A Bombardier Challenger 300, N300ER, encountered an upset when the flight crew disconnected the autopilot while suffering from a nose-up mistrim situation. The mistrim was the result from a failure of the autopilot stabilizer trim control. The upset resulted in a roller coaster exceeding +4g and -2g, eventually brought under control by the flight crew. Tragically, a passenger was fatally injured in the upset.
The NTSB report makes no mention of PRI STAB TRIM FAIL alert being activated. For some reason, the flight crew chose to follow this unannunciated failure checklist. The first action was to select Stabilizer Trim to OFF. This action would immediately disconnect the autopilot.
Severe mistrim is a dangerous situation. Autopilot disconnect with severe mistrim has resulted in loss of control and catastrophe. While the flight crew managed to regain control and recover the airplane safely, the loss of life was tragic. The flight crew should have expected an upset and prepared the cabin occupants before taking action. Furthermore, the pilots should have reviewed the appropriate checklists which would have prepared them for the upset. The NTSB final report will analyze these circumstances when determining probable cause.