Introduction
On 31 December 2013, a Cessna 525A Citation CJ2+ (registration N380CR) stalled while climbing to its planned cruising altitude of FL430 approximately 5.7 nm north-west of Coventry, Warwickshire. The aircraft, powered by two Williams FJ44-3A-24 turbofan engines, sustained structural damage to both wings and its HF antenna, but the pilot and sole passenger were uninjured.
History of the Flight
The aircraft was kept in a heated hangar at Leeds Bradford Airport. The owner planned to fly with one passenger to Palma de Majorca, Spain. Before departure, the pilot conducted a pre-flight inspection and noted no defects. The flight departed and climbed continuously towards FL430 with the autopilot engaged in vertical speed mode. As the aircraft passed FL410, the indicated airspeed decreased below 150 kt. The pilot noticed the lower-than-expected airspeed and reduced the rate of climb to 500 ft/min. However, the speed continued to drop.
Between FL420 and FL430, the pilot looked down at a tablet-style electronic device to check a forecast winds chart. During this brief distraction, the aircraft pitched severely nose-down and rolled to the right. The pilot thought the stick shaker may have activated once. The aircraft departed from controlled flight, performing five 360° rolls to the right. The pilot regained control briefly but the aircraft stalled again, and during recovery, the wings were overloaded. The pilot eventually regained full control, re-established communications with ATC, and landed at Leeds Bradford Airport without further incident.
Aircraft Examination
Post-flight inspection revealed damage to both wings consistent with overload in positive symmetrical bending. Five ribs in the outboard wingbox of each wing were buckled, and the bonded joints between ribs and wing skins had failed. The upper and lower wing skins were permanently deformed. Despite structural disruption, no fuel leaked. The damage was consistent with symmetrical pullout loads between +3.6g and +5.4g. Additionally, the HF antenna had broken and flailed, denting the fin leading edge. Weight and balance calculations showed the aircraft was within limits.
Recorded Information
The aircraft was not fitted with a flight data recorder but had an Aircraft Recording System (AReS) that recorded ARINC 429 data. The data confirmed the stall sequence and showed that the angle of attack (AOA) sensing system had 'stuck' in flight, and the stall warning system did not operate prior to the stall. The AOA signal had frozen, and an invalidity marker was present. The AOA system is designed to freeze at the last known value when the signal is invalid.
Weather
The UK Met Office provided an aftercast. There was occasional stratus at 600 ft with tops at 1,000 ft, broken cumulus and stratocumulus between 2,000 ft and 8,000 ft, and alto-cumulus above. The freezing level was at about 4,300 ft. Above that, no cloud up to about FL290, then cirrus likely above. Absolute cloud tops were around FL350. No significant wind shear was reported. There was a slight temperature rise from -57°C to -55°C between FL400 and FL430.
Conclusion
The pilot successfully landed the aircraft despite the structural damage. The investigation determined that the stall occurred because the pilot was not monitoring indicated airspeed, and the AOA sensing system had stuck, preventing the stall warning from activating.