What happened
On February 15, 1983, at approximately 11:00 MST, a DHC-6 operated by Sierra Pacific Airlines as Transwestern Flight 868 was performing a scheduled commuter service from Boise to Hailey, Idaho. While executing the final approach to runway 31, the aircraft was positioned roughly 1.7 miles south of Friedman Memorial Airport.
As the aircraft reached an altitude of about 800 feet over Bellevue, the pilot decreased engine power to begin the configuration process for landing. During this maneuver, the flight crew experienced a sudden loss of elevator control. The aircraft entered a steep descent with the nose pitched down. Although the pilot attempted to regain pitch control by increasing power, the plane struck a highway in a nosedown position with the right wing dipping. Following the impact, the aircraft veered off the road and collided with a 4-foot snowbank, causing the airframe to break apart. There was no fire at the scene.
The flight was carrying six passengers and two crew members. Of the eight people on board, one passenger sustained minor injuries, while the remaining seven occupants suffered serious injuries.
Findings
Investigations determined that the primary cause of the accident was the separation of the control rod from the torque tube. This failure occurred at a connection point where the airline's maintenance department had utilized an unauthorized and unsecured bolt. The error went unnoticed by the company's inspection personnel.
Additional contributing factors included the operator's failure to maintain the necessary separation between its maintenance and inspection departments, violating the FAA-approved maintenance program. Furthermore, the FAA failed to identify this deviation from approved procedures during its oversight inspections.