ATS during descent: tuning needed? (Solved)

[RonMont]

I have now completed a few dozen flights with the Challenger.

I notice that during descent, the ATS speed control is quite erratic in maintaining the FCP selected speed.

For example, speed set to 240 under FL100, from current speed of 290: upon reaching 240 after the deceleration, the throttles get increased a few knots before reaching 240 and push the aircraft to ~248, then throttles get reduced by the ATS.

Again, upon reaching between 243 and 240, the throttles engage again, pushing the aircraft to ~248, and back down again.

I believe the ATS needs some fine tuning, as it is currently too aggressive, pushing the nose up and down, into a clear oscillation pattern.

This is happening no matter what the weather is, and annoys me to a point where I often take manual control of the throttles.

One thing I also notice, relating to all this, is the amber GND Spoilers alert on the MFD, as soon as you overspeed by as little as 3 or 4 knots; is this also happening in the real thing?

I reckon there might be some settings related to those sensitivities somewhere in the settings, of which I am not aware; so I will be happy if someone can point me in the right direction.

Thanks in advance for any help in resolving this.

[bpcw0001]

To my knowledge, the ATS behavior cannot be tweaked by users and there are no settings for it.

Still, I've never experienced any of this, the ATS just works fine. As for going over the speed during descent and the GND spoiler thing, I need to check this. I do get the "SPEED MSG" alert though if the speed doesn't come down.

However:
Ever since upgrading from X-Plane 12.05 to 12.06 and 12.07, I occasionally face quite the contrary issue: the ATS reacts way too slow to keep speeds on approach, typically when going "gear down flaps 30" or later with "flaps 45" and the respective speeds set manually, the ATS sometimes (not always!) lets the speed drop way below the selected value, so I end up disconnecting the ATS and taking over the throttles (N1 target is selected, ATS displays "SPEED" mode, so no pilot error here).

Edited October 4, 2023 by bpcw0001

[RonMont]

4 hours ago, bpcw0001 said:

To my knowledge, the ATS behavior cannot be tweaked by users and there are no settings for it.

Thanks for your reply.

I was not referring to a user tweak, but rather from the developers of the aircraft.

[ois650]

ATS isn't perfect and it's been tweaked to behave like the actual thing, although we've had no reports of ATS causing oscillations in the descent - video of this behaviour would be useful. 

Ground spoiler CAS message with overspeed is a new one - again, video would be useful here should it happen again.

Thanks

[RonMont]

On 10/5/2023 at 1:36 PM, ois650 said:

ATS isn't perfect and it's been tweaked to behave like the actual thing, although we've had no reports of ATS causing oscillations in the descent - video of this behaviour would be useful. 

Ground spoiler CAS message with overspeed is a new one - again, video would be useful here should it happen again.

Thanks

Here's a video of it. Note that the behaviour starts happening when getting close to the 240 knots /10,000' restriction.

Sorry it is only 1 minute, but I just downloaded this screen recorder, and it is limited to 1 minute videos in it's free version; but the behavior will continue all the way down to landing.

Thanks for looking into it, as I really find it annoying. I usually disconnect AT and try to control my speeds/spoilers manually, which takes a lot of concentration off the route and approach.

 

[Pils]

2 hours ago, RonMont said:

Sorry it is only 1 minute, but I just downloaded this screen recorder, and it is limited to 1 minute videos in it's free version

https://obsproject.com/

[RonMont]

1 hour ago, Pils said:

https://obsproject.com/

Thanks a lot... will take a look at that one :-)

 

[RonMont]

@ois650

Here's a longer one; you can clearly see the porpoising, and the N1 pitching up and down to catch up.

Something's not right for sure.

Thanks to @Pils for pointing me to OBS, which allowed me to record a longer video.

 

[Pils]

46 minutes ago, RonMont said:

Thanks to @Pils for pointing me to OBS, which allowed me to record a longer video.

No problem, but a wider angle is needed, please.

[Pils]

1 hour ago, RonMont said:

Here's a longer one; you can clearly see the porpoising, and the N1 pitching up and down to catch up.

Are you deploying the spoilers somehow?

Edited October 10, 2023 by Pils

[Pils]

On 10/4/2023 at 1:18 PM, RonMont said:

I was not referring to a user tweak, but rather from the developers of the aircraft.

The aircraft seems to work fine for basically everyone else, so... Let’s figure out what’s happening for you first.

Edited October 9, 2023 by Pils

[skiselkov]

The oscillation is being caused by a positive feedback with the ground spoilers deploying whenever the ATS commands idle thrust and pulls the thrust levers back to the idle stop. This causes the nose to either drop or rise, which then feeds back into the speed excursion, the ATS changes power settings again, the vertical guidance starts hunting for the VPATH and the whole cycle repeats.

Ground spoiler deployment can only happen if:

1. Ground spoiler arming switch is either in AUTO or ARM, and
2. Thrust levers are idle, and
3. Either weight-on-wheels is detected, or wheel spin-up is detected

Neither of 3 should theoretically be possible, but seeing a recording of the aircraft from the outside to see if the wheels are spinning in the wheel bins would be helpful. We do tell X-Plane to STOP the wheels from spinning when retracted, but if there's some sort of physics bugs going on inside of X-Plane's wheel logic, then it might be spinning the wheels anyway. This would trigger the PSEU wheel spin-up signal and that might be causing the ground spoiler deployment in flight. I can tell you that for sure we set both the l_brake_add and r_brake_add datarefs to non-zero values to tell X-Plane to NOT DO THAT, but if it is ignoring our input, then it's out of our hands.

[RonMont]

12 hours ago, Pils said:

No problem, but a wider angle is needed, please.

I thought showing you the PFD and MFD was what you needed. What would you like to see? If I record from my normal sitting position, the instruments are unreadable.

[RonMont]

12 hours ago, Pils said:

Are you deploying the spoilers somehow?

I get the GND spoilers alert popping up in the MFD, during that porpoising, but I am not deploying them manually, unless there is a serious overspeed as compared to the requested speeed.

But I think @skiselkov put the finger on the problem, with the Ground Spoiler switch: mine is in the "ARM" position.

I will test on my next flight with the switch at OFF. I did not think that could make them deploy automatically during flight.

[RonMont]

11 hours ago, skiselkov said:

The oscillation is being caused by a positive feedback with the ground spoilers deploying whenever the ATS commands idle thrust and pulls the thrust levers back to the idle stop. This causes the nose to either drop or rise, which then feeds back into the speed excursion, the ATS changes power settings again, the vertical guidance starts hunting for the VPATH and the whole cycle repeats.

Ground spoiler deployment can only happen if:

1. Ground spoiler arming switch is either in AUTO or ARM, and
2. Thrust levers are idle, and
3. Either weight-on-wheels is detected, or wheel spin-up is detected

Neither of 3 should theoretically be possible, but seeing a recording of the aircraft from the outside to see if the wheels are spinning in the wheel bins would be helpful. We do tell X-Plane to STOP the wheels from spinning when retracted, but if there's some sort of physics bugs going on inside of X-Plane's wheel logic, then it might be spinning the wheels anyway. This would trigger the PSEU wheel spin-up signal and that might be causing the ground spoiler deployment in flight. I can tell you that for sure we set both the l_brake_add and r_brake_add datarefs to non-zero values to tell X-Plane to NOT DO THAT, but if it is ignoring our input, then it's out of our hands.

Thanks for this precious information; I always have that switch at ARM, and never thought it could trigger auto deployment of the spoilers during flight.

I will test that on my next flight, having the switch at OFF.

I will also check the spinning of the wheels during flight.

Thanks a lot!

[skiselkov]

17 minutes ago, RonMont said:

I will test that on my next flight, having the switch at OFF.

AUTO is the normal position. The checklists should make that clear.

[RonMont]

1 minute ago, skiselkov said:

AUTO is the normal position. The checklists should make that clear.

Yes, I just reviewed the checklist, and you are right indeed. Will test with the switch set at AUTO, and as I said, will also check the wheel spinning too.

I will also make sure I press the brake pedals after gear retraction, in case.

[RonMont]

@skiselkov@Pils

Hi guys!

Finally had a chance to test the descent yesterday; had the smoothest and most pleasant descent ever in my 231 hours on the CL60.

The problem was indeed what @skiselkov diagnosed: having the GND SPOILERS set at AUTO instead of ARM did the trick.

Thank you very much guys for taking the time to troubleshoot my issue, and pointing me in the right direction.

You have no idea how much I appreciate!

Have a great day!

[Pils]

12 hours ago, RonMont said:

The problem was indeed what @skiselkov diagnosed: having the GND SPOILERS set at AUTO instead of ARM did the trick.

That’s good to hear, but we’re the wheels spinning? The logic even in ARM shouldn’t have deployed them in flight.

[RonMont]

9 hours ago, Pils said:

That’s good to hear, but we’re the wheels spinning? The logic even in ARM shouldn’t have deployed them in flight.

I've made 3 more flights since, and observed the wheels: they were NOT spinning; was wondering the same as you: why then did the GND spoilers deploy?