Accident Overview
During an arrival for an instrument landing system (ILS) approach to runway 21R, the aircraft encountered icing conditions for approximately 9.5 minutes. As the aircraft was vectored for the approach, the Seattle Air Route Traffic Control Center (ARTCC) controller used an expanded radar range and did not provide precise positioning of the aircraft to the final approach course. The flight crew attempted to continue with a steep, unstabilized approach for landing.
Deviations from Approach Path
Recorded radar data showed that the aircraft was well to the right of the ILS course line and well above the glide slope as it passed the outer marker/final approach fix (FAF). The aircraft did not intercept the localizer course until it was about 1.5 miles inside the FAF. Additionally, it was still well above the ILS glide slope when recorded altitude data was lost at about 2.5 miles from the airport.
Final Descent and Crash
The tower had closed, but the controller observed the aircraft in a higher than normal rate of descent in a wings level attitude. Before reaching the runway, the aircraft nosed over and crashed in a steep descent.
Ice Accumulation Evidence
There was evidence that ice had accumulated on the airframe, including the horizontal stabilizers, which may have resulted in a tail plane stall. The accident investigation is referenced in NTSB/AAR-91/06.