Dual-Channel Stabilizer Trim Failures on a CL-600 (N97325)

Initial Incident and Crew Response During the first occurrence, the aircraft was climbing through 23,000 feet toward flight level 290 when a stab trim/mach trim caution illuminated, indicating both channels 1 and 2 were inoperative. The pitch trim system could not be moved. The captain kept airspeed below 215 knots and initiated a return to the departure airport. The crew took turns at the controls, with the first officer applying about 30 pounds of backpressure on the yoke to counteract trim forces. The captain landed the aircraft uneventfully. Maintenance initially suspected the captain's stab trim disconnect switch was stuck in the cutout position and replaced it. ## Second Incident and Maintenance Findings Thirty-two days later, the same aircraft experienced a second stab trim failure during descent from flight level 290. Both stab trim channels and mach trim failed, displaying an “AP Pitch Trim” message. After about two minutes, the captain reset both stab trim channels and then the mach trim, and the aircraft landed uneventfully. Maintenance removed and replaced the Horizontal Stabilizer Trim Control Unit (HSTCU). ## Data Review and Physical Examination Flight data recorders were removed and downloaded. For the first occurrence, the stabilizer trim discrete changed from “on” to “off” 17 minutes into the flight. For the second occurrence, the discrete changed from “on” to “off” for about two minutes at 96 minutes into the flight, then returned to “on.” The stab trim disconnect switch and HSTCU were tested and functioned within manufacturer specifications. Physical examination found the HSTCU access panel door cam-type fastener’s shoulder guide missing from one of two door latching fittings, with the shoulder half loose inside the HSTCU circuitry area. Engineers noted the circuit boards had a heavy coat of protective/sealing varnish and that each trim channel was on a separate circuit board, making simultaneous failure from a single piece of debris unlikely. ## Database Search and Aircraft History A search of the FAA Service Difficulty Report database for Stab Control System anomalies in the CL-600 from 1995 to July 10, 2003, identified 67 dual-channel stab trim failures. Twenty-seven percent were attributed to cockpit trim disconnect switch issues, 52 percent to HSTCU/HSTA/MCU issues, and 21 percent were unresolved or could not be duplicated on the ground. The aircraft in question, serial number 7325, was not listed in the database as having reported flight control discrepancies.