Incident
Shortly after departure, the flight crew received STAB TRIM and MACH TRIM caution messages on the Electronic Information Control Advisory System (EICAS). The horizontal stabilizer control system would not reset. The crew declared an emergency, and the flight returned to the departure airport and landed without further incident.
Maintenance Actions
Company maintenance personnel removed and replaced the horizontal stabilizer trim control unit (HSTCU) and motor control unit (MCU) from the airplane. The airplane was subsequently returned to service. The HSTCU and MCU were tested and examined per manufacturer specifications. No anomalies were noted that would have contributed to the caution messages.
Investigation
The failure of the horizontal stabilizer control system was not determined.