Flight Details
The pilot, with more than 3,340 hours of pilot-in-command experience in the accident aircraft make and model, departed on a cargo flight in an SA227 turboprop. The pilot was very familiar with the destination airport and its ILS approach procedure. Before departure, the primary (NAV 1) ILS receiver was deferred due to unreliable performance noted the previous evening, leaving only the secondary (NAV 2) ILS receiver available for ILS use.
Approach
The pilot arrived in the destination terminal area and was given vectors to intercept the ILS localizer. Radar data showed the aircraft intercepting and tracking the localizer accurately throughout the approach. Mode C altitude readouts indicated the aircraft approaching from below the glideslope at the required intercept altitude of 4,100 feet, passing through and above the glideslope, then initiating a relatively constant descent at an angle exceeding the glideslope angle of -3.5 degrees.
Weather at the destination airport was reported as 400-foot overcast. The decision height for the ILS/DME runway 21R approach was 270 feet. The aircraft passed through the tops of trees in level flight about 530 feet above the airport elevation and slightly under 3 nautical miles from the runway threshold.
Low Altitude Alert and Impact
The tower controller issued a low altitude alert, which the pilot acknowledged, reporting a descent through 2,800 feet. This altitude was confirmed by Mode C radar readout. At that point, the aircraft was well below the ILS glideslope and approximately 13 seconds from impacting the trees.
Post-Crash Examination
Post-crash examination of information captured from the left and right HSI units and an RMI revealed that the NAV 1 receiver was most likely set on the ILS frequency, and the NAV 2 receiver was most likely set on Spokane VORTAC, a terminal navigation facility located slightly right of the aircraft's nose and 14 nautical miles southwest of the destination airport.