What happened
A cargo flight operating an SA227 turboprop was conducting an ILS approach to runway 21R under low visibility conditions, with an overcast ceiling of 400 feet. The pilot, who possessed extensive experience in this specific aircraft type, was navigating the terminal area using vectors to intercept the localizer. While radar data confirmed the aircraft was tracking the localizer accurately, altitude readouts indicated the aircraft was approaching from below the required glideslope. As the aircraft descended, it maintained a descent rate steeper than the -3.5 degree glideslope angle.
During the descent, the aircraft passed through the tops of trees at an altitude of approximately 530 feet above the airport elevation, roughly 3 nautical miles from the runway threshold. Although the tower issued a low altitude alert, which the pilot acknowledged while reporting a descent through 2,800 feet, the aircraft continued its descent toward the terrain. The accident resulted in no survivors and the destruction of the aircraft.
Findings
Investigations into the navigation equipment revealed that the primary ILS receiver (NAV 1) had been deferred from service the previous evening due to unreliable performance. Consequently, the aircraft was relying on the secondary (NAV 2) receiver for ILS operations. Post-crash analysis of the HSI and RMI units indicated that the NAV 1 receiver was incorrectly tuned to the ILS frequency, while the NAV 2 receiver was tuned to the Spokane VORTAC, a facility located 14 nautical miles away. This incorrect frequency tuning caused the aircraft to deviate significantly from the glideslope.