Flight Path and Communication
The airplane was traveling southwest, level at 8,000 feet, and was operating either in or above instrument meteorological conditions. The pilot was instructed by air traffic control to change radio frequency. After checking in with the next sector controller, no further transmissions were received from the pilot. The airplane subsequently descended to 7,400 feet, then climbed to 8,500 feet over a period of 4 minutes. It remained at 8,500 feet for about one minute, then descended to and maintained an altitude between 8,100 and 8,200 feet for another minute. Following this, the airplane executed a descending turn to the right, with rates of descent reaching approximately 16,000 feet per minute.
Wreckage Distribution
The wreckage path extended 1,900 feet in length. The first components found were parts from the airplane's tail section. Approximately 500 feet from the beginning of the path, a section of the right wing was located. About 400 feet beyond that point, the entire left wing was discovered. Both the stabilator spar and the left wing separations exhibited downward bending.
Vacuum System Examination
The airplane was equipped with a vacuum-driven attitude indicator (AI) and directional gyro (DG). A standby vacuum system was also installed, which required a reduction of engine power to operate properly. Two low-vacuum annunciator lights were present: one with a filament that exhibited characteristics consistent with its being illuminated at the time of the accident; the other was a diode type that was tested and found operational. Post-accident examination revealed that the vacuum pump flex coupling was sheared. The vacuum-driven instruments did not exhibit any rotational scoring. The vacuum pump coupling was manufactured in 1979. The manufacturer recommended replacing the coupling every 6 years. There was no regulatory requirement to change the coupling, nor was there any regulatory requirement for redundant systems in case of vacuum system failure.