What happened
A Learjet 35A, registration C-GDJH, was performing a medical evacuation flight from Vancouver to Terrace, British Columbia, when the crew encountered unexpected control difficulties. While climbing through flight level 290, the aircraft began a right-hand turn with a 5-degree bank despite the autopilot being engaged. Upon disengaging the autopilot, the crew discovered the ailerons were immobile.
As the right bank increased to approximately 20 degrees, the crew applied various control inputs without success. After notifying Air Traffic Control and consulting the control malfunction checklist, the crew applied significant force to the controls. Following several attempts, the ailerons regained movement, and the bank angle reduced to 15 degrees. By the time full control was restored, the aircraft had completed a 215-degree turn. The aircraft subsequently returned to Vancouver and landed safely without further incident.
Prior to the flight, the aircraft had been parked outdoors in heavy rain for several hours. The surface temperature was 8 degrees Celsius, and the freezing level was approximately 5000 feet.
The investigation
Investigators examined the aileron brush seals, which are designed to prevent aileron buzz at high Mach numbers. Maintenance personnel found excessive water and traces of ice within the seals. A follow-up inspection two days later revealed that the seals were visibly worn and matted, and several drainage channels were distorted.
Review of service difficulty reports indicated that aileron control issues occur roughly once a year across all Learjet models, typically following periods where aircraft are soaked by rain and then climb into freezing air. The investigation also looked into maintenance practices, noting that while the manual requires cleaning and lubricating the seals every 300 hours, it lacks specific criteria for replacing worn seals or defining the amount of grease that constitutes an "excessive" application.
Findings
- Water became trapped in worn and matted aileron brush seals and froze during flight, resulting in a loss of aileron control.
- The accumulation of water was facilitated by the degraded state of the seals and potentially by an excessive amount of grease, which may have blocked the drainage channels.
- The maintenance manual provided insufficient guidance regarding the inspection of worn or damaged seals and lacked an objective definition for proper lubrication levels.
- While the Pilot's Operating Manual mentions the risk of frozen seals, the control malfunction checklist does not provide specific recovery procedures for this condition.
- There is no guidance in the operating or maintenance manuals regarding the potential for structural damage to the control system caused by the application of extreme force to break free frozen controls.
Safety action
Following the incident, Canada Jet Charters reduced the lubrication interval for the seals from 300 to 100 hours. Additionally, the TSB issued an Aviation Safety Information Letter to Learjet Inc. to address grease application, seal wear, potential for control system damage, and necessary amendments to operating manuals.