What happened
On 10 December 2021, a Beech 1900D, registration F-GLNH, was operating a commercial flight from Toulouse-Blagnac to Metz-Nancy-Lorraine. While climbing through FL 200, the crew observed the "A/P TRIM FAIL" warning light illuminate. Upon disconnecting the autopilot, the crew discovered that the elevator control had become extremely stiff and the elevator trim was difficult to move, causing the aircraft to adopt a steep nose-up attitude.
To maintain level flight, the crew had to apply abnormal amounts of force to both the elevator control and the manual trim wheel. The pilots managed to stabilize the aircraft by adjusting engine power and pitch, eventually deciding to descend through several flight levels to warmer air. During the approach to Metz-Nancy-Lorraine, the crew continued to fly the aircraft manually. As the aircraft descended and the temperature rose, the ice began to melt; during the landing flare, the crew noted that the elevator control had returned to normal operation with no remaining friction.
The investigation
The BEA investigation focused on the potential for ice to accumulate within the aircraft's control systems. Investigators identified several areas where moisture could collect and freeze, specifically where elevator control cables pass through pulleys in the vertical stabilizer and through pressure seals in the bulkhead.
Maintenance records and inspections revealed that water had been observed in the wing box structures and under the cabin floor. The investigation also examined the operator's training and safety management systems, noting that while a specific "STUCK ELEVATOR TRIM" procedure existed in the aircraft's supplemental flight manual, it was not included in the crew's primary emergency checklists or recurrent training programs.
Findings
- The primary cause of the control stiffness was ice formation on the elevator control cables and/or trim actuators, resulting from moisture accumulation in unheated areas of the fuselage that froze during the climb through cold air masses.
- Heavy rain prior to takeoff likely allowed water to seep into the fuselage and accumulate in cable pathways.
- The crew relied on basic airmanship to manage the flight because the specific procedure for stuck elevator trim was not readily available in their paper emergency checklists.
- The operator's safety management system had not effectively shared information regarding a similar occurrence involving the same aircraft type that had happened in 2020.