Bombardier CL-600-2B19 N97325

The airplane was climbing through 23,000 feet for flight level 290 (29,000 feet) when the stab trim/mach trim caution illuminated on the instrument panel indicating both channels 1 and 2 were inoperative, and subsequently the pitch trim system could not be moved. The captain maintained the airplane's airspeed below 215 knots, and initiated a return to the departure airport. The flight crew took turns at the controls. To maintain the airplane in the proper attitude, the first officer had to maintain about 30 pounds of backpressure on the yolk to counteract the trim forces. The captain landed the airplane uneventfully. Maintenance crews believed the failure was in the captain's stab trim disconnect switch, which they suspected was stuck in the cutout position. The switch was replaced with another unit. A second stab trim failure event occurred 32 days later in the same airplane. The captain reported that during a descent from flight level 290, both channels of stab trim and mach trim failed with a corresponding "AP Pitch Trim" message. After about 2 minutes, the captain was able to reset both stab trim channels and then the mach trim. The airplane landed uneventfully. The maintenance crew removed and replaced the Horizontal Stabilizer Trim Control Unit (HSTCU) to correct the discrepancy. The airplane's flight data recorders (FDR's) were removed, and the data downloaded. The data revealed that 17 minutes into the flight on the first occurrence the stabilizer trim discrete changed from 'on' to 'off.' The data for the second occurrence revealed that 96 minutes into the flight the stabilizer trim discrete changed from 'on' to 'off' for about 2 minutes, and then back to 'on.' The stab trim disconnect switch and the HSTCU were tested, and both functioned within the manufacturer's specifications. Under physical examination it was noted that the HSTCU's access panel door cam-type fastener's shoulder guide was missing from one of the two door latching fittings. The shoulder half was located loose inside the HSTCU circuitry area of the unit. Engineers described the electrical circuit boards as having a heavy coat of protective/sealing varnish that would protect the circuits from shorts caused by debris. Additionally, each trim channel is on a separate circuit board in the HSTCU. The simultaneous failure of both trim channels due to a single piece of debris is unlikely. A search of the Federal Aviation Administration Service Difficulty Report (SDR) 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 of the dual trim failures were identified as cockpit trim disconnect switch issues; 52 percent were identified as HSTCU/HSTA/MCU issues; and 21percent were not resolved or could not be duplicated on the ground. The airplane in question, serial number 7325, was not listed in the results as having any reported flight control discrepancies in the database.

Quoted verbatim from the NTSB record.