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.

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)

 

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.

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 !

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

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

 

sigh...:-(

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

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

 

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

Rob-

 

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

 

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

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.

Dear PMDG pilots, developers et al,

 

I was just commenting with Kyle on another thread about this "super autobrake" behaviour. Autobrake 1 will mostly always suffice for my landing-and-stopping operations (typically Vref 137 KIAS and LW 220 tons for the 77L). When I want to roll all of a long runway I don't even select Autobrake and only use thrust reversers then. BTW, I also use the FSUIPC friction correction.

Now, one thing just crossed my mind reading this thread. What is the guarantee that the "Decelaration Rates" numbers seen in FSX are accurate in regard to the ones provided by Boeing? It's known along the years that flight modelling numbers don't quite match to reality and have to be "tweaked" in order to provide an as-much-realistic-as-possible behavior, even though the numbers will be somewhat screwed.

It would be nice to also have a comparative test in terms of breaking distances (and not only times), in comparison to the real ones, too. Maybe I'll do that myself with the aid of some friends who are Triple-Seven rated pilots, and the performance/handling-qualities data.

Thanks for your comments.

Dear PMDG pilots, developers et al,

 

I was just commenting with Kyle on another thread about this "super autobrake" behaviour. Autobrake 1 will mostly always suffice for my landing-and-stopping operations (typically Vref 137 KIAS and LW 220 tons for the 77L). When I want to roll all of a long runway I don't even select Autobrake and only use thrust reversers then. BTW, I also use the FSUIPC friction correction.

Now, one thing just crossed my mind reading this thread. What is the guarantee that the "Decelaration Rates" numbers seen in FSX are accurate in regard to the ones provided by Boeing? It's known along the years that flight modelling numbers don't quite match to reality and have to be "tweaked" in order to provide an as-much-realistic-as-possible behavior, even though the numbers will be somewhat screwed.

It would be nice to also have a comparative test in terms of breaking distances (and not only times), in comparison to the real ones, too. Maybe I'll do that myself with the aid of some friends who are Triple-Seven rated pilots, and the performance/handling-qualities data.

Thanks for your comments.

Have you actually read this thread before making this statement? Please read RSR's post #17 above. It has all the answers you need. Autobrake commands a fixed deceleration rate. PMDG have tested their model to ensure these rates are satisfied. For runway stopping with autobrake nothing has to be "tweaked". The autobrakes will modulate brake pressure to give the required decel rate. This will also compensate for reverse thrust and speedbrake. So if you select autobrake 1 you will get a 4 ft/sec² deceleration rate. That defines your stopping time and distance and you can work those out from the standard kinematic equations.

 

Stopping time (sec) = V_I (ft/sec) / deceleration rate (ft/sec²)

Stopping distance (feet) = 0.5 * V_I * Stopping time

 

V_I is the initial speed when brakes are applied.

 

The problem comes with the excessive friction in FSX. That reduces the amount of braking needed to give the commanded deceleration rate. At low commanded decel rates this will mean that the brakes may actually be completely released and still decel rate is too high. So it is unavoidable that low autobrake settings will give quicker than expected stopping time and distance. Paradoxically I found that using the FSUIPC friction mod stopping time and distance is actually decreased, not increased as you might expect (see my post #15 for details). I don't know what dynamic modifications PMDG have made to compensate for FSX friction errors which the FSUIPC mod may be in conflict with.

 

If you are comparing stopping distances with the QRH note that these numbers assume 50 ft threshold crossing height and are from the threshold, not the touchdown point.

 

According to the QRH (which is real world data), at 220 tonnes autobrake 1 will stop you at about 7450 ft from the threshold (400 ft further with no reverse thrust). That is well short of the end of a long runway. The PMDG 777 may be stopping sooner than that due to FSX limitations, but it's not massively out. It's representative enough for a desktop sim.

 

 

Have you actually read this thread before making this statement?

 

Honestly, based on the other thread (where I linked to this one and RSR's answer in Post #7), it seems someone is on a little bit of a witch hunt for some reason.

No witches hunting on my part, I just strive to have a realistic simulation (up to the practical limitations). I'm sorry if this stuff bothers you, but as costumer, and more than that, a community member, I suppose I have the right to discuss the issue and ask for solution, just like others do.

Anyway this is interesting to find out that other equivalent planes, when set to Autobrake 1, don't suffer from the so called excessive friction inherent to FSX. I know the excessive friction is unfortunately there, but not to the point of being more effective than Autobrake 1 itself. :-)

 

 

No witches hunting on my part, I just strive to have a realistic simulation (up to the practical limitations). I'm sorry if this stuff bothers you, but as costumer, and more than that, a community member, I suppose I have the right to discuss the issue and ask for solution, just like others do.

 

Right - up to the practical limitations. The practical limitations have already been explained in the post we're all pointing to here, and have been closely matched to the Boeing values (also as already mentioned).

 

You certainly are allowed to discuss this here (you'll note that the posts have remained and the topics haven't been locked), but I'm not sure what the point is. RSR himself has already clearly addressed it. The question itself doesn't quite bother me (though the anecdotal nature of "it doesn't feel right" bothers my scientific/statistics-driven side). It's the refusal to accept the answer.