Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem; Flight Crew Overrode Automation
Aircraft; Environment - Non Weather Related
Upon departing ZZZ we pulled weather for GEG; it was 1000 rvr fzfg. We set up for the CAT III into Runway 21; at the top of descent we got the ATIS and the weather had improved WE NOW HAD 6000+ rvr and ovc 100. We decided to shoot the CAT III anyway for practice; we turned final at 10 miles out and had the runway insight. We continued down the approach with no anomalies; about 500 ft. we looked a little left of course but continued at around 200 ft. the FO (First Officer) announced that we were a little left. I disconnected the auto pilot shifted the aircraft back to the right and landed without incident. I feel we were about 100 ft. left of center line and would have hit the left edge runway lights had we not taken control of the aircraft. I hate to think what would have happened if we were in actual CAT III conditions as we probably would not have had time to correct the problem.
Air carrier Captain reported during a CAT III approach in VMC the aircraft began tracking to the left of the course; requiring the Pilot Flying to disconnect the auto pilot and continue the approach to landing.