Incident Overview
On a medical evacuation flight from Vancouver to Terrace, British Columbia, a Learjet 35A (registration C-GDJH) encountered aileron control difficulties shortly after takeoff. While climbing through flight level 290 with the autopilot engaged, the aircraft initiated an uncommanded right turn with 5 degrees of bank. The autopilot was disengaged, but the ailerons were immovable. Despite various control inputs, the ailerons remained stuck, and the right bank increased to approximately 20 degrees. Fuel contents were balanced. Air Traffic Control was advised of the problem, and the control malfunction checklist was consulted.
After four or five attempts, the ailerons moved slightly, reducing the bank to about 15 degrees. Continued force gradually restored full aileron control, by which time the aircraft had turned roughly 215 degrees to the right. The flight returned to Vancouver, where the descent and landing were uneventful.
Pre-Flight Conditions and Post-Flight Examination
Before the flight, the aircraft had been on standby, fueled and parked outdoors for several hours during heavy rain. The surface temperature was 8°C, and the freezing level was about 5,000 feet. The autopilot was engaged after passing through flight level 180.
Due to the nature of the malfunction and environmental conditions, freezing of the controls was suspected. After landing, Canada Jet Charters maintenance personnel examined the aileron brush seals and found excessive water and traces of ice. The seals were cleaned, dried, and re-lubricated. A re-examination two days later showed the brush seals were visibly worn and matted, with some drainage channels distorted.
Background and Maintenance Issues
Service difficulty reports and Bombardier/Learjet information indicated that aileron control problems on Learjet aircraft occur about once a year, typically after exposure to water followed by a climb into freezing conditions. Frozen aileron brush seals were suspected as causal.
Brush seals are fitted to prevent aileron buzz at high Mach numbers. When worn and matted, the drainage channels (spaced about every 1.5 inches) lose definition and cannot freely pass water. On the occurrence aircraft, these seals were worn. Maintenance procedures called for cleaning with a dry cloth and lubricating with silicone grease every 300 hours, with a practice of generous application. The Learjet 35A maintenance manual cautions against excess greasing as it may block drainage channels but provides no quantitative definition of excess. It also does not establish criteria for replacing worn or damaged seals.
Experienced Learjet pilots are aware that hand-flying during takeoff and climb into freezing temperatures after rain can reduce the risk of frozen ailerons by keeping them in motion. The Pilot's Operating Manual mentions the possibility of frozen ailerons, but the control malfunction checklist does not specifically address the condition.
Investigation Conclusions
The Transportation Safety Board concluded that water collected in the aileron brush seals while on the ground and subsequently froze when the aircraft climbed above the freezing level, effectively freezing the ailerons. Wear and matting of the seals, and possibly excessive grease, affected the seal channels so they could not freely pass water, which then froze. Maintenance guidance was insufficient, leading to inattention to wear and a lack of understanding of over-greasing effects.
Probable Cause
Water became trapped in the worn and matted aileron brush seals and froze during flight, resulting in a loss of aileron control.
Contributing Factors
- An excessive amount of grease may have been used, increasing the risk of water retention.
- The maintenance manual did not address issues with worn or damaged seals or define over-greased vs. properly greased seals.
- The pilot's operating manual and control malfunction checklist did not provide specific recovery actions or address frozen aileron brush seals.
Other Findings
The possibility of damaging the aileron control system while applying abnormal forces to break free frozen brush seals is not addressed in operating or maintenance manuals.
Safety Actions Taken
- Canada Jet Charters reduced the lubrication interval from 300 to 100 hours.
- Bombardier revised maintenance manual procedures for all Learjet models to include inspection criteria for worn or damaged seals and more complete lubrication instructions.
- An article on proper brush seal maintenance appeared in Bombardier's customer newspaper, Infoservice, to be published semi-annually.
- On 7 May 2001, the TSB issued an Aviation Safety Information Letter to Learjet Inc. regarding grease quantity, wear and drainage, potential damage from forcing controls, and possible amendments to operating manuals.
This report concludes the Transportation Safety Board's investigation, authorized for release on 3 October 2001.