Seattle-Tacoma Intl Airport
Aircraft Accident/Incident Report

Seattle, Washington 98168
Saturday, July 18, 1998 7:00 PDT

The flight was cleared for an instrument landing system (ILS) approach. The ceiling was 200 feet overcast, visibility was 1 mile (runway visual range was greater than 6,000 feet) and the first officer was the pilot flying (according to company operating procedures, the minimum ceiling for first officer flying is normally 500 feet.) Air traffic control (ATC) radar indicated that the aircraft was left of the localizer course and/or below glide path, outside of the company's stabilized approach limits, for the majority of the final approach segment below 1,000 feet above touchdown. The aircraft broke out approximately at minimums (200 feet above touchdown), left of (and diverging away from) the runway centerline. The captain called, "push it down, push it down, push it down", and asked, "you got it or you want me to get it?". The first officer replied, "I can get it", and the captain said, "OK." The aircraft pitched down and banked up to approximately 14 degrees in correcting back to the runway. A high sink rate existed in the last few seconds before touchdown; the flight data recorder registered a vertical acceleration of about 1.8 G at touchdown. The aircraft's number 1 main landing gear wheel separated from the aircraft at or shortly after touchdown. The separated wheel entered the airport ramp area and struck two parked trucks and a baggage cart in front of the main passenger terminal. Post-incident examinations of the aircraft's number 1 wheel retaining nut disclosed that the nut threads were worn approximately 0.030 inch beyond engineering drawing specifications. FAA guidance directs FAA principal operations inspectors (POIs) to ensure their operators' operations manuals contain stabilized approach criteria as well as required actions in the event of deviations ("i.e. missed approach or go/around [sic]"). The company's DC-8 operations manual states only that deviations from the stabilized approach profile "are cause for consideration to abandon the approach."

The flight crew's failure to perform a missed approach upon failing to attain and/or maintain proper course/runway alignment and glidepath on final approach, resulting in a high-sink-rate landing and subsequent separation of a main landing gear wheel from the aircraft. Factors contributing to the incident included: low ceiling; the first officer's failure to attain and/or maintain proper course/runway alignment and glidepath on approach; insufficiently defined company procedures for responding to deviations from a stabilized approach profile; inadequate FAA principal operations inspector approval of company operating procedures; and a worn main landing gear wheel retaining nut.