Flight History
The flight was in radio contact with air traffic controllers at the FAA Memphis Air Route Traffic Control Center (ARTCC) while level at 9,000 feet. The pilot checked in on frequency, and the last radio contact occurred at 0447. Radar contact was lost at 0520. The airplane impacted rough terrain at a steep angle. Two other company pilots flying in trail reported radio contact with the pilot of N840FE about 5 minutes before the accident, stating he sounded fine and reported no problems.
Radar Data and Airspeed Changes
Radar data showed a gradual decrease in ground speed from 158 knots at 0512 to 125 knots at 0519:40. The heading and altitude remained constant during this period. After the 0519:40 radar hit, the next hit was coast, and the flight disappeared from radar. According to an NTSB Radar Data Study, calculated flight parameters indicated a slow reduction of airspeed over the final 8 minutes at altitude, followed by an abrupt sharp nose-down pitch attitude with a rapid increase in airspeed. At the time radar contact was lost, calculated airspeed was below 102 knots and angle of attack had increased to 8.8 degrees, with a subsequent large reduction in pitch angle, angle of attack, and flight path angle.
Weather and Icing Conditions
The NTSB Meteorological Factual Report indicated that at 0515, about 7 minutes before radar loss, the radiative temperature in an area centered on Clarksville showed a mean of -6.26°C (21°F) with a minimum of -6.66°C (19°F). Archive Level II Doppler weather radar data from a sweep beginning at 0508:10 showed that N840FE tracked into a weather echo from 0510:34 to 0516:28. The aircraft entered a weak weather echo about the same time the airspeed began decreasing at 9,000 feet and remained in it for several minutes. These data indicated that in-flight airplane icing conditions were encountered.
Examination of Aircraft Systems
Examination of the engine Power Analyzer and Recorder (PAR) revealed no exceedences or caution timing events in progress before power removal at impact. The PAR computer appeared operational until impact. Examination of the autopilot was inconclusive due to impact damage, and engagement status could not be determined. The aircraft was equipped with leading-edge deicing boots on the wings, elevators, struts, and a cargo pod deicing capability. Lights were installed to illuminate the wing leading edges for ice detection during night operations, but the aircraft lacked an ice detection device. A Cessna Airworthiness Directive (AD 96-09-15) applicable to this airplane, which the company complied with on December 12, 1996, outlined procedures and limitations for severe icing conditions, requiring incorporation into the Airplane Flight Manual.
Probable Cause
The pilot did not maintain control of the airplane due to undetected airframe ice, resulting in an inadvertent stall and subsequent impact with the ground. Contributing factors were flight into clouds, below-freezing temperatures, and the inability to detect ice because the aircraft lacked a system to show ice buildup on portions of the airframe not visible from the cockpit.