Incident Description
A Bombardier CL600-2B19 regional jet encountered aileron control stiffness during cruise flight. The aircraft had been pushed back in moderate to heavy rain and held on the taxiway for about 30 minutes before takeoff in heavy rain. After leveling off at flight level 280, the "AP TRIM IS LWD" caution annunciator illuminated. The first officer disconnected the autopilot and found the aileron controls "stiff and binding." The captain took control and also experienced stiffness. Applying "light to moderate" roll input, the captain felt a snap, after which aileron control returned to normal. The captain executed turns without further problems, re-engaged the autopilot, and landed uneventfully.
Similar Incidents
On the same date, two other CL600-2B19 aircraft from the same airline experienced aileron stiffness after taking off from the same airport in similar weather conditions. Both landed without incident, though one diverted to an alternate airport.
Investigation Findings
The investigation revealed that approximately 40 Bombardier CRJ CL-600-2B19 aircraft had reported in-flight aileron control stiffness after standing or taxiing in rain, followed by climbing to freezing temperatures. The stiffness ceased upon descending to above-freezing temperatures. Testing identified that aileron circuit binding occurred in the area of the Brake and Position Sensor Unit (BPSU) bay. Horizontally oriented pulleys in the aileron control circuit would seize, and the aileron cable, after 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 pulley orientation, used without difficulty for over 20 years in transport category aircraft, combined with the dished cross section of the pulley face, allowed water retention when parked. Drying of pulley grease over time, along with water migration from rain, could temporarily cause bearing seizure at altitude. A test on a dry-bearing pulley showed that water applied to the pulley face pooled until the pulley moved, then flowed into the bearing.
Manufacturer Response
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 pulleys, grease aileron cables, and replace aging and seized pulleys led to over 60 seized and deteriorating pulleys being found fleet-wide. No further aileron control stiffness events occurred on aircraft that underwent the new inspection and maintenance procedures.