FTW02IA097

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

History of flight

On March 14, 2002, approximately 0921 central standard time, a Canadair CL-600-2B19 airplane, N960CA, operating as Comair Flight 3860, experienced a failure of the horizontal stabilizer control system after departing Tulsa International Airport (TUL), Tulsa, Oklahoma. The airplane was registered to First Union National Bank of NC Trustee, Charlotte, North Carolina, and operated by Comair Inc., of Cincinnati, Ohio. The airline transport rated captain, commercial rated first officer, one flight attendant, and 42 passengers were not injured. Visual meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan was filed for the 14 Code of Federal Regulations Part 121 scheduled passenger flight. The flight originated from TUL at 0845 and was destined for Cincinnati/Northern Kentucky International Airport (CVG), Covington, Kentucky.

According to information provided by Comair, the flight originally departed TUL for CVG at 0610; however, returned to TUL at 0710 due to a reported flight control problem. After landing without incident, company maintenance personnel inspected the airplane. A review of the aircraft logbook revealed that the following discrepancy was recorded by the crew: "After takeoff mach trim/stab trim caution msgs illuminated. Would not reset." The following action was recorded by company maintenance personnel in the aircraft logbook: "Reset stab channels, performed ops [check] of stab trim & mach trim, no defects noted at this time, ops good per F/M fault reset pg. 104." The airplane was returned to service.

The flight then departed TUL for CVG at 0845, and subsequently, returned to TUL at 0924. In a written statement, the crew reported, "On takeoff stab/mach trim caution messages illuminated right after becoming airborne. QRH [Quick Reference Handbook] procedures followed; system would not reset. Emergency declared. Uneventful landing."

For company convenience and troubleshooting, company maintenance personnel removed and replaced the horizontal stabilizer trim control unit (HSTCU), and motor control unit (MCU) from the airplane, and the airplane was returned to service. No additional discrepancies or anomalies were noted with the airplane on the subsequent flight from TUL to CVG. The HSTCU (part number 7060-7, serial number 135), and MCU, (part number 7062-3, serial number 654), were retained by the NTSB for further examination.

Personnel information

The captain held an airline transport pilot certificate with a rating for multi-engine land airplanes, and a commercial pilot certificate with a rating for single-engine land airplanes. The captain held type ratings for the CL-65 and EMB-120. On February 28, 2002, the captain was issued a first class medical certificate with the restriction "must have available glasses for near vision."

The first officer (FO) held a commercial pilot certificate with multi-engine land, single-engine land, and instrument airplane ratings. The FO also held an airplane single-engine, multi-engine, and instrument flight instructor ratings. On January 3, 2002, the FO was issued a first class medical certificate with no limitations or restrictions.

Aircraft information

According to the Canadair Regional Jet Airframe/Electrical Line Maintenance Training Manual, the Canadair CL-600-2B19 horizontal stabilizer control system supplies pitch trim to the airplane. The system monitors the operation of the stabilizer pitch actuating system through the HSTCU. The horizontal trim actuating system receives the commands from the HSTCU allowing the operation of the horizontal stabilizer trim actuator (HSTA) through the MCU.

The HSTCU is supplied by two independent aircraft electrical power sources. The HSTCU contains two channels, channel 1 and channel 2, and a built-in test check. Channel 1 and Channel 2 each contain a priority code. The code is used to find the priority between various input commands. The priority structure, in order of highest to lowest, is as follows: pilot's manual trim, copilot's manual trim, auto pitch trim, auto-trim (from pitch feel control unit), and Mach trim command. The HSTCU is mounted below the cabin floor on the left side of the avionics compartment.

The MCU is an electronic unit that controls the speed and the direction of the motors in the HSTA. The MCU supplies protection to the HSTA motors from electromagnetic interference, over current and overheat conditions. If overheating occurs in the MCU, the MCU removes electrical power from that HSTA motor and sends a channel failure signal to the HSTCU. The HSTCU disengages that HSTCU channel, and engages the backup HSTCU channel. The MCU is attached to the vertical stabilizer structure.

The pilot and copilot manual trim switch is located on the outboard top end of the flight control wheel, and the manual autopilot disconnect is located near the trim switch. The trim channel engage switch, Mach trim engage switch, and pitch trim control panel are located in the flight compartment on the center pedestal. The pitch trim control panel contains two trim channel engage switches, identified as CH 1 and CH 2, and the Mach trim engage switch, identified as INOP. The horizontal stabilizer control system is operated when either trim channel engage switch is pushed in. The Mach trim function of the HSTCU operates through either control channel of the HSTCU. When the Mach trim engage switch is pushed in, the Mach trim function control inputs operate the HSTA.

As previously mentioned, the HSTCU can obtain control inputs from the pilot and copilot manual trim switches, the autopilot, the pitch feel control unit, and the Mach trim. The HSTCU sends these inputs through the active channel to the MCU, and the MCU then controls the HSTA. The HSTCU gets Mach trim data from the air data computer. The Mach trim function of the HSTCU monitors the validity of this data and sends the control inputs to the MCU to operate the HSTA, if the data is invalid, the Mach trim is disengaged. The HSTA contains feedback sensors which supply position and rate of movement information for the horizontal stabilizer to the motor control unit. The MCU then supplies position and speed indications to the HSTCU.

The "STAB TRIM" caution message illuminates on the Electronic Information Control Advisory System (EICAS) when either channel of the HSTCU have failed. The "MACH TRIM" caution message illuminates on the EICAS when the Mach trim function has failed. At the time of the incident, the airplane, serial number 7117, had accumulated 14,113.6 total hours.

Test and research

On May 14, 2002, the HSTCU and MCU were tested and examined at the facilities of SFIM, Inc., Grand Prairie, Texas, under the supervision of NTSB representatives, an FAA representative, a Bombardier representative, and SFIM technicians. Aviac/Sagem of France manufactured the above referenced components.

SFIM technicians connected the HSTCU to a test bench to verify the fault memory of the unit . The bench test revealed that at some point, not necessarily during the flight, the unit identified a failure of the Mach Trim Speed. The failure was recorded in Channel 1. The HSTCU was then tested on the Automated Test Bench, and a failure of the unit's door switch was noted. According to Bombardier, the failure of the door switch was not related to the anomaly experienced by the crew. The door switch was adjusted, and no additional failures were noted. The unit was opened and a visual inspection was performed. No anomalies were noted with the unit during the visual inspection.

The MCU was connected to a test bench, and tested at various modes of operation. According to SFIM technicians, during operation on the airplane, the MCU operates at 28 voltage direct current (VDC). During the bench test, various supply voltage loads were applied to the MCU with no anomalies n