What happened
On 6 November 2015, a Saab 2000, registration G-LGNR, was operating a passenger flight from Manchester to Inverness. During the climb, while the aircraft was accelerating through 150 kt, the crew observed the aircraft entering an unusual attitude, characterized by a nose-up pitch and a left roll. Shortly after, a roll mistrim indication appeared on the primary flight display, signaling untrimmed forces within the aileron system.
Upon disconnecting the autopilot, the pilot experienced a significant jolt and found the flight controls to be unresponsive and "slopt." The crew observed via the electronic displays that both ailerons appeared to be deflected upward. In an attempt to rectify the issue, the crew followed a malfunction checklist and pulled the roll handle to separate the left and right aileron control systems. This action resulted in reduced aircraft controllability. The crew declared a MAYDAY and initiated an emergency return to Manchester. During the approach, the crew extended flaps, which further complicated the aircraft's handling, leading to terrain warnings before a safe landing was achieved.
The investigation
The investigation focused on analyzing flight data from the Quick Access Recorder (QAR) and examining the aircraft's control systems. Investigators found that the aircraft was experiencing two distinct faults. First, the rudder trim position data recorded on the QAR showed rapid, erratic changes that were inconsistent with the actual rudder surface position. The data suggested a restriction was preventing the rudder pedals from moving freely, a condition that ceased once the autopilot was manually disconnected.
Second, the investigation identified an issue with the left aileron position transducer. While the electronic displays indicated significant upward aileron deflection, the flight path remained stable, suggesting the cockpit displays were providing erroneous information. The investigation also noted that the crew relied on electronic synoptic pages rather than visually verifying the physical position of the ailerons through the cockpit windows.
Findings
- The aircraft experienced a restriction of the rudder pedals, which likely caused the flight to be out of balance during the initial phase of the flight.
- An erroneous signal from an aileron position transducer provided the crew with misleading information regarding the physical position of the ailerons.
- The crew did not visually confirm the aileron positions, a step that was not explicitly required by the cockpit malfunction checklist.
- The extension of flaps during the emergency approach increased the pilot workload and further degraded aircraft controllability.
Safety action
Following the investigation, the manufacturer committed to updating the 'Aileron system open failure' section of the malfunction checklist. The revised instructions will explicitly instruct pilots to verify an open failure by visually observing that the ailerons do not follow the control wheel input.