Accident Overview
On an IFR arrival, Flight 4821 was cleared to intersection 17 northeast of the airport at 6,000 feet, then cleared for an ILS approach to runway 23. Radar service was terminated when the flight was 6.5 miles east of the intersection. Radar data indicated that the aircraft crossed and then bracketed the localizer. The flight intercepted the glide slope from below approximately 7 miles outside the outer marker and subsequently deviated above the glide slope. About 2 miles outside the outer marker, the aircraft was at a full fly-down deflection and entered a descent varying between 1,200 and 2,000 feet per minute. The aircraft struck a wooded mountain top 2.0 miles inside the outer marker (3.9 miles from the runway) at an elevation of 2,280 feet. The minimum altitude at the outer marker was 3,600 feet, and the glide slope elevation at the point of impact was approximately 2,900 feet.
Investigation Findings
Investigators found evidence of an inadequate electrical ground path between the radome and fuselage. Combined with prevailing weather conditions, this may have produced electrostatic discharge, known as precipitation static. Post-accident tests were not conclusive, but the safety board believed that the glide slope indications might have been unreliable due to precipitation static interference.
Probable Cause
The official probable cause was failure of the captain to establish a stabilized approach, his inadequate cross-check of instruments, his descent below specified minimum altitude at the final approach fix, and failure of the copilot to monitor the approach. Factors related to the accident were weather conditions and possible precipitation static interference, caused by inadequate grounding between the radome and fuselage that could have resulted in unreliable glide slope indications.
Casualties and Survivors
Two occupants survived the accident, while two others—one pilot and one passenger—were killed.