I was doing some endurance flight testing on my Pixhawk powered Align M690L hexa, and I noticed regular spikes in the pitch that were causing much higher current draw from the battery than I expected.
Further investigation yielded that the desired acceleration (in my case specifically DAccY) had regular spikes. The discontinuities were also obvious in desired velocity. I am not certain the way guidance works on the ArduCopter and what is computed from what, so it is difficult to get root cause. However, the spikes are very regular, and are clearly a source of the guidance (potentially poor PID settings?).
The spikes were exhibited in front forward auto flight, but not in the RTL back-forward portion of the flight. I am not certain whether this means that this is caused by flying forward, or if the guidance code is subtly different b/w RTL and waypoint flight.
Attaching some pictures of the dacc, dvel, and the pitch.