What happened
During a night flight in instrument meteorological conditions, a pilot was performing a precision instrument approach when the aircraft type struck the ground approximately four miles before reaching the runway. Radar data and air traffic control records indicate that although the pilot had been vectored onto the final approach course, the aircraft failed to establish itself on the glide slope. Instead, the pilot began a controlled descent below the required glide path while advancing toward the airport.
When the aircraft was five miles from the destination, a tower controller received an audible alert from the Minimum Safe Altitude Warning (MSAW) system. Although the controller promptly alerted the pilot to the low altitude, the collision occurred only seconds later. An inspection of the onboard navigation and instrument approach equipment showed no malfunctions prior to the accident.
Findings
Investigations into the MSAW system parameters revealed that the tower controller's audible alarm was only configured to trigger for aircraft within a 5-nautical-mile radius of the airport. Because the handoff from the approach controller to the tower controller occurred when the aircraft was 10.7 miles away, a 5.7-mile gap existed where both controllers could see visual alerts, but only the approach controller could hear an audible alarm.
Crucially, the approach controller failed to notify the tower controller of the low altitude alert due to a misunderstanding of the alarm settings and a change in FAA policy. The approach controller believed the MSAW parameters were set to a 10-mile radius and assumed the aircraft was no longer her responsibility following the frequency change. Additionally, the investigation noted that the audible tone of the MSAW alarm used by the approach controller was not sufficiently distinct to ensure she recognized the severity of the alert.