Mooney M20C — Cashiers, NC

What happened

The pilot obtained a weather briefing and filed his instrument flight rules flight plan the night before the cross-country flight. About 30 minutes after departure, an air traffic controller advised the pilot that he had overcorrected a turn. Shortly after, the pilot advised the controller that his attitude indicator was not functioning. The controller asked the pilot if he was in the clouds, and the pilot responded that he was in instrument meteorological conditions. The controller attempted to help the pilot get to clearer air, but the pilot continued to have difficulty controlling the airplane and maintaining an assigned course. Radar contact and radio contact were lost shortly thereafter.

The wreckage was located in mountainous terrain, and all major components and control surfaces were accounted for at the accident site. Various parts of the airplane were scattered through the treetops and the wreckage debris field. While the airplane’s attitude indicator was not recovered, the directional gyroscope and vacuum pump were, and they were each examined in detail.

Examination of the vacuum pump revealed that the shear coupling that connected the driveshaft between the engine accessory drive gear and the vacuum pump driveshaft assembly likely fractured before impact. The condition of the fractured shear coupling was consistent with the drive side rotating after fracture while in contact with the separated (and hence, stationary) driven side. A transverse fracture as observed through the shear coupling would have disengaged the two-piece driveshaft, which would not have allowed the pump to rotate. Additionally, all six of the vacuum pump’s vanes were intact but the rotor was fragmented into four relatively large pieces, and a relatively small area had fragmentated into smaller pieces. Given this information, the vacuum pump was likely not rotating at impact, as a greater degree of fragmentation would be expected if the pump were rotating. The interior surface vacuum pump housing displayed wear consistent with vane impact, dragging, and debris contamination within the housing at some point during the pump’s operational lifetime. One of the vanes exhibited heat tinting, and portions of the rotor’s exterior circumference also exhibited wear scarring. These observations were all consistent with the presence of forces resisting the pump’s normal rotation. It is likely that these resistive forces ultimately resulted in the failure of the rotor or the shear coupling. Additionally, examination of the directional gyroscope revealed that the rotor and its housing displayed signatures consistent with ingestion of dirt particles or foreign debris during operation, though none of the signatures conclusively supported its operational status at the time of the accident.

Given these observations, it is likely that the vacuum pump ceased operating during the flight, which would have rendered the airplane’s vacuum-driven attitude indicator and directional gyroscope inoperative. A definitive cause for this failure could not be determined, though it is possible that the system had been contaminated with debris, which may have contributed to the failure. Because the operational status of the directional gyroscope was inconclusive, it could not be determined whether the pilot attempted to use the airplane’s standby vacuum system or whether that system was functional at the time of the accident.

Because the pilot was operating the airplane under instrument flight rules and in instrument meteorological conditions, he would have primarily relied on the airplane’s instruments to maintain control and orientation of the airplane. It is likely that following the inflight failure of the vacuum pump, the pilot’s ability to control the airplane continually degraded, based on his communications with air traffic control and the airplane’s flight track as observed by the air traffic controller. While the pilot might have used the airplane’s standby vacuum system or attempted to maintain control of the airplane using partial instrument techniques, this would have been difficult given his lack of recent instrument flight experience. Ultimately, it is likely that the pilot succumbed to spatial disorientation and lost control of the airplane.

While toxicological testing of specimens collected from the pilot’s remains following the accident were positive for the presence of ethanol, there was no evidence available to suggest that it was the result of ingestion rather than post-mortem production.

An editorial "what led to it / how to avoid it" analysis for this accident is generated separately and will appear here.