What happened
During 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 passed in close proximity to an approximately 290-foot-high air traffic control (ATC) tower that was under construction on the airport grounds.
The flight crew reported that during the initial approach, the autopilot was coupled to the #1 NAV, and all cockpit instruments indicated the aircraft was on course and on the glide slope. Upon reaching decision height, the crew initiated a missed approach because they did not observe the necessary visual cues; the tower subsequently notified them that they were left of course. Visibility during the event was reported as 1/4 statute mile with freezing fog and a vertical visibility of 100 feet. On a second attempt, the first officer hand-flew the approach using the #2 NAV, resulting in a successful landing without further incident.
The investigation
A post-incident flight inspection of the localizer and glide slope revealed no problems with the ILS signal. However, troubleshooting of the aircraft's course deviation circuitry led maintenance personnel to discover a defective relay within the switching matrix for the flight management system (FMS), which had been installed under an FAA Supplemental Type Certificate. Technicians observed that physically tapping the area around the FMS switching matrix or the relay itself caused the captain's course deviation indicator (CDI) to jump. The issue was resolved by installing a replacement relay.
Inflight system testing also identified that the autopilot exhibited erratic behavior in pitch, characterized by porpoising about the glide slope. This was traced to an anomaly found during an autopilot pitch computer self-test, which was corrected by replacing the fore and aft pitch accelerometers. Additionally, maintenance personnel found that replacing the BNC connectors at NAV 1 and NAV 2 significantly improved system efficiency. No anomalies were found regarding the first officer's CDI indications or false glide slope indications.