Fatal Crash After Random Offboard Reject

Hello all,

We were flight testing last Friday and had a fatal crash. The setup is as follows.
Flight Controller: Pixhawk 2.1
Firmware version: 1.7.3
TX: RadioLink AT9
RX: Radiolink R9DS
Telem: RFD900+


We have a companion computer where we change to offboard mode to send position setpoints, but not during the mission. I suppose that something triggered OFFBOARD mode, and since the COM_OBL_RC_ACT was set to 0 which is POSCTL, the copter switches to POSCTL for a split second and then drops to STAB. Since the throttle was set to 0 from the TX, copter crashed with 0 thrust in about 1 seconds since it was about 10 meters above ground. While it was dropping, it switches to POSCTL and back to STAB again.

  1. Might this offboard attempt be related to some noise on TX or RX side? The RC_MAP_OFFB_SW was set to 9 so we also have an offboard switch on the transmitter. But never touched it really.
  2. The TX flap was in the default position which is STAB when the offboard reject happens, but since the COM_OBL_RC_ACT is set to POSCTL, shouldn’t it stay that way? It seems that the RC tx somehow overrides all the commands(twice). This could be a critical bug which might need handling.