times are not affected by speedbrake deployment so that part of the autobrake model is working correctly.

 

I didn't see any unusually effective autobraking at high speeds. At about half the stopping time ground speed was about half what it was at brake application, so deceleration is roughly constant. Also the speed trend vector is about the same length during braking, indicating a constant deceleration rate.

 

Stopping distances were correspondingly shorter with the FSUIPC patch ON, but consistently the stopping test with speedbrake used slightly less distance than that without, even though the stopping times were almost the same. Without detailed data it's not worthwhile speculating why this may be. The effect is small anyway.

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Gents-

 

 

We have covered this in great detail in other posts- and I am too lazy to make up for others being too lazy to search for it....  :ph34r:

 

The autobrake logic in the 777 targets the published deceleration rates as defined in the FCOM.  During testing of the airplane, i can activate a little window that tells me what the target decel rate is, and what the actual decel rate is- along with how much energy the brakes are absorbing, how much pressure is being applied to each brake unit, etc.  We have tested this rather exhaustively and it does work very well to adhering to those deceleration rates in almost all circumstances.

 

The circumstances where you will see increasing variation from the book values is with light weights and heavy weights.

 

The FSX ground friction model does wind up playing a role here, because there comes a point where the latent ground friction induces a greater decel rate than the brakes are calling for- and during these preiods our logic winds up removing the brake pressure entirely.  This of course has an impact on the energy absorption model for the brake system, but is a result of a ground surface contact model that can best be desribed as "hard surfaces in FSX are coated with a substance the consistency of peanut butter."

 

If you set no autobrake and never touch the brakes in FSX- the airplane will stop quite nicely given enough runway.  In the real world you would continue to roll for fantastic distances.

 

I know it is hard to believe given the complete lack of detail and accuracy in our products- but we really do drill pretty deeply into these things...  B)

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I know it is hard to believe given the complete lack of detail and accuracy in our products- but we really do drill pretty deeply into these things...  B)

 

Robert, I've considered several of the "friction fix" solutions, and I'd like to know if the brake logic will adapt to it or if it'll screw it up.

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I would like to embrace 777simmer for his participation in this forum, an incredible source of pure diamonds for us all. Thanks a lot and Merry Christmas mate !

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I would like to embrace 777simmer for his participation in this forum, an incredible source of pure diamonds for us all. Thanks a lot and Merry Christmas mate !

Wow, golden would have been sufficient but pure diamonds....wow just wow....thanks haha :-)

 

I appriciate the kind words and at the same time feel it is little unfair towards the many others that contribute here on an allmost daily basis with their knowledge.

Knowledge about all kinds of aviation aspects, FSX and PCs.......not just the 777!

 

When I joined AVSIM I was not sure at first if this place was the right one for me.

There seemed to be a lot of bikkering over who knows better and not much honest help.

Maybe it is just me, but it is my impression that things are much better these days.

 

So I would like to thank everybody for making this a civilized, informative, and enjoyable forum to be part of :-)

 

Merry.......mmm........I saw a funny Tshirt recently with the following print: "Happy whatever doesn't upset you". lol

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I guess that just proved me wrong with this being a civilized forum.

 

sigh...:-(

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guess some one is going to have a break on this forum  when Robert wakes up :P

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I guess that just proved me wrong with this being a civilized forum.

 

sigh...:-(

 

Mischief managed. That was completely uncalled for, so it's gone.

 

guess some one is going to have a break on this forum  when Robert wakes up :P

 

Yeah, that would be my bet. It's hidden until then...

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Rob-

 

Shoot me a PM when you have a few minutes free.

 

Oh- and Happy Whatever Doesn't ###### You Off.  B)

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Gents-

 

 

We have covered this in great detail in other posts- and I am too lazy to make up for others being too lazy to search for it....  :ph34r:

 

The autobrake logic in the 777 targets the published deceleration rates as defined in the FCOM.  During testing of the airplane, i can activate a little window that tells me what the target decel rate is, and what the actual decel rate is- along with how much energy the brakes are absorbing, how much pressure is being applied to each brake unit, etc.  We have tested this rather exhaustively and it does work very well to adhering to those deceleration rates in almost all circumstances.

 

The circumstances where you will see increasing variation from the book values is with light weights and heavy weights.

 

The FSX ground friction model does wind up playing a role here, because there comes a point where the latent ground friction induces a greater decel rate than the brakes are calling for- and during these preiods our logic winds up removing the brake pressure entirely.  This of course has an impact on the energy absorption model for the brake system, but is a result of a ground surface contact model that can best be desribed as "hard surfaces in FSX are coated with a substance the consistency of peanut butter."

 

If you set no autobrake and never touch the brakes in FSX- the airplane will stop quite nicely given enough runway.  In the real world you would continue to roll for fantastic distances.

 

I know it is hard to believe given the complete lack of detail and accuracy in our products- but we really do drill pretty deeply into these things...  B)

 

The friction tweak is to fix the unrealistic thrust required to move the airplane. So the autobrake distance may have been configured by your team to work as designed, but below 30 knots the ground friction is completely unrealistic and requires the great fsuipc script to fix.

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