PMDG 777 Nose Wheel Steering after pushback

[CapnTibbs]

Hi guys,
On the past couple of flights I've done in the 777, after disconnecting the tow bar, the nose wheel is stuck to the left and when I even nick the rudder the aircraft just takes off to the left with full right rudder barely being enough to keep the aircraft going straight. I'm 99.9% sure that GSX is causing the problem and that the problem is not isolated to the 777. I've tried a number of tactics, using Quick Edit pushbacks and putting waypoints in them, disconnecting my rudder pedals when the nose wheel is centered, turning off the hydraulics during the pushback to let the nose wheel go freely, nothing has fixed the issue. I've scoured the forums here looking for suggestions and fixes and found absolutely nothing.

I know what the issue is and what is causing it, I just need a fix for it.

Thanks

[virtuali]

I've scoured the forums here looking for suggestions and fixes and found absolutely nothing.

The problem is, nobody ever reported it before, that's why you can't find anything.

I know what the issue is and what is causing it, I just need a fix for it.

That's your problem: being "sure" GSX is the problem, when it likely isn't because, if it was, you would have found similar reports, considering how popular BOTH GSX and the PMDG 777 are. And if it really was a GSX problem, it would have been fixed already.

While it's possible that *WHILE* GSX is pushing, you might see some visual flickering of the rudder pedals in the VC, which is caused by the PMDG internal simulation of the rudder that is "fighting" with the GSX truck setting the rudder at the same time ( which could be easily fixed by PMDG if they did what other developers like FS Labs, Aerosoft or Leonardo SH that have similar custom systems they momentarily turn off while GSX is pushing, checking a variable GSX exposes just for that reason ), AT THE END of the Pushback, when it says "bypass pin has been disconnected", GSX will stop acting on the rudder so, if it's "stuck" for you, it must be caused by something else.

Of course, before posting this, I did my usual checking and, sure enough, there are no issues I can see. See the below video I just made, which shows how the rudder works normally at the end of the pushback.

[Captain Kevin]

I've seen it happen a couple times before, but the problem is it's not consistent, so I don't know what would cause it. 5:30 into this video, you'll see it works fine, but 1:24:00 in, you'll see the rudder goes to the left after pushback.

https://www.twitch.tv/videos/1168786851

In this one, no issue at 6:10, but it happens at 3:03:41.

https://www.twitch.tv/videos/1192932152

In this one, no issue at 5:10 or 1:22:04.

https://www.twitch.tv/videos/1194765726

In this one, no issue at 5:32. Not sure at 2:36:07. It may have happened at 4:39:08. No issue at 6:01:48.

https://www.twitch.tv/videos/1197054206

[virtuali]

5:30 into this video, you'll see it works fine, but 1:24:00 in, you'll see the rudder goes to the left after pushback.

What I see in that video is:

1:23:12 - GSX says "tow truck disconnected, bypass pin removed", which means it has stopped acting on the rudder.

1:23:15 - Immediately thereafter, the Flight controls check page shows the rudder is working perfectly fine, further confirming GSX is not controlling it anymore.

1:24:25 - The nose gear looks turned so, whatever happened between the previous confirmation of the rudder working normally and now, can't possibly be GSX, which at this time has already recalled the truck, so it's not doing anything.

In this one, no issue at 6:10, but it happens at 3:03:41.

Same here.

The controls page shows the rudder centered when the GSX truck is already moving out, so clearly it has released the rudder.

In fact, if you notice the external view at that time, the nose gear is turned almost completely to the left, but the rudder it's not.

The rudder is the only thing ( during the pushback, only ) which GSX controls so, the real issue here seems that is some cases, what is likely a custom variable PMDG set up for the nose steering system, which is a non-standard system ( which GSX never dare to touch, of course ), has lost its sync with the standard variable for the rudder, possibly because their code doesn't expect the rudder to be controlled by another app, even if it's only for a short while. Maybe it doesn't always happen, because the code is eventually able to re-establish the sync, but that's just a guess.

[Captain Kevin]

One thing to add here is that I have PMDG set to use the tiller, so the nose gear would be controlled by the tiller rather than the rudder.

[CapnTibbs]

Just to clarify. Here's a photo from after I parked up. The rudder is neutral and there is no pressure being applied on the rudder here. Obviously before the pushback it didn't look like this.

[Captain Kevin]

Are you using the tiller option in the 777 or not. I am, so I'm just curious as to whether that has anything to do with it.

[CapnTibbs]

Interestingly enough I tested that hypothesis, and the situation got much worse. On the landing rollout through about 50-60kts just as I was about to vacate the runway the aircraft violently and uncontrollably pivoted to the left. Many expletives were said when that happened. That was definitely unexpected.

[Captain Kevin]

Interestingly enough I tested that hypothesis, and the situation got much worse. On the landing rollout through about 50-60kts just as I was about to vacate the runway the aircraft violently and uncontrollably pivoted to the left. Many expletives were said when that happened. That was definitely unexpected.

That tells me literally nothing. I don't know if you had it on and disabled it or if you had it off and enabled it. That also has nothing to do with the pushback.

[CapnTibbs]

I don't see how that tells you nothing but that's fine I'll try a different way of explaining it. I normally don't use the tiller axis on the 777 but I turned it on in the air to see if that would keep the nose wheel tracking straight once I was on the ground. When I dropped the gear, the nose wheel had straightened out so I figured that had sorted the problem but it clearly hadn't. As I said before, on the landing rollout through about 50-60kts just as I was about to vacate the runway the aircraft violently and uncontrollably pivoted to the left.

Obvious question, seeing I did try resetting Couatl if resetting Couatl doesn't fix the problem then it isn't a problem with GSX?

[Captain Kevin]

I don't see how that tells you nothing but that's fine I'll try a different way of explaining it. I normally don't use the tiller axis on the 777

That's the part I was trying to figure out because up until now, I had no clue if you even used the tiller axis or not because this point wasn't clear and I'm just a truck driver, not a mind reader.

but I turned it on in the air to see if that would keep the nose wheel tracking straight once I was on the ground. When I dropped the gear, the nose wheel had straightened out so I figured that had sorted the problem but it clearly hadn't. As I said before, on the landing rollout through about 50-60kts just as I was about to vacate the runway the aircraft violently and uncontrollably pivoted to the left.

You really can't use the tiller at more than 20 knots or the plane just loses it. Something in the PMDG coding somewhere. I don't know what you had set for the tiller or if having nothing set for the tiller could cause it. The rudder can still steer the nose gear, but up to I think 7 degrees, so you'd really only use it for a high-speed exit after landing.

Obvious question, seeing I did try resetting Couatl if resetting Couatl doesn't fix the problem then it isn't a problem with GSX?

I got nothing.

[virtuali]

Just to clarify. Here's a photo from after I parked up. The rudder is neutral and there is no pressure being applied on the rudder here. Obviously before the pushback it didn't look like this.

As I've said, GSX ONLY acts on the rudder and ONLY during pushback so, if the rudder looks neutral at the end of the pushback, it's clear GSX has done what it's supposed to do.

IF the rudder was still deflected at the end of the Pushback, it might have been a GSX issue that, for some reason, failed to release control over it when it's supposed to. But since it seems from all your reports this is not the case, it's very likely the real issue is with the PMDG Nose steering system ( which GSX DOES NOT touch ) being confused by the fact the airplane code wasn't the only thing that controlled the rudder during pushback.

And that's precisely why GSX has a public documented variable to inform airplane developers that GSX is pushing, so they can momentarily turn off any custom simulation of the rudder/steering or anything else ONLY THEM CAN POSSIBLY KNOW that might be affected if something external is pushing.

It's a simulation of what happens in real life with the bypass pin: a mechanical system that decouples the steering from the hydraulic system, allowing the tow truck to be in control.  Our public variable is sort like a "software bypass pin", and other developers like FS Labs, Aerosoft or Leonardo SH, which ALL have their own custom system to simulate steering and hydraulics in general, which they know should be turned off during pushback but only them can possibly know what should be change in their custom code to support that, that's why the proper and safe way to do this is to have GSX telling the plane is pushing, and rely on the plane code doing what it needs to be done to ensure no strange issues like these happens.

When airplane developers use custom code to create non-standard systems not available in the sim (or if they redo systems in a different way), there's no way for us to access those, unless they have specific hooks in an SDK made for that airplane but, while PMDG doe have an SDK for the 777, it mainly provides with events to trigger button/switches in the VC, so it's mostly useful for cockpit builders, sure I can't find anything about temporarily disable the hydraulic simulation or control the steering, which I guess should be required to eventually fix the problem from GSX side.

[CapnTibbs]

Okay Umberto thanks for the explanation. I've already sent in a support ticket to PMDG but I haven't heard back from them

[CapnTibbs]

After some back and forths with PMDG and some self testing in the sim, I'm pretty sure the issue might well have been as simple as an intermittent USB port triggering at just the wrong moment.