Hi, enabling crash detection prevents hand launch in Auto mode despite setting CRASH_ACC_THRESH higher than maximum detected IMU.AccX during hand launch. It results on a motor starting for half a second and disarming. I think that crash detection should be inactivated during the the early first phase of a mission Takeoff when TKOFF_THR_MINACC is set (or at least that the value of CRASH_ACC_THRESH be augmented by the value of TKOFF_THR_MINACC during that phase) . That said, I don’t understand why in my case crash is detected whereas IMU.AccX max (25,35m/s/s) is much lower than my CRASH_ACC_THRESH setting (50m/s/s).
Thanks
Hi, having in mind that Ardupilot developers usually work good 
, I tried to find where I could be wrong. One idea is that I may misunderstood the value I have to enter in CRASH_ACC_THRESH. Reading the parameter description, I though I had to set it an absolute value (positive) and put 50m/s/s. The exemple mention:
But maybe has this parameter has to be set a Negative value?
Thanks
Looking at Complete Parameter List — Plane documentation the allowable range is 10 - 127 so not negative.
The docs for TKOFF_THR_MINACC suggest an acceleration threshold of 15 for a hand launch which means that they expect the acceleration of a hand launch to be greater than that.
Yes, this is where the my problem come:
I use those parameters value:
- CRASH_ACC_THRESH = 50
- TKOFF_THR_MINACC = 10
If I shake the plane at 25m/s/s (max reported by IMU.AccX) then the motor start as it should but half a second after it disarms (enough for me, the first time it happened, to launch the plane in the air, and see it falling down like a crap… 
). This should not happen as I am under crash detection threshold ? I am sure it is due to crash detection system as when I disable it I have no problem with hand-launch.
No clue. Maybe post a blackbox log and someone with more experience than I can help debug?
@Allister TKOFF_THR_MINSPD is set to zero as I hand-launch. This parameter is more for bungee-launch or pusher planes to avoid prop hitting ramp, bungee or arm. In my case it is a puller so I don’t need it to be away when motors start. I only put it in takeoff mode, arm it and then shake it in IMU AccX direction so motors start. Then I just have to lauch it facing wind. All of that works well. The problem only occurs in Auto Mode during first waypoint execution (TAKEOFF) when crash detection is enabled. Normal takeoff (using takeoff mode) does not take crash detection into account. In Auto mode, the only workaround to hand-launch takeoff is to disable crash detection.
Post log files from the crash and from a good launch.
Exactly the same experience:
Hand launch in Auto mode despite setting CRASH_ACC_THRESH higher than maximum detected IMU.AccX during hand launch. It results on a motor starting for half a second and disarming.
Log:
https://drive.google.com/file/d/12xrdHlxKanhnPnUB2gk9LsdNnP_Fkc_D/view?usp=sharing