Loss of altitude during auto mission

I was asked to help analyzing the altitude loss which occurs during auto missions with a copter.

The behavior happens sometimes, it was not a one time event.

The copter is a 3DR Y6 Copter with Pixhawk and uBlox GPS.

LOG file and flight plan:
dropbox.com/sh/a37n0rq4gu3u … GxvXxtic….

I had a look, but can’t spot anything (the altitude loss occurs at around 3.5 minutes).

  • vibrations are ok
  • RC/TX connection is ok
  • no pilot input during this time
  • GPS is ok
  • all in auto mode during this time
  • the mission is defined to be at a height of 200 meters, so there should be no loss of altitude
  • the loss of altitude is visible in the LOG file
  • desired altitude decreases (why??), this leads the copter to loose height

I am stuck with this one,
any help is appreciated

thanks a lot


I think the dropbox link is not working,
here again

dropbox.com/s/7d48x36uu1yle … 4.log?dl=0
dropbox.com/s/pvrjo00s5phbg … 2.txt?dl=0

Channel 1 is maxed out for a big chunk of your log. It maxes out at the time your alt drops.

I understand des alt chases alt down if alt can not be maintained.

If the copter has a weak motor/drive train (chan 1 in this case), the other outputs are limited in order to maintain attitude…therefore altitude can not be maintained for a given air speed. Your pitch can not match des pitch either as it is limiting pitch to try to maintain vertical thrust.

Something looks wrong with channel 1 out put to me, could abe a number of things, weak motor, slipping prop, bad esc cal, wiring, a mechanical issue…etc