Narrative:

With autoplt engaged on heading 220 degrees magnetic at 4000 ft MSL, 170 KIAS, aircraft was east of runway 18L centerline, on vector to intercept localizer, and cleared for ILS runway 18R, cvg approach. As aircraft approached extended centerline, localizer course came alive, but as autoplt captured the localizer, instead of turning left (from 220 degrees to 184 degrees, the inbound course) the aircraft rolled rapidly right into a 30 degree angle of bank turn. Pilot disconnected the autoplt and reversed the aircraft roll back left, while tower controller made a call that the aircraft was overshooting the localizer centerline. Maximum angle of bank was about 30 degrees, and maximum heading deviation was to 270 degrees magnetic. There were no TCASII advisories, and aircraft stayed level at 4000 ft MSL, +/-100 ft. Tower supervisor advised aircraft heading of 130 degrees and maintain altitude 4000 ft. Aircraft was vectored out of the approach pattern and set up for another vector to reintercept the ILS to runway 18L at cvg. Autoplt was re-engaged on downwind, and once again operated normally until localizer intercept, when once again the autoplt started a right roll away from course. Autoplt was immediately disconnected, and an uneventful hand-flown ILS approach was conducted to runway 18L. It should be noted that this entire sequence of events occurred in IMC conditions, with light mixed ice. Conditions at the airport were 2500 ft overcast, winds 270 degrees at 18 KTS gusting 29 KTS. Also, this airplane had been flown by this flight crew on 4 different legs the preceding day, with no instrument or autoplt deviations noted, including an approach exactly as described to the same runway, using the same autoplt system, but without the abnormal turn away from course that occurred on this flight. This event shows a classic example of how the role of the pilot is crucial in ensuring the proper operation of automated system. It also is a classic example of the requirement to never become complacent, even during repetitive, seemingly routine operations.

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Original NASA ASRS Text

Title: B737-200 CREW ATTEMPTED AN AUTO-COUPLED APCH. ON LOC INTERCEPT, AUTOPLT TURNED ACFT THE WRONG WAY. LCL CTLR ISSUED VECTORS FOR ANOTHER APCH.

Narrative: WITH AUTOPLT ENGAGED ON HEADING 220 DEGS MAGNETIC AT 4000 FT MSL, 170 KIAS, ACFT WAS E OF RWY 18L CTRLINE, ON VECTOR TO INTERCEPT LOC, AND CLRED FOR ILS RWY 18R, CVG APCH. AS ACFT APCHED EXTENDED CTRLINE, LOC COURSE CAME ALIVE, BUT AS AUTOPLT CAPTURED THE LOC, INSTEAD OF TURNING L (FROM 220 DEGS TO 184 DEGS, THE INBOUND COURSE) THE ACFT ROLLED RAPIDLY R INTO A 30 DEG ANGLE OF BANK TURN. PLT DISCONNECTED THE AUTOPLT AND REVERSED THE ACFT ROLL BACK L, WHILE TWR CTLR MADE A CALL THAT THE ACFT WAS OVERSHOOTING THE LOC CTRLINE. MAX ANGLE OF BANK WAS ABOUT 30 DEGS, AND MAX HEADING DEV WAS TO 270 DEGS MAGNETIC. THERE WERE NO TCASII ADVISORIES, AND ACFT STAYED LEVEL AT 4000 FT MSL, +/-100 FT. TWR SUPVR ADVISED ACFT HEADING OF 130 DEGS AND MAINTAIN ALT 4000 FT. ACFT WAS VECTORED OUT OF THE APCH PATTERN AND SET UP FOR ANOTHER VECTOR TO REINTERCEPT THE ILS TO RWY 18L AT CVG. AUTOPLT WAS RE-ENGAGED ON DOWNWIND, AND ONCE AGAIN OPERATED NORMALLY UNTIL LOC INTERCEPT, WHEN ONCE AGAIN THE AUTOPLT STARTED A R ROLL AWAY FROM COURSE. AUTOPLT WAS IMMEDIATELY DISCONNECTED, AND AN UNEVENTFUL HAND-FLOWN ILS APCH WAS CONDUCTED TO RWY 18L. IT SHOULD BE NOTED THAT THIS ENTIRE SEQUENCE OF EVENTS OCCURRED IN IMC CONDITIONS, WITH LIGHT MIXED ICE. CONDITIONS AT THE ARPT WERE 2500 FT OVCST, WINDS 270 DEGS AT 18 KTS GUSTING 29 KTS. ALSO, THIS AIRPLANE HAD BEEN FLOWN BY THIS FLT CREW ON 4 DIFFERENT LEGS THE PRECEDING DAY, WITH NO INST OR AUTOPLT DEVS NOTED, INCLUDING AN APCH EXACTLY AS DESCRIBED TO THE SAME RWY, USING THE SAME AUTOPLT SYS, BUT WITHOUT THE ABNORMAL TURN AWAY FROM COURSE THAT OCCURRED ON THIS FLT. THIS EVENT SHOWS A CLASSIC EXAMPLE OF HOW THE ROLE OF THE PLT IS CRUCIAL IN ENSURING THE PROPER OP OF AUTOMATED SYS. IT ALSO IS A CLASSIC EXAMPLE OF THE REQUIREMENT TO NEVER BECOME COMPLACENT, EVEN DURING REPETITIVE, SEEMINGLY ROUTINE OPS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.