Narrative:

During approach, we received a TA on TCASII indicating traffic at 3 O'clock and above. We were just beginning to enter IMC. Captain was flying the airplane and I was looking for traffic. We both then noticed erratic fluctuations on localizer needle. Shortly after, we received an RA on TCASII commanding descent and almost immediately were instructed by ATC to turn left immediately. Captain promptly responded and I got a quick glance of a jetliner in the upper right cockpit window. We then entered the clouds. The localizer indications then resumed normal operation. Workload was heavy at this time. I crosschecked both ILS indications and both appeared to give normal indications. We landed normally. After parking at the gate, I asked the captain if he thought we should contact ATC by telephone to report and discuss incident. He said no. He then explained to me that the ILS receivers on the particular aircraft we were flying had a history of presenting fluctuating signals from time to time but that the problem had never been fully rectified though attempts were made. I was unaware of this. The captain did report the localizer fluctuations to the avionics shop. I do not know at this time what corrective action, if any, had been taken. It was difficult to recall details since workload was heavy, but I believe the erroneous signals given by the localizer needle caused us to drift to the right of course.

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Original NASA ASRS Text

Title: ACR HAS TCASII RA AT SAME TIME AS ERRATIC FLUCTUATIONS ON LOC NEEDLE. ATC GIVES IMMEDIATE R TURN.

Narrative: DURING APCH, WE RECEIVED A TA ON TCASII INDICATING TFC AT 3 O'CLOCK AND ABOVE. WE WERE JUST BEGINNING TO ENTER IMC. CAPT WAS FLYING THE AIRPLANE AND I WAS LOOKING FOR TFC. WE BOTH THEN NOTICED ERRATIC FLUCTUATIONS ON LOC NEEDLE. SHORTLY AFTER, WE RECEIVED AN RA ON TCASII COMMANDING DSCNT AND ALMOST IMMEDIATELY WERE INSTRUCTED BY ATC TO TURN L IMMEDIATELY. CAPT PROMPTLY RESPONDED AND I GOT A QUICK GLANCE OF A JETLINER IN THE UPPER R COCKPIT WINDOW. WE THEN ENTERED THE CLOUDS. THE LOC INDICATIONS THEN RESUMED NORMAL OP. WORKLOAD WAS HVY AT THIS TIME. I XCHKED BOTH ILS INDICATIONS AND BOTH APPEARED TO GIVE NORMAL INDICATIONS. WE LANDED NORMALLY. AFTER PARKING AT THE GATE, I ASKED THE CAPT IF HE THOUGHT WE SHOULD CONTACT ATC BY TELEPHONE TO RPT AND DISCUSS INCIDENT. HE SAID NO. HE THEN EXPLAINED TO ME THAT THE ILS RECEIVERS ON THE PARTICULAR ACFT WE WERE FLYING HAD A HISTORY OF PRESENTING FLUCTUATING SIGNALS FROM TIME TO TIME BUT THAT THE PROB HAD NEVER BEEN FULLY RECTIFIED THOUGH ATTEMPTS WERE MADE. I WAS UNAWARE OF THIS. THE CAPT DID RPT THE LOC FLUCTUATIONS TO THE AVIONICS SHOP. I DO NOT KNOW AT THIS TIME WHAT CORRECTIVE ACTION, IF ANY, HAD BEEN TAKEN. IT WAS DIFFICULT TO RECALL DETAILS SINCE WORKLOAD WAS HVY, BUT I BELIEVE THE ERRONEOUS SIGNALS GIVEN BY THE LOC NEEDLE CAUSED US TO DRIFT TO THE R OF COURSE.

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.