What happened
The aircraft was operating under instrument flight rules during nighttime conditions at an altitude of 7,000 feet mean sea level within instrument meteorological conditions. The pilot reported that the directional gyro had become inoperative while the aircraft was outbound from the Pulaski VOR on airway V45, heading southeast. At that time, the turn and bank indicator and the magnetic compass were confirmed to be operative.
Shortly after reporting the instrument failure, the pilot was unable to maintain the assigned course. The aircraft eventually reversed its direction of travel. While air traffic control attempted to assist the pilot in returning to the correct course, the aircraft descended and collided with the ground. Witnesses located near the accident site reported poor visibility, low clouds, and fog at the time of the incident.
The pilot had previously flown a four-hour leg to drop off passengers and was returning to his home base. The aircraft had been in flight for approximately three hours on this return leg.
The investigation
Examination of the engine-driven vacuum pump did not reveal conclusive evidence of mechanical failure. Although the aircraft was equipped with a standby vacuum system, the operation of this backup system could not be confirmed. The aircraft's capability to continue instrument meteorological conditions could not be accurately determined due to extensive fire and impact damage sustained during the collision.
Findings
Contributing factors included the inoperative directional gyro, the pilot's inability to maintain course, and adverse weather conditions including poor visibility, low clouds, and fog. The failure to utilize or confirm the operation of the standby vacuum system may have played a role.