What happened
Shortly after departing, the flight crew of the aircraft received STAB TRIM and MACH TRIM caution messages on the Electronic Information Control Advisory System (EICAS). During this period, the horizontal stabilizer control system failed to reset. In response to these indications, the crew declared an emergency. The aircraft returned to the departure airport and landed without further incident.
The investigation
Following the event, company maintenance personnel removed and replaced both the horizontal stabilizer trim control unit (HSTCU) and the motor control unit (MCU) from the aircraft. Subsequent testing and examination of the HSTCU and MCU were conducted according to manufacturer specifications; however, no anomalies were identified in these components that would have contributed to the caution messages. The specific cause for the failure of the horizontal stabilizer control system was not determined.