Comair Bombardier CL600-2B19 incident

The Bombardier CL600-2B19 regional jet experienced aileron control stiffness during cruise flight. The airplane was pushed back during a period of moderate to heavy rain. After pushback, the airplane was held on the taxiway for about 30 minutes while the rain continued. Takeoff also occurred during a period of heavy rain. After leveling off at flight level 280, while in cruise flight, the "AP TRIM IS LWD" caution annunciator illuminated. The first officer disconnected the autopilot, and found the aileron controls to be "stiff and binding." The captain then took control of the airplane, and also found the aileron controls to be stiff and binding. The captain applied "light to moderate" roll input and felt a snap. Aileron control then operated normally. The captain executed left and right turns with no additional problems or binding noted. The crew re-engaged the autopilot, and the flight continued to an uneventful landing. On the same date, two other CL600 regional jets from the same airline also experienced aileron stiffness after taking off at the same airport and in similar weather conditions. Both airplanes landed uneventfully. One of the airplanes diverted to an alternate airport. The investigation revealed that approximately 40 Bombardier CRJ CL-600-2B19 regional jets experienced in-flight aileron control stiffness after standing/taxiing in, or taking off during rainy conditions, then climbing to freezing temperatures. When the jets descended to above-freezing temperatures, the aileron control stiffness ceased. Testing revealed that aileron circuit binding occurred in the area of the Brake and Position Sensor Unit (BPSU) bay. Typically, within the bay, horizontally-oriented pulleys in the aileron control circuit would seize, and the aileron cable, after being subjected to water soaking, would freeze to the pulley. Water ingress to the bay most likely occurred via an aging self-contouring sealing pad on an access panel directly above the pulleys. The horizontal orientation of the pulleys, which had been used without difficulty in transport category aircraft for over 20 years, and the dished cross section of the pulley face allowed the retention of water when the airplane was parked. In addition, the drying of pulley grease over time, combined with water migration from rain, could temporarily allow the bearing to seize when the airplane reached altitude. A test on a dry-bearing pulley revealed that when water was applied to the pulley face, it remained pooled until the pulley was moved, when it flowed into the bearing. Prior to the incidents, the manufacturer had not issued specific instructions to inspect pulleys in the BPSU bay. Subsequent service letters and airworthiness directives to inspect the pulleys, grease the aileron cables and replace aging and seized pulleys, resulted in over 60 seized and deteriorating pulleys being found fleet wide. There have been no further aileron control stiffness events on aircraft which had undergone the new inspection/maintenance procedures.

Verbatim NTSB analysis and probable cause from the NTSB dataset