Incident Description
After departure, the incident airplane was climbing toward its cruising altitude in a stratified region of precipitation within a convective system, and in conditions favorable for the accumulation of structural icing. At some point, the pitot/static system began accumulating ice because the air data heat system had not been activated or was not functioning. The condition first manifested itself as a "RUDDER LIMIT FAULT" warning due to icing of the rudder limiting system pitot tube. The icing continued to accumulate on the other probes of the air data system, degrading its ability to reliably determine the airplane's airspeed.
Flight Crew Actions
About 19,000 feet, the flight crew disengaged the autopilot and pushed the pitch control column forward, and the airplane entered a descent. The flight crew initially applied uncoordinated control inputs, in the process reaching nearly 100 pounds of differential force on the pitch control column, while attempting to recover the airplane. During this period, the airplane's pitch continued to oscillate through 5 cycles, for a duration of 8 minutes, reaching altitudes as low as 10,600 feet and as high as 23,300 feet. During the oscillations, the airplane's indicated airspeed varied greatly, between 54 and 460 knots; however, airplane systems tests and aircraft performance data show that the recorded, as well as the displayed, airspeed indications were adversely affected by the icing conditions. Once regaining control of the airplane, the crew diverted and made an uneventful landing.
Post-Incident Findings
Post-incident testing of the airplane's mechanical and electronic systems revealed no abnormalities that would have accounted for the unreliable airspeed indications or the loss of control reported by the flight crew. Post-incident computer modeling also confirmed that the airplane performed in a manner consistent with all deviations from normal flight having been initiated or exacerbated by the control inputs of the flight crew. Review of flight data recorder, cockpit voice recorder, and flight crew interviews revealed that the flight crew's actions during the event were in part contradictory with the operator's training and operational procedures. Specifically, the crew initially failed to properly identify and respond to the erroneous airspeed indications that were presented and failed to coordinate their recovery of the airplane to controlled flight.