What happened
While cruising at flight level 400, the crew of a Learjet 45, registration D-CNMB, encountered an autopilot failure warning. Following established checklists, the pilots transitioned to manual flight. Upon doing so, the aircraft entered a series of difficult-to-control, jerky left-hand rolls. These movements were further intensified by Pilot Induced Oscillations (PIO).
During the struggle to stabilize the aircraft, the crew noticed that the spoiler movements did not correspond to their manual control inputs. To regain control, the pilots pulled one of the two circuit breakers to deactivate the spoiler function. This action successfully stopped the oscillations, allowing the aircraft to become stable and maneuverable for the remainder of the flight.
The investigation
SUST examined the aircraft's flight data and maintenance history to determine why the manual flight caused such instability. The investigation focused on the flight control systems, specifically the lateral control loop and the spoiler deployment mechanism. Investigators also reviewed the aircraft's maintenance records, including recent work on the RVDT sensor block and the implementation of Bombardier Service Bulletin SB 45-27-20.
Findings
- The investigation established that a maintenance error had caused the spoiler functions to be inverted, meaning the left spoiler deployed when the right was commanded, and vice versa.
- The aircraft had been modified per a service bulletin to improve spoiler system reliability, but this modification removed the functionality of the Control Wheel Master Switch (MSW), which previously allowed pilots to force spoilers down during uncommanded motions.
- The existing "Roll or Yaw Axis Uncommanded Motion" checklist had not been updated to reflect this loss of MSW functionality, rendering the checklist ineffective for the specific problem encountered.
- The inversion error had gone undetected by previous crews because the flight control page display groups the left and right spoiler position indexes around a single vertical axis, making them easily confused.
- The flight data recorder (FDR) lacked primary flight control surface deflection parameters, forcing investigators to reconstruct the aircraft's dynamic behavior indirectly through spoiler and roll data.