What happened
The aircraft, Boeing 737-400 registered as N98SW, was conducting a VOR/DME approach to Runway 15R at Seattle-Tacoma International Airport. The pilot acknowledged receiving Automatic Terminal Information Service (ATIS) data and field condition reports indicating braking action was "fair to poor." However, the crew was not advised of three other pilot reports citing braking action as "poor to nil" or simply "poor." Due to possible wind shear, the flight crew extended 35 degrees of flaps instead of the standard 50 degrees. The autothrottle/speed control system was engaged throughout the approach and flare to manage speed, which remained approximately 10 knots high per company standard operating procedures.
The aircraft touched down approximately 2,500 feet beyond the displaced threshold, leaving about 6,690 feet of runway remaining. No long-range runway distance markers were available at the time. Despite using all deceleration devices to slow the aircraft, it continued past the end of the runway and crashed over a sea wall.
The investigation
Inspection of the wreckage revealed that precipitation had frozen to form glaze ice on the aircraft. The investigation highlighted systemic issues regarding runway condition reporting. ATIS information had not been updated for two hours prior to the incident. Additionally, fourteen passengers were on board; nine had not volunteered braking action reports, and the tower did not request them during continued precipitation.
Findings
The National Transportation Safety Board identified several contributing factors. Federal regulations lacked guidance for airport management regarding the measurement of runway slickness and the correlation of slick runway surfaces with aircraft stopping distances. Regulations also failed to extend authorized runway lengths for icy conditions. Airport management did not inspect or improve runway conditions after reports of poor or nil braking action were received.
Safety message
The incident underscores the critical need for accurate and timely runway condition reporting, particularly during freezing precipitation events where glaze ice can significantly reduce braking effectiveness.