ASN Wikibase Occurrence # 231967
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Narrative:While the Learjet 45 was cruising at FL400, the warning "AP AIL MISTRIM1" alerted the pilots that an incorrect aileron trim setting was affecting the autopilot's operation. Following the checklist procedures, the flight crew switched to manual control, and the aircraft began rolling abruptly to the left, which the captain had difficulty controlling. The copilot declared a distress situation (Mayday) to air traffic control. He noticed that the spoileron deflections did not correspond to the manually given roll commands and deactivated the spoiler function by pulling one of its two circuit breakers. The roll oscillations stopped, and the aircraft became stable and maneuverable. The pilots diverted to Geneva Airport (LSGG), and the landing occurred without incident with the flaps retracted.
|Date:||Wednesday 18 December 2019|
|Fatalities:||Fatalities: 0 / Occupants: 2|
|Aircraft damage:|| None|
|Location:||15 km SE of Briançon -
|Phase:|| En route|
|Departure airport:||Gerona-Costa Brava Airport (GRO/LEGE)|
|Destination airport:||München-Franz Josef Strauss Airport (MUC/EDDM)|
|Investigating agency: ||STSB|
|Confidence Rating:|| Accident investigation report completed and information captured|
The investigation into the serious incident determined that the electrical rotary variable differential transformer (RVDT) sensors, located in the left pilot's control column, had been incorrectly installed 1 year and 8 months earlier, causing a reversal of the spoileron functions: the left-wing spoileron deployed instead of the right-wing spoileron and vice versa. During this period, the aileron servo motor was subjected to resisting torques that induced play in its operation, degrading linearity and consequently the stability of the roll control system. As a result, self-sustaining lateral oscillations of limit cycles were generated, triggering the "AP AIL MISTRIM" warning.
The serious incident resulted from a loss of aircraft control induced by pilot-induced oscillations (PIO) following disengagement of the autopilot affected by an incorrect aileron trim setting.
- Due to an undetected maintenance error made 1 year and 8 months before the serious incident, the spoileron functions were reversed, delaying the response of the manual roll controls.
- On the flight control synoptic page, the position indexes for the left and right spoilerons are grouped around a single vertical axis and can be easily confused when checking the control surface deflections before takeoff.
- Following the application of Bombardier Service Bulletin SB 45-27-20 ATA-27-60, the function of button MSW, which allowed the spoilerons to be deflected downward, was removed without informing the flight crew.
| || |
|Investigating agency: ||STSB |
|Report number: || |
|Status: ||Investigation completed|
|Duration: ||3 years and 9 months|
|Download report: || Final report|
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