A stuck angle of attack (AoA) sensor and the pilot's failure to monitor the flight instruments while climbing were factors in the Dec. 31, 2013 wing overstress accident involving a Cessna Citation CJ2+ after it departed controlled flight, according to a newly released report from the UK's Air Accidents Investigation Branch (AAIB). The light jet was being flown single pilot and departed from controlled flight just as it approached its assigned cruising altitude of FL430 over Coventry in central UK.
The autopilot was engaged at the time of the incident, the pilot having selected the “vertical speed” mode for the climb. According to the AAIB, the aircraft stalled, then completed five 360-degree rolls before the pilot briefly regained control. The pilot again stalled the jet before eventually returning to level flight after losing 16,000 feet of altitude.
The pilot managed to land the jet safely, with no injuries to himself or the passenger. However, the wings were substantially damaged, as the pilot overstressed the aircraft during the recovery. The AAIB reported that the onboard Cessna AReS data recording system captured a maximum load of 3.25 gs during the event. Inspection of the aircraft revealed damage consistent with pullout-maneuver loads of between 3.6 gs and 5.4 gs.
According to the AAIB, several factors contributed to this loss-of-control incident. The Citation’s AoA sensing system stuck, so no standard stall warning was available to the pilot. Another factor was the pilot’s failure to monitor the flight instruments during the climb, causing him to miss indications of the aircraft’s decaying speed and increasing pitch attitude before the stall.
The pilot told the AAIB he became distracted during the climb while attempting to gather upper-level wind data from a portable tablet electronic device he had placed on the unoccupied right seat. While he was focused head-down with the device, he heard a click—possibly autopilot disengagement—before the aircraft pitched severely nose-down and rolled to the right. He also thought the stick shaker might have activated as the aircraft pitched down. Additionally, the pilot said the recovery was hampered by his inability to interpret the primary flight display attitude indicator, which he described as presenting information that he could not recall having seen before.