Seattle-Tacoma International Airport
Aircraft Accident/Incident Report

Seattle, Washington 98168
Tuesady, January 16, 2001 8:42 PST

On a coupled instrument landing system (ILS) approach to runway 16R at Seattle-Tacoma International Airport, the aircraft deviated approximately 0.4 nautical mile east of the final approach course. During missed approach, the aircraft flew east of parallel runway 16L and in proximity to an approximately 290-foot-high air traffic control (ATC) tower under construction on the airport. The crew subsequently made a second approach and landing attempt without further incident. The flight crew reported that the incident approach was flown with the autopilot coupled to the #1 NAV, and that all instruments indicated on course and on glide slope during final approach. The crew reported that at decision height, they initiated a missed approach when they did not observe the proper visual cues, and that the tower subsequently informed them that they were left of course. On the second (successful) approach attempt, the first officer hand-flew the approach using the #2 NAV. Visibility at the time was reported as 1/4 statute mile with freezing fog, with a vertical visibility of 100 feet. A post-incident flight inspection of the localizer and glide slope did not disclose any problems with the ILS signal. During post-incident troubleshooting of the aircraft's course deviation circuitry, maintenance personnel discovered a defective relay in the switching matrix for the aircraft's flight management system (FMS), which was installed under an FAA Supplemental Type Certificate. Whenever the area around the FMS switching matrix or the defective relay itself was physically tapped, the captain's course deviation indicator (CDI) was observed to jump. When a replacement relay was installed, this anomaly was not observed. Inflight system testing also disclosed that the autopilot was erratic in pitch, with porpoising about the glide slope. Maintenance personnel subsequently discovered an anomaly during the autopilot pitch computer self test, which was resolved by replacing the fore and aft pitch accelerometers. Maintenance personnel further discovered that by replacing the BNC connectors at NAV 1 and NAV 2, significant improvements in system efficiency were attained. No anomalies with the first officer's CDI indications, or false "on glide slope" indications, were found during any post-incident troubleshooting.

A malfunctioning relay in the aircraft's flight management system (FMS) switching matrix and associated false "on course" indication on the captain's course deviation indicator (CDI), resulting in proper localizer course alignment not being obtained or maintained and subsequent flight in close proximity to the new control tower. The reason for the reported false "on course" indications on the first officer's CDI was not determined. Factors contributing to the incident included low ceiling and obscuration, weak signal received by both localizer receivers due to faulty BNC connectors, and the new control tower.