Accident Sequence
The aircraft was operating under an IFR clearance and climbing through a cloud layer when it broke up in flight following an in-flight upset. As the airplane began to intercept a victor airway, climbing at about 2,000 feet per minute (fpm) through 6,700 feet, it started a series of heading and altitude changes inconsistent with its ATC clearances. It turned right and climbed to 8,600 feet, then turned left and descended to 8,000 feet. Subsequently, it turned right and climbed to 8,500 feet, where it began a rapidly descending right turn. At 1034:33, while descending through 7,000 feet, the pilot advised ATC, "four Juliet victor I just lost my needle give me..." No further transmissions were received. The last radar return showed the aircraft descending through 3,200 feet at about 11,000 fpm. Analysis of radar data indicated the airplane was near Vmo at the last Mode C return. Ground witnesses observed the aircraft emerge from clouds in a high-speed spiral descent just before it broke up approximately 1,000 feet above ground level. Examination of the wreckage showed all structural failures resulted from overload.
Weather Conditions
Weather conditions included multiple cloud layers from 4,000 to 13,000 feet, with a freezing level around 7,000 feet mean sea level. An AIRMET was in effect for occasional moderate rime to mixed icing in clouds and in precipitation below 18,000 feet.
Cockpit Configuration
The aircraft was equipped with full flight instruments on both sides of the cockpit; however, the flight director system attitude director indicator and horizontal situation indicator were only on the left side. The aircraft was configured for flight into known icing conditions, including heated pitot tubes (left and right), static sources, and stall warning vanes. During the on-scene cockpit examination, except for the pitot heat switches, all cockpit controls and switches were found in positions consistent with the aircraft's phase of flight prior to the upset. The right pitot heat switch was in the ON position, while the left switch was OFF. The left pitot heat switch toggle lever was noticeably displaced to the left due to impact. With the exception of the left pitot heat, all anti-ice and de-ice system switches were configured for icing conditions.
Pitot Heat Switch Examination
The pitot heat switches, which function as both toggle switches and circuit breakers, were removed and sent for laboratory testing. Low power stereoscopic examination found the right switch intact, while the left switch's toggle lever mechanism was broken loose from its housing. Microscopic examination of the left switch's housing fracture surface revealed embedded debris and wear marks indicative of an old fracture predating the accident. The broken left switch could be electrically operated by manually holding the toggle lever in the ON or OFF position. Electrical contact resistance measurements of the left switch varied between 0.3 and 1.4 ohms, and the switch was intermittently open when in the ON position. Disassembly revealed particulate debris in both switches, but the left switch contained a significant amount of large coarse fibrous lint-like debris. The flexible copper conductor in the left switch's circuit breaker section had several broken strands, and the electrical contacts were dirty. The laboratory report concluded that the left switch's toggle was bent left during the impact, but the housing fracture predated the accident and allowed internal accumulation of debris. The combined effects of the broken housing, misaligned toggle mechanism, dirty contacts, and large coarse lint debris prevented reliable electrical switching and presented the opportunity for intermittently open electrical contacts. Continuity of the pitot and static plumbing to instruments was verified. Electrical continuity from bus power through circuit breakers and switches to the pitot and static heating elements was established, and the heating elements operated when connected to a 12-volt battery.
Probable Cause
The official findings stated: "The pilot's loss of control and resulting exceedance of the design stress limits of the aircraft, which led to an in-flight structural failure. The pilot's loss of control was due in part to the loss of primary airspeed reference resulting from pitot tube icing, which was caused by the internal failure of the pitot heat switch. Factors in the accident were the pilot's distraction caused by the airspeed reading anomaly and spatial disorientation."