EMB ERJ 170/175 ER/LR
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Overcame Equipment Problem; General Physical Injury / Incapacitation
Aircraft
While in cruise at FL350 at Mach .75 and 54 minutes into the flight (in VMC conditions; in still air; with no turbulence being alerted to us; the flight crew by ATC or by PIREPS) the autopilot was engaged until the autopilot disengaged and the aircraft did an abrupt pitch up and stick shaker occurred. The aircraft climbed approximately 200 feet during this event. The aircraft was returned back to FL350 and autopilot was reengaged. No EICAS message occurred however we reviewed the pitch trim runaway checklist even though there was no EICAS message. From the time the autopilot disengaged to the stick shaker was less than 2 seconds. Recovery of the aircraft was immediate with myself (pilot flying) and the FO pushing on the yoke forward for this stall recovery incident. ATC was not notified of any flight deviation since the total incident lasted less than 20 seconds from the start to recovery back at FL350. ATC did not question our altitude change.I contacted the FAs and no passengers were injured. FA #2 was injured to where she sustained a bloody lip and twisted foot. I asked if she was ok or needed immediate medical care. She indicated she was not in need of medical care thus we continued to ZZZ which was less than 1 hour away. Once the aircraft was in cruise back at FL350; as indicated above; I evaluated the aircraft stability; flight characteristics and safety of the passengers and since there were no issues; I did not declare an emergency. After the event the aircraft preformed as usual and autopilot was reengaged and speed brakes were used. After the event I monitored the pitch trim indicator on the EICAS and it reflected between 2.5 and 2.7 on the trim with the autopilot engaged.After the event the maintenance personnel that inspected the aircraft indicated the initial cause was a disagreement with the elevator servo. In looking at the event a week later; it appears over time at cruise; the aircraft reconfiguring to a pitch up attitude due to an elevator/servo disagreement and the autopilot disengaged since it was not able to hold a level altitude with this configuration. Thus when the autopilot disengaged the aircraft was configured in a pitch up attitude and we pitched up to a stick shaker notification. The recovery of this event was an immediate response from myself and the FO. The recovery of the aircraft was what we were taught in recovery of a stall at high altitude. This was my first sequence after recurrent to which a high altitude stall was demonstrated in the simulator. The only difference was in the simulator the airspeed was reduced and in this real life situation the aircraft did not lose airspeed but was placed in an immediate pitch up attitude.
EMB175 Captain reported an autopilot disengagement and abrupt pitch up at FL350. Later; maintenance inspection revealed a disagreement with the elevator servo.