Created Thursday, Jan 12th 2023 20:15Z, last updated Thursday, Jan 12th 2023 20:15Z
A West Atlantic British Aerospace BAe ATP, registration SE-MAP performing flight NPT-31E from London Stansted,EN to Belfast International,NI (UK) with 2 crew, was climbing out of London when the aircraft encountered some icing however insufficient to use the de-ice boots. The aircraft climbed to FL200, the aircraft began to show some "hunting" with the trim wheel persistently twitching while trying to maintain altitude. Being in turbulent conditions the crew did not consider this abnormal. In the descent towards Belfast the autopilot struggled to maintain a selected vertical speed prompting the crew to disengage the autopilot subsequently detecting that the elevator was stiff to move and jerky, the aircraft however remained controllable. After safe landing the crew observed that the elevator control became more free over time and a few minutes the feel had returned to normal.
The British AAIB released their final report reporting the cause of this serious incident could not be determined.
However a safety action was taken: "During these investigations the operator identified that inspection of the wiring in the steering columns was challenging due to access constraints. Consequently, a new procedure has been developed to enable a more extensive inspection. The operator is pursuing the completion of the steering column inspections across its ATP fleet as a priority."
The AAIB summarized the operator's investigation:
The crew confirmed that the autopilot had disconnected as they had received the audio warning and the autopilot engaged indications on the control panel and PFD had cleared. This was also confirmed by the data downloaded from the Flight Data Recorders. During the subsequent fault finding the engineers found the elevator primary servo and lever assembly sounded noisy and was not able to drive the elevator through the full range of motion. The elevator trim servo motor also did not drive through its full range. The primary elevator servo and lever assembly and elevator trim servo were all replaced and sent to the operator’s component testing bay in Sweden for examination and testing. However, despite extensive testing the bay could not find any fault with the components.
The aircraft flew a further six cycles without incident. When on stand the flight crew discovered an elevator trim defect whilst depowering the aircraft. During testing, the electric trim switch was very slow to spring back to centre but when cleaned the spring action was restored. It was not possible to determine if this fault was related to the previous elevator control issue, but the cause was likely due to ingression of cleaning agents into the yoke switches. The operator has now ceased the disinfection of cockpit for ATP fleet due to the potential risk of inducing electrical problems and introduced a procedure for removing and residual cleaning agents from the yoke switches.
