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. 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 noted that the flight controls felt unresponsive and "sloppy." Electronic displays indicated that both ailerons were deflected upwards. Seeking to resolve the issue, the crew implemented an aileron system open failure checklist, which involved pulling the roll handle to separate the left and right aileron control systems. This action resulted in reduced controllability. The crew declared a MAYDAY and returned to Manchester. During the approach, the crew extended flaps, which further complicated the aircraft's handling. Despite experiencing TAWS warnings regarding glideslope and terrain proximity near the end of the approach, the aircraft landed safely.
The investigation
The AAIB examined flight data from the Quick Access Recorder (QAR). The investigation identified that the aircraft was flying out of balance during the initial phase of the flight. Data showed that the rudder trim position changed rapidly and erratically, and the rudder pedals appeared to be subject to a restriction that prevented movement. This restriction ceased only after the autopilot was manually disconnected.
Furthermore, the investigation found that the aileron position indications on the secondary display were unreliable. While the displays suggested significant upward deflection of the ailerons, the aircraft's flight path remained stable, suggesting that at least one aileron position transducer was providing erroneous data. The investigation also noted that the crew did not visually verify the actual position of the ailerons from the cockpit, relying instead on the electronic synoptic page.
Findings
- The aircraft experienced two distinct faults: an erratic rudder trim position recording and a faulty output from the left aileron position transducer.
- A restriction of the rudder pedals likely caused the aircraft to fly out of balance, a condition that resolved once the autopilot was disconnected.
- The crew relied on electronic flight instrument indications rather than a visual external check of the aileron positions.
- The cockpit malfunction checklist did not instruct the crew to visually verify aileron movement during an open failure procedure.
- The crew's workload was increased by the selection of flaps, which negatively impacted controllability, and by late-stage TAWS warnings.