Approach and Icing Conditions
During arrival for an ILS runway 21R approach, the aircraft encountered icing conditions for approximately 9.5 minutes. The flight crew was being vectored by Seattle ARTCC for the approach.
Controller Vectoring and Flight Path
The controller used an expanded radar range and did not provide precise positioning of the aircraft to the final approach course. 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). It did not intercept the localizer course until about 1.5 miles inside the FAF. The aircraft was still well above the ILS glide slope when recorded altitude data were lost at approximately 2.5 miles from the airport.
Final Descent and Loss of Control
The flight crew attempted to continue a steep, unstabilized approach for landing. 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. Evidence indicated that ice had accumulated on the airframe, including the horizontal stabilizers, which may have resulted in a tailplane stall.
Fatalities
All six occupants were killed.