Loss of control Serious incident Learjet 45 D-CNMB, Wednesday 18 December 2019
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Date:Wednesday 18 December 2019
Time:13:45 LT
Type:Silhouette image of generic LJ45 model; specific model in this crash may look slightly different    
Learjet 45
Owner/operator:MHS Aviation
Registration: D-CNMB
MSN: 45-024
Fatalities:Fatalities: 0 / Occupants: 2
Other fatalities:0
Aircraft damage: None
Category:Serious incident
Location:15 km SE of Briançon -   France
Phase: En route
Nature:Unknown
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
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.

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.

Causes
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.

Contributing Factors:
- 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.

Accident investigation:
cover
  
Investigating agency: STSB
Report number: 
Status: Investigation completed
Duration: 3 years and 9 months
Download report: Final report

Sources:

https://www.bea.aero/en/investigation-reports/notified-events/detail/event/incident-grave-du-learjet-45-immatricule-d-cnmb-survenu-le-18122019-en-croisiere-enquete-deleguee/
https://www.sust.admin.ch/inhalte/AV-berichte/D-CNMB.pdf

Revision history:

Date/timeContributorUpdates
06-Jan-2020 08:39 harro Added
22-Sep-2023 08:10 harro Updated

Corrections or additions? ... Edit this accident description

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