What happened
On 13 April 2019, a Cessna Citation CJ1+, registration N680KH, departed Bournemouth Airport on a private flight to Rotterdam. Shortly after takeoff, while climbing through 3,000 ft, the pilot noticed a light vibration and an alert indicating that the 'ATLAS' system was inoperative. Moments later, the aircraft experienced a sudden, uncommanded left roll at a rate of approximately 4 degrees per second. The roll reached a maximum angle of 75 degrees, causing the autopilot to disconnect and the aircraft to descend rapidly at a rate of 4,500 ft/min.
Despite the pilot applying full right aileron and rudder, the aircraft remained difficult to control. The pilot eventually managed to reduce the airspeed and level the wings to a 30-degree bank, but the aircraft continued a descending left turn. The aircraft eventually returned to the airfield, performing a very tight approach that observers described as appearing to be on its side, before landing safely. There were no fatalities and no injuries among the four people on board.
The investigation
The AAIB investigation focused on the 'Active Technology Load Alleviation System' (ATLAS) installed on the aircraft. This modification uses supplementary control surfaces to manage wing loading. Investigators examined flight data, including the cockpit voice recorder and maintenance diagnostic systems. The investigation revealed that a short circuit in the left ATLAS Control Unit caused one of the control surfaces to fail in a fully deflected upward position.
Furthermore, the investigation found that the pilot, who had recently purchased the aircraft, was unaware of the specific supplementary procedures required for this modification because the relevant flight manual supplement was missing from the aircraft's documentation. The investigation also noted that previous certification flight tests had not fully captured the severity of the aircraft upset caused by such a failure.