Flight Details
The private pilot and passenger departed on an 875-nautical-mile cross-country flight. The airplane climbed to a cruise altitude of 24,000 feet mean sea level, using the autopilot as indicated by radar data. The pilot was familiar with the route, which passed over largely unpopulated hilly terrain. The clouds were well below the cruising altitude, providing reliable external visual cues. No turbulence was reported in the area.
Events Leading to Upset
About 1 hour 40 minutes after departure, the pilot contacted air traffic control and requested to "leave frequency for a couple of minutes." No further radio transmissions were made. Approximately 20 seconds after the last transmission, the airplane banked to the right, entered a spiral descent, and subsequently broke apart. At no time did the pilot indicate any difficulty or request assistance. Just prior to the deviation, the pilot had recorded engine parameters in a flight log, which appeared consistent with normal operation.
Investigation Findings
During the breakup, portions of the wings, horizontal stabilizers, and elevators separated. Analysis of fracture surfaces, debris field distribution, and radar data indicated that the rapid descent exceeded the airplane's design stress limits, leading to in-flight structural failure. Examination of the engine components and surviving primary airframe parts revealed no mechanical malfunctions or failures that would have precluded normal operation.
The airplane was equipped with a supplemental oxygen system. The pilot likely had his mask plugged in and available, unstowed behind his seat; the passenger's mask was stowed under her seat. The autopilot could be disengaged by depressing the appropriate mode switch, pushing the disengage switch on the control wheel, or turning off the autopilot switch. All autopilot servos had a clutch mechanism allowing manual override at any time.
It is likely that the pilot's request to leave the frequency was to leave his seat to attend to something. While doing so, he may have inadvertently disconnected the autopilot and was unable to recover control before the deviation occurred.
Probable Cause
The pilot's failure to regain airplane control following a sudden rapid descent during cruise flight, which resulted in an exceedance of the design stress limits of the aircraft and led to an in-flight structural failure.