What happened
A Bombardier CL600-2B19 regional jet experienced aileron control stiffness while in cruise flight at flight level 280. Prior to the event, the aircraft had been pushed back and held on the taxiway for approximately 30 minutes during moderate to heavy rain. Takeoff was also conducted during heavy rain conditions. While cruising, the "AP TRIM IS LWD" caution annunciator illuminated. Upon disconnecting the autopilot, the first officer found the aileron controls to be stiff and binding. The captain took control and confirmed the stiffness. After applying light to moderate roll input, the captain felt a snap, after which the aileron control operated normally. The crew performed left and right turns without further issues, re-engaged the autopilot, and completed an uneventful landing.
On the same date, two other CL600 regional jets from the same airline also experienced similar aileron stiffness after taking off from the same airport under similar weather conditions. Both aircraft landed without further incident, though one diverted to an alternate airport.
The investigation
The investigation identified that approximately 40 Bombardier CRJ CL-600-2B19 regional jets had experienced in-flight aileron control stiffness after taxiing or taking off in rainy conditions and then climbing into freezing temperatures. The stiffness ceased once the aircraft descended back into temperatures above freezing.
Testing determined that binding in the aileron control circuit occurred within the Brake and Position Sensor Unit (BPSU) bay. It was found that horizontally-oriented pulleys in the circuit would seize, as the ailerable cable, having been soaked with water, would freeze to the pulley. Water ingress into the bay likely occurred through an aging self-contouring sealing pad on an access panel located directly above the pulleys. The design of the pulley face allowed water to be retained while the aircraft was parked. The combination of water migration from rain and the drying of pulley grease over time caused the bearings to seize when the aircraft reached altitude. Tests showed that water applied to a dry-bearing pulley would pool on the face until movement forced the water into the bearing.
Findings
- Water ingress via an aging sealing pad allowed moisture into the BPSU bay.
- The horizontal orientation of the pulleys and the dished cross section of the pulley face caused water to pool during rain.
- Water freezing on the pulleys caused the aileron control stiffness.
- Prior to these incidents, there were no manufacturer instructions to inspect these specific pulleys.