Friction in P3D V4

[light_blue_yonder 15]

*I know a lot of people have brought this up before, this post is NOT a complaint about the software.*

Hello fellow captains,

I know a lot of people have brought up the friction problem since the FSX days and continues to till this day. I managed to talk to an Air Canada 777 crew on a flight from Vancouver to Beijing about this, and I thought I'd share what they said. I don't remember their exact words and some things in quotation marks are paraphrased. These were the questions I asked:

 

1. How much throttle did it take to break away on that day with our weight?

Captain's answer: "None, you release the brakes and it creeps forward." According to this captain, we were "really heavy" and "only slightly below max. takeoff weight." He said that yes, you do add throttle to start moving with that kind of weight, but you don't *need* throttle to start a creeping forward motion. Of course, if you don't add power, you probably won't go much faster than a slight creep, he added; he's never tried that, so he gave me the speculation only.

Neither F/Os gave an answer.

 

2. How much power does it take to make the tightest possible turn and keep rolling?

Captain answered that he always added a little power in a tight turn, because most aprons aren't 100% flat, there are almost always bumps taxiing into a gate that need a little power to overcome when the nose gear is turned to one side.

Right seat F/O answered that at the end of most flights, you don't need any power to turn by following the taxi lines. You only absolutely need power when doing the overshoot-and-turn-hard technique (as he called it).

The second F/O simply agreed with them and did not give any extra information.

 

3. When lightly loaded (I used their occasional YEG-YVR relocation flight as an example), how fast can you go on ground idle power in a straight line?

Captain answered that he's personally never done it, the shortest 777 flight he's commanded was YYZ-YUL. He said exact weights vary, but you don't need power if you don't need to pick up speed quickly. However, in big airports like YYZ, he generally uses power to accelerate in a straight line to help the ground controllers out when it's busy. When you pull back to ground idle though, the airplane will continue to accelerate (fastest he's gone was ~30kts before braking).

The right seat F/O answered that he's never done flights that short in a 777 and doesn't know.

The F/O in the jump seat answered that he's done a YYC-YVR relocation flight and you don't need any power to taxi and the airplane does accelerate, albeit a bit slowly. He did not elaborate further.

 

4. What autobrake do you need to exit runway 26R at M6 landing at YVR?

Captain said autobrake 2 or 3 depending on weight, because that's what they use at most airports not because he remembers exactly what they use in my particular scenario.

I said I can exit on M4 (one highspeed exit earlier than M6) with autobrake 1 in the sim, he said that would be "unlikely." (Interesting how he didn't say that it's definitively impossible.)

Neither F/Os gave an answer.

 

Finally, the Captain added that what he said changes between the 200LR and the 300ER, but it's not immensely significant in real life.

The jump seat F/O added that the training simulators they use also have this problem, but a previous airline he flew with used a simulator that ran a third-party friction code that was a little closer to real life. He did not give details on the airline, simulated aircraft type, or the name of the third-party code.

 

I think the above information is fairly interesting and most of it contradicts the behavior inside the sim. That said, I don't develop the software, so I don't know what goes on behind the scenes. I'm just putting this out here and see if anyone finds this interesting.

Edit: the whole conversation took about 5 minutes, so it wasn't an elaborate discussion about these things, just the rudimentary information shown above.

Edited January 6, 2019 by flightsim91

[downscc 1,327]

I hope you find nothing but agreement that the FSX/ESP/P3D ground friction model is make believe.  There is hope, PMDG took the ground friction and rolling resistance stuff outside of the simulator box for the QOTSII and I expect they'd do the same for future major updates to 737/777 and new products.  Don't hold your breath.  This stuff takes time.

 

[light_blue_yonder 15]

Agreed. I used to think that PMDG should've done it from the beginning, BUT now I feel like Lockheed Martin should be addressing these issues, not forcing third-party developers to do it.

Anyways... just putting that stuff up there just in case anyone is wondering. I'm sure PMDG knows this and is doing something about it. They can't ever be fast enough though. 😉

[light_blue_yonder 15]

One more question: does this mean that the current 777 landing braking behavior is unrealistic? (In reference to the autobrake question.) Or does it simply mean that I'm landing a lot lighter than Air Canada typically does?

Edited January 7, 2019 by flightsim91

[downscc 1,327]

Autobrake 1 is unlikely to be used real world because when thrust reverse effect is added the braking is on and off and this is a big no no because the brakes are rated for number of applications.  So while autobrake 1 might be okay at your weight and landing conditions the real world choice would favor autobrake 2.  Most of my sessions in the 777 have used autobrake 2 or 3.

[light_blue_yonder 15]

Makes sense, thanks! I guess I will have to confirm what the typical real world landing weight range is next time I fly. But still, leaving this post up just for the information at the beginning.

[light_blue_yonder 15]

*UPDATE*

Two more Q&As on another recent flight of mine on this topic.

1. How fast can you go on idle thrust?

The flight departed from Beijing with a long wait in line for takeoff. I noticed we rolled a long time on idle thrust, so the question was asked.

The reply was fluctuating between 5 and 6 knots at around 10 tons below MTOW. However, all three pilots agreed that a) it took a long time to reach that speed and b) the aircraft doesn't always do that on taxiways with even a slight slope. In fact, the captain said that when on idle thrust and heavy, the airplanes kind of lurch forward to a crawl when the brakes are released and then slowly coasts to a stop within half a minute or so (his guess), and you wouldn't get very far.

2. Does the aircraft slow down when taxiing with idle thrust?

Again, at that MTOW, the general answer is yes. The higher the speed, the more it slows down. Again, the captain gave his experience saying that around 10kts it doesn't really slow down, it kind of stabilizes (again, varies with taxiways). Above 10kts it tends to go back to around 10kts, and significantly above 10kts, it sometimes slow fairly sharply, requiring the occasional burst of power to keep the speed.

[kevinh 409]

Having fixed other long term issues with FSX I’m amazed Lockheed Martin haven’t addressed this in P3D. There are a couple of third party fixes available, but it would be far better if LM did it once and for all. The problem with developers fixing this is that friction coefficients in the sim vary with the surface defined in the scenery. So different adjustments need to be made for each surface and for rain, ice and snow contamination. Correcting the friction coefficient tables in the base simulator is the obvious and most complete fix.