Last Friday, I was preparing for a rather simple auto mission. I set everything up, did a last minute change to CH7_OPT, then positioned my IRIS on a nice place for takeoff. I armed in LOITER and raised the throttle. The IRIS lifted off to about 50cm and started drifting to the right.I tried to steer against with no effect. I switched through modes but couldn't get back control. The IRIS bounced off a tree, then another tree and finally got stuck in a hill. Still no reaction to sticks, mode, nothing. I didn't have stabilize mode configured as flight mode, so I jumped to my GCS, switched to STAB through AP2 and finally, the throttle went zero and I could disarm.

What has happened? Logfile analysis revealed that all THR_* PIDs were zero. Some more testing turned out that APM Planner 2 apparently doesn't always correctly refresh the extended tuning page, so when I did the last minute change to CH7_OPT, I accidentally also wrote the zero THR_* PIDs to the vehicle. Of course, with all those zero, the IRIS couldn't do anything with my control inputs in any mode in which the altitude hold controller was active.

That whole thing could have gone much worse. I only broke the 2 front props but without the trees to bounce off from, the IRIS could have made it to the street or somewhere else.

This is a textbook example why it is extremely important to be thorough when setting up your vehicle.

  • When changing the configuration, always double check all parameters you are writing! Not only the ones you changed but the whole screen!
  • Always have stabilize configured as a flight mode for your TX switches! It can be the last resort to save your vehicle or - more important - prevent it from causing harm to property or people!
  • When using copter 3.3 or higher, it's a good idea to have the new motor kill function configured on a switch. Better dump the copter somewhere than let it go out of control.