I have a custom quadcopter frame that is controlled by a Pixhawk. While testing the frame in auto flight, I have observed extensive pitch oscillations when the quad is in horizontal motion while descending as shown in the descent phase of the plot below (figure 1). I say horizontal motion since the problem exists when flying forward, sideways or backward while descending in auto mode.
These oscillations are not always present during this stage of a flight but come up randomly. During multiple flights through the same flight path, you can see from the plot below (figure 2) that these oscillations are not present for the descent part of this flight.
The system was tested with manual and auto tuning to correct this problem to no use. I have had excellent flights with the same system and same tuning values in all other stages of flight. Stabilize, Alt Hold and Loiter were also found to be excellent.
I have looked online but have not found any discussion on this topic. Any help would be greatly appreciated. A copy of the logs for this flight are attached.