Event Sequence
The airplane was in cruise at flight level 210 in instrument meteorological conditions with turbulence and a wind from approximately 255 degrees at 70 knots. At about 1600 CST, the airplane drifted off course. Over the following ten minutes, unrecognized heading changes occurred. At 1610:09, the pilot stated, “. . . I've lost my gyro.” Six seconds later, the controller issued a no-gyro vector. At 1611:29, as the airplane was still turning, the pilot said, “we've lost aLL our instruments . . . please direct me towards VFR.” He was cleared to descend to 14,000 feet. Around the same time, he stated, “we're in trouble” and “we've lost all vacuum.” No further radio transmissions were received.
The airplane entered a steep, downward spiraling right turn. The left outer wing panel separated upward and aft (in flight) due to overload and impacted the left stabilizer. The airplane crashed, and parts that separated were found over a four-mile area.
Investigation Findings
Examination revealed evidence that the horizontal situation indicator (HSI) heading card can fail without the HDG flag appearing. Although the pilot reported loss of instruments and vacuum, inspection of the airplane showed that the engine, flight controls, electrical system, pitot/static system, and vacuum systems exhibited continuity. No malfunction was found that would have led to loss of pressurization or hypoxia.
Probable Cause
The official probable cause was: "Spatial disorientation of the pilot, and his failure to maintain control of the airplane, which resulted in his exceeding the design stress limits of the airframe. A factor relating to the accident was: turbulence in clouds."