Medium Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness; Troubleshooting
ATC Issue All Types; Deviation - Track / Heading All Types
Aircraft Equipment Problem Dissipated; Flight Crew FLC Overrode Automation; Flight Crew Took Evasive Action
Aircraft; ATC Equipment / Nav Facility / Buildings
The flight was cleared to intercept the [Runway] 23L localizer between CLANG and ISOLA. We had a positive localizer ID and a stable reception of localizer and glideslope. The localizer needle seemed to be stable at first. No oscillations were noted before interception. The FGC was set to the NAV mode. When the autopilot captured the localizer; the localizer needle started to fluctuate about 2-3 degrees either side of the approach course and the autopilot chased the needle resulting in continuous large S-turns on the final between 20 and 10 miles. The pilot flying switched to HDG mode to dampen the oscillations. The localizer needle was stable once inside the ZEROX FAF. The glideslope was stable the whole time. I have no suggestions how to fix the localizer oscillation problems.
An ERJ pilot reported that in the NAV Mode the aircraft S-turned while following the IND 23L localizer oscillations between 10 and 20 miles; but stabilized in the Heading Mode. The localizer oscillations ceased inside ZEROX.