I´m not sure what exactly caused such a sudden descend.
Plane: History of 169 flights/missions. 167 successful and 2 problems (logs links posted here).
1200 mm (47 in) wingspan flying wing; 1,7kg; 3DR Pixhawk mini.
The plane entered Descend Mode while executing a mission @ 230m AGL (about 1000m ASL).
I can see that the altitude setpoint goes to 0m ASL causing the plane to rush to the ground.
I have already updated the firmware on the board.
The airframe was ok regarding vibrations, flight characteristics and magnetic interference.
The day the plane misjudges the altitude setpoint, we had thermal column of ascending air on the area.
Maybe this is the start of the problem, or maybe the fact that the plane was running older firmware.
you should try the newer 1.8.x firmware. There was a lot of bad stuff for fixed wing in older firmware, especially certain EKF2 and PX4 failsafe behavior. In particular this change allowed relaxing a number of settings for fixed wing vehicles: