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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.


Rob Robson

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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...


Kyle Rodgers

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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.


ki9cAAb.jpg

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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.


Rob Robson

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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.


Kenny Lee
"Keep climbing"
pmdg_trijet.jpg

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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.


Kyle Rodgers

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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.


Rob Robson

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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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