Crazy Pitching in FLC mode

[tomghaynes]

Hi there,

I have an issue when using FLC to climb in the P180. The aircraft pitches violently up and down to try and pitch for the selected speed. The oscillations get so bad within a few seconds that the autopilot eventually puts the plane into a nosedive and disconnects. Pitch trim seems to be nearly all the way down as well.
I notice that the aircraft also pitches very sharply/aggressively when changing from V/S to FLC mode.

[Taguilo]

Hi,

Do you have the same issue with every flight, in different wx conditions?
Any other detailed info you can provide will be very helpful.

Thanks!
Tomas

[sharpless281]

Mind does that as well, but I do not loose the AP setting, unless I switch t VS. I kind of gave up on it and just use VS have not tried in on the 180 yet

[Emi]

I'm seeing the same as the OP. During the pitch downs I even end up with negative G forces.

They seem to happen mainly when there's very small speed variatinos due to wind. P3Dv5, even with default weather, often lets the airspeed oscillate by about a knot in winds (which is realistic btw, you see the same in real aircraft) and to me it looks like the AP is aggressively trying to chase the speed, leading to this pitching and finally resulting in AP disconnects.

The same aggressive pitching happens when changing the target speed for FLC mode. It is most noticable at higher altitudes, but happens close to the ground as well.

[matpietsch]

That is exactly the issue I described. Same in the king air.

[Taguilo]

I think that issue is related to a P3D v5.1 wx engine behavior. Mostly if also present in the KA, where it was working fine in v4.5 and is the same code. Is it possible that only happens when entering a cloud layer?

I will try to replicate it and eventually see what could be the triggering situation.

Thanks!
Tomas

[tomghaynes]

Yeah, I noticed that it will happen with changing winds/turbulence.
Any sort of change in the IAS (even by a couple of knots) causes the flight directors to start to aggressively chase the selected speed, resulting in the pitching becoming out of control and then nosediving.
In clear weather conditions it doesn't happen, but when flying with real weather FLC mode is often un-useable because of this.
I may be able to record a video tomorrow to try and demonstrate.

[matpietsch]

Did some further testing: Weather can throw it off (so reducing your ASP3D settings for tubenlence does help) but also something simple as a turn in HDG mode will send it go crazy,

[Floh124!]

I can confirm this fault (or better to say, have the same problem). And I can't also see no relation to weather. Sometimes it starts pitching hardly and wild for no obvious reason.

Best regards
Florian

[Emi]

It is unrelated to any kind of weather. Happens in clear skies as well.

Here's a video, maybe it helps you. Skies aren't 100% clear in this one, unfortunately when it happened on the previous flight which had CAVOK conditions I was too late to turn on the camera.
https://youtu.be/iTm4HZJWl5I

[Taguilo]

I reproduced the video conditions, same FLC speed, altitude, etc and couldn't get the same results as yours. It didn't fail.
BTW, do you have your spoilers mapped to an axis in your hardware?
I'll keep trying to see if I can find a specific situation that might cause that.

Thanks!
Tomas

[Emi]

Thanks for your efforts Tomas,

I do not have spoilers mapped to an axis.

The problem does not seem to be specific to any set of conditions I could figure out yet - it happens on almost any departure.
The scenario is usually always this:
I stabilize the airplane pretty much on the flight director, make sure I'm not in a turn or any situation where I'd need constant pressure on the control column in either direction. Everything is stable.
Then I engage the AP. It will usually continue to fly the target for about a second or two, then it will start to rapidly pitch. Mostly upwards for a second, then downwards until reaching about -5° of pitch, which is the point when the AP disconnects.

What I *could* imagine is that very slight variations in airspeed play a role here. Active Sky and P3Dv5 insert those very small airspeed variations when there's wind (not even turbulence).
This said however, when I'm stable at 160kt after takeoff with FLC engaged and wind the speedbug up to 200kt for my climb the issue sometimes happens as well.

[Taguilo]

I think this issue is mostly related to Active Sky.
Can you do some tests using only P3D default weather? (Fair, Building storms, etc).
Anyway, I will try to improve FLC by damping unusual speed's instant variations.

Thanks!
Tomas

[Emi]

Some good news Tomas: Since I installed the update from the 26th the pitching is gone. Now it's pitching perfectly fine in climb.
I have encountered one attempt to climb in descend though which disconnected the AP.
I was descending at about 1500fpm and selected FLC mode with a target speed 3kt below my present speed.
This caused the AP to start a climb and the moment the VS became positive the AP disconnected.

[Taguilo]

Hi Emi,

Thanks for advising!. It seems you had the original version installed.
Regarding FLC in descents, what you comment is also weird. I suggest you keep testing in different descent situations. However, bear in mind descents in FLC are very unusual in RL; especially when following VNAV directives. Typical modes are VPATH and VVS; also VS is used when not in VNAV mode.

Thanks!
Tomas