On February 24, 2004, approximately 0930 Pacific standard time, the First Officer of a Southwest Airlines 737-300, N368SW, inadvertently aligned the aircraft for a landing on Taxiway Tango at Seattle-Tacoma International Airport (SEATAC), Seattle, Washington. There were no injuries to the flight crew, three flight attendants, or any of the 89 passengers, and there was no damage to the aircraft, which is owned and operated by Southwest Airlines Company. The scheduled Part 121 domestic passenger flight, which departed Oakland, California, about 85 minutes prior to the incident, was being operated in visual meteorological conditions at the time of the subject event. The flight had been on an IFR flight plan, and had been cleared for a visual approach to Runway 16 Right at SEATAC.

According to the flight crew, while they were west of the airport, the flight was cleared for the Bay Visual Approach to Runway 16 Right. The First Officer, who was flying at the time, made a right turn over Elliott Bay and lined up on what he believed to be Runway 16 Right. When the aircraft was about one mile from the end of the taxiway, the first officer noticed a yellow X located just off the northern end of the surface he intended to land on. Upon realizing he was lined up on the wrong surface, he initiated a sidestep to Runway 16 Right. He subsequently completed an uneventful landing on Runway 16 Right, and taxied to the gate for a normal deplanement of the passengers. According to recorded radar tracking data, at the time of the turn to initiate the sidestep, the aircraft was approximately 600 feet above the ground (AGL).

In a post-incident interview, the First Officer said that he did not realize he was lined up to land on the wrong surface until he saw the yellow X. He also said that there were varying degrees of reflection and glare at different areas of the airfield environment, and that the size and shape of the taxiway made it look very much like a runway. He further stated that although he was eventually able to see the Runway 16 Right identification markings, he never did clearly see any markings that indicated Taxiway Tango was a taxiway. When advised that there was also a temporary lighted-X near the permanent yellow X, he responded that the only X he had seen was the solid yellow X. When told that the Runway 16 centerline lights were on in the dim position, he said that he did not remember seeing them, even after executing the sidestep maneuver.

In a post-incident interview with the Captain, he stated that he knew which surface was the runway and which was the taxiway soon after the aircraft rolled out on final, but at the time he first visually acquired the runway environment, it looked to him like the First Officer was lining up on Runway 16 Right. The Captain explained that since they had been cleared to land long, his attention was diverted from the runway environment while he explained the landing/deceleration technique he wanted the first Officer to use, and then waited for the First Officer's response. He added that almost immediately after the First Officer confirmed that he understood his instructions, the First Officer made a comment and started a shallow left turn. At that point, the Captain briefly checked the airspeed, and then returned his attention to the runway/taxiway environment. He said that it was at that point in time when he realized that the First Officer had lined up to the right (west) of Runway 16 Right, and was now correcting to it. He also mentioned that he too briefly saw the yellow X at that point in time. The Captain said that he thought briefly about a go-around, but since the bank angle was shallow, and the runway very nearby, he elected to let the First Officer continue the sidestep.

At the time of the event, the airport's paved surfaces were wet from a recent rain shower, and although the Captain was aware of the note on the airport diagram advising crews not to mistake Taxiway Tango for a landing surface, this note was not discussed prior to or during the approach sequence. He said that because the ILS was out, they had no localizer backup. Both crew members mentioned that there was a need for a better/clearer identification of the runways, but that some type of markings making it clear that Taxiway Tango is not a runway was the most important action that could be taken.

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