AltHold Algorithm Bug

Hello Friends
I posted my problems in connection with overshooting in the AltHold mode a few months ago on DIYDrones. But nobody really listen…
Today I did some aerial footage for a commercial. It was windy but I saw no reason to don’t start. I waited in the the Loiter Mode until I get the order to fly forward. Suddenly my X8 coax copter nearly did a backflip… I switch immediately to the Stabilize Mode and the multicopter leveled horizontally. After that I landed and finished the drone shots.
I went home and did a comparison test between the AltHold Mode and the Stabilize Mode. The goal was to fly forward and to slow down the flight speed by push the transmitter stick to the neutral position. So, no aggressive brake maneuver.
Here it is in the AltHold Mode, you can clearly see that the multicopter overshoots. :

And the same maneuver in Stabilze:

You can clearly see a difference between the flight modes. I got this problem also during shoots early this year. We said, it must be the weather conditions, maybe to windy… But no, in my opinion it’s a problem with the AltHold mode and all other modes who relate to them.
I hope you guys find a solution for this. It’s only a matter of time til a multicopter crashes and hopefully nobody get injured…

Here is my setup:

V 3.3.3 / 3DR Pixhawk
X8 Coax

Here is the log from the test: Dropbox - - Simplify your life

BTW, I have the same problem with other X8 coax multicopter…

Thanks a lot

I too had a copter that flew just fine until one day I was flying into windy conditions and it would suddenly take off. The funny thing was it was only going into the wind and not away. For a while this was kind of fun until I flew it in very windy conditions and it shot up 20 meters into the air and flipped over mid flight over me. I quickly switched to stabilize and cut the throttle. I was able to give it a little throttle and get it to right itself and then get it to softly land.

The problem was that I mounted the Pixhawk directly on the copter with a piece of Velcro. That way if I wanted to move it I could just pick it off. It turns out that as the copter flies into the wind it generated a huge amount of vibration in the Z axes that misleads the flight controller into thinking its falling and corrects buy giving it full throttle.

I thought the flight controller should not do this but that’s what it does and still does. I then mounted the unit on a foam platform and the problem was gone.

I looked at your logs and see the same conditions. The vibrations in the Z axes are high and I can see the throttle going to full.


Hi Mike

Thanks a lot for your opinion. The strange thing is, that that I have those problems since maybe 6 month. Before I never got issues like this. I’m not sure if it’s since the hole EKF thing was implemented…

The vibrations in the Z axis are still in the acceptable range from -5 to -15 during the massive overshooting… So I’m not sure if this is really the problem.

But I know that high vibration cause strange problems…