Incident Overview
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 the subsequent 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 then performed a second approach and landing without further incident.
Flight Crew Actions
The flight crew reported that the incident approach was flown with the autopilot coupled to the #1 NAV receiver, and that all instruments indicated on course and on glide slope during the final approach. At decision height, they initiated a missed approach when they did not observe the proper visual cues. 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 receiver.
Environmental Conditions
Visibility at the time was reported as 1/4 statute mile with freezing fog, and a vertical visibility of 100 feet.
Post-Inspections and System Checks
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.
Further Maintenance Findings
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. Additionally, 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.