Background
Prior to an instrument flight, the pilot received a preflight briefing from the Gainesville Automated Flight Service Station. The briefer advised of potential occasional moderate turbulence between 24,000 and 37,000 feet and noted a current Convective SIGMET for embedded thunderstorms over southern Mississippi.
Flight Events
The flight was in cruise at 24,000 feet when it encountered moderate to severe turbulence and heavy rain. The airplane subsequently entered a descending right turn, losing altitude from 24,000 feet to 3,100 feet in 2 minutes and 10 seconds. Radar contact was lost after this descent. The airplane was located 8 hours and 26 minutes after the accident along a crash debris line extending between 1.31 and 1.53 miles northwest of Arlington, Alabama.
Examination Findings
Airframe components recovered from the accident site were submitted to the NTSB Materials Laboratory for examination. The examinations revealed that all failures were consistent with overstress fracturing, with no evidence of pre-existing conditions or fatigue damage. Further examination indicated that the airframe design limits were exceeded. According to the Pilot's Operating Handbook, the maximum structural cruising speed is 173 knots indicated airspeed (KIAS) or 170 knots calibrated airspeed (KCAS). The design maneuvering speed is 135 KIAS or 133 KCAS. The co-pilot airspeed indicator at the crash site indicated 180 KCAS.
Probable Cause
According to the official findings, the probable cause of this accident was the pilot's inadequate in-flight planning/decision and his failure to maintain aircraft control, resulting in an in-flight encounter with a thunderstorm and exceeding the design limits of the aircraft.