What happened
On 27 November 2021, a BAe ATP, registration SE-MAP, was operating a cargo flight from London Stansted to Belfast International Airport. The flight occurred during the period of Storm Arwena, which brought turbulent weather to the region. During the initial climb from Stansted, the aircraft encountered minor icing, though conditions did not necessitate the activation of anti-icing boots.
While cruising at 20,000 feet, the pilot observed the aircraft hunting for altitude and noted that the trim wheel was twitching continuously. Given the turbulent atmosphere, this was not initially viewed as an anomaly. However, during the descent toward Belfast, the autopilot began struggling to maintain the commanded vertical speed. Upon transitioning to manual flight for the final approach, the crew encountered significant resistance in the elevator controls. The elevator was stiff and required two hands to operate, moving in a jerky manner. Despite this mechanical difficulty, the crew managed to land the aircraft safely at Belfast International.
The investigation
Following the incident, investigators reviewed data from the Flight Data Recoded, which confirmed that the autopilot had disconnected following an audio warning. Maintenance inspections by the operator's engineers revealed that the primary elevator servo and lever assembly were noisy and unable to move through their full range. The elevator trim servo motor also exhibited restricted movement. Both components were replaced and sent to a testing facility in Sweden, though extensive testing there failed to replicate the fault.
Subsequent observations noted that the elevator control feel had gradually returned to normal during taxiing. During a later flight, the crew identified a separate defect regarding the electric trim switch, which was slow to return to its center position. This was attributed to the ingression of cleaning agents into the yoke switches.
Findings
- The primary cause of the elevator stiffness could not be definitively established, as component testing failed to find faults in the replaced servos.
- Ingression of cleaning agents into the yoke switches was identified as a likely cause for the observed trim switch sluggishness.
- Access constraints were identified as a barrier to effectively inspecting the wiring within the steering columns.