Generic hexarotor fly away pixhawk 2.1 1.5.5


We are using a pixhawk 2.1 with px4 1.5.5 in LPE with a Here GPS.
We have experienced a fly away right after the takeoff. We use companion board to command the UAV.

From the log, we encounter a GPS timeout right as the problem begin. However we still received GPS data. During the flyaway, the data does not seems to correspond to the actual UAV position.

The LPE position in X and Y seems OK (but jumping back a little, as if it was trying to make somme position correction using GPS data ?). During the fly away, the tilt and roll angle are a little shaky.

the crash occured in nederlands the 21th of february 2018, the uav just took off, trying to reach 5m height but at 2.5 meters it started pitching seriously (it almost hit the ground but a tether help to change the angle). The uav kept flying at 2 meters above the ground still tilted forward and drifting). I had to kill it remotely to stop it.Unfortunately, I do not have the right to post the log of this flight due to GPS informations.

It does not looks like a motor problem as no motor received the order to go to full throttle. This drone has perfectly flown for more than one hour prior to the crash. Also two other drone sharing the same architecture have flown 5 hours and 2 hours whithout any issue.

Here is a log of a crash experienced in november 2017 which seems related to the same issue (GPS timeout observab le in MAVGAnalsys right when the UAV start flying away)

Does someone has an idea of what might have happened ? In the futur, we have plan to use EKF2 with px4 1.7.X and 1.8 (as the magnometer sensors has changed in the here GPS since some time). In between, we have to keep flying with LPE 1.5.5

Thank you.