What happened
The airplane was traveling southwest, level at 8,000 feet, while operating either in or above instrument meteorological conditions. After receiving instructions to change radio frequencies, the pilot checked in with the next sector controller; however, no further transmissions were received from the aircraft.
Flight data indicated the airplane descended to 7,400 feet and then climbed to 8,500 feet over a four-minute period. The aircraft maintained 8,500 feet for approximately one minute before descending to and maintaining an altitude between 8,100 and 8,200 feet for another minute. Following this, the airplane entered a descending right turn with descent rates reaching approximately 16,000 feet per minute.
The wreckage path spanned 1,900 feet, beginning with debris from the tail section. Approximately 500 feet from the start of the path, a section of the right wing was located, and 400 feet further, the entire left wing was found. The separations of the left wing and the stabilator spar showed evidence of downwards bending.
The investigation
The aircraft was equipped with a directional gyro (DG) and an attitude indicator (AI) driven by a vacuum system. A standby vacuum system was also present, though its proper operation required a reduction in engine power. Investigators found that one low-vacuum annunciator light showed filament characteristics consistent with being illuminated at the time of the accident.
Post-accident examination revealed that the vacuum pump flex coupling had sheared. The vacuum-driven instruments showed no evidence of rotational scoring. The coupling had been manufactured in 1979, and while the manufacturer recommended replacement every six years, there were no regulatory requirements for such a replacement schedule or for the installation of redundant systems to handle a vacuum system failure.