LVL CHG mode pitch up after IAS/MACH switch

[daemotron]

I have noticed a behaviour of the LVL CHG mode where I'm not sure wether this is inteded or needs some fine-tuning:

I usually use the LVL CHG mode for climbing, and as long as IAS is selected, everything works fine; the climb rate decreases with increasing altitude (down to ~1,000 fpm with 285 Kt IAS approaching FL290). The moment however I switch from IAS to MACH (without changing the selected speed, just a switch from e. g. 285 Kt IAS @FL290 to M.74), the plane pitches up, increasing VS up to 3,000 fps and only pitching down once the speed has decreased to M.70, then levelling and regaining speed before resuming a "normal" climb profile.

I noticed this particularly during the last two flights where I had significant tail wind conditions.

Situation where speed has fallen to M.70 and plane starts pitching down:

Speed recovery:

IXEG_FMS_debug.txt

GizmoLog.txt

Log.txt

[Nils]

Hi, is the IAS to Mach conversion working as intended for you, i.e. does the magenta speed bug stay at your current speed with the click the C/O button?

As always, a short video is very useful for us to analyze the problem.

[daemotron]

Hi Nils, yes, the speed bug stayed in the same position. I'll try capturing a video to show the effect.

[daemotron]

OK, here is the video:

The effect is not as strong as during my previous flight; here the pitch is only maintained until M.72 is reached, but the noticeable pitch up happens.

Jump to 1:55 of the video, and you can see the cut over from IAS to MACH, and subsequently the V/S increasing to 3,000 fpm, staying there for half a minute, and then slowly decrease.

NB I was apparently wrong - it has nothing to do with tailwind (as this video was taken in a headwind situation) - but both situations had some turbulence in common, so maybe that's a trace...

IXEG_FMS_debug.txt

GizmoLog.txt

Log.txt

Edited December 9, 2016 by daemotron
forgot adding log files...

[Nils]

Many thanks. The pitch control regulator needs some fine tuning when tracking Mach number. Some pitch up is to be expected when making the cut over from IAS to Mach since you are effectively starting to climb at lower airspeed, but the regulator was overshooting, particularly in gusty conditions. Will tune and include in future update. @daemotron

Edited December 10, 2016 by Nils