Narrative:

We were cleared for an approach to 26R in atl. The approach had been briefed and the first officer was PF. He elected to shoot a coupled approach and set up to do so. After receiving approach clearance, first officer armed the system to capture the ILS. He then switched his navigation display to arc mode with captain in missed approach mode. Both navigation receivers were on 110.1. ILS 26R. Autoplt captured the localizer signal and began tracking. Aircraft began chasing the localizer signal left and right. Copilot disconnected autoplt and began hand flying localizer signal. We were outside FAF when approach called and told us to turn 30 degrees right and reintercept localizer. A quick check of first officer raw data showed that we were on course but we turned to assigned heading anyway. Captain then switched his navigation display to arc mode and noted that his display did indeed show us well left of course. About the same time the comparator light came on ILS. We asked to be pulled off approach to sort out which ILS was giving wrong information. During second approach, it was determined that copilot's #2 navigation was getting bad information so the digital flight guidance was switched to #1 and captain flew approach to landing. Approach control was asked to monitor our course which they did. On arrival, maintenance replaced #2 navigation receiver.

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Original NASA ASRS Text

Title: ACR HAS NAV EQUIP PROB. EXECUTES MISSED APCH WHILE TROUBLESHOOTING, THEN LANDS.

Narrative: WE WERE CLRED FOR AN APCH TO 26R IN ATL. THE APCH HAD BEEN BRIEFED AND THE FO WAS PF. HE ELECTED TO SHOOT A COUPLED APCH AND SET UP TO DO SO. AFTER RECEIVING APCH CLRNC, FO ARMED THE SYS TO CAPTURE THE ILS. HE THEN SWITCHED HIS NAV DISPLAY TO ARC MODE WITH CAPT IN MISSED APCH MODE. BOTH NAV RECEIVERS WERE ON 110.1. ILS 26R. AUTOPLT CAPTURED THE LOC SIGNAL AND BEGAN TRACKING. ACFT BEGAN CHASING THE LOC SIGNAL L AND R. COPLT DISCONNECTED AUTOPLT AND BEGAN HAND FLYING LOC SIGNAL. WE WERE OUTSIDE FAF WHEN APCH CALLED AND TOLD US TO TURN 30 DEGS R AND REINTERCEPT LOC. A QUICK CHK OF FO RAW DATA SHOWED THAT WE WERE ON COURSE BUT WE TURNED TO ASSIGNED HDG ANYWAY. CAPT THEN SWITCHED HIS NAV DISPLAY TO ARC MODE AND NOTED THAT HIS DISPLAY DID INDEED SHOW US WELL L OF COURSE. ABOUT THE SAME TIME THE COMPARATOR LIGHT CAME ON ILS. WE ASKED TO BE PULLED OFF APCH TO SORT OUT WHICH ILS WAS GIVING WRONG INFO. DURING SECOND APCH, IT WAS DETERMINED THAT COPLT'S #2 NAV WAS GETTING BAD INFO SO THE DIGITAL FLT GUIDANCE WAS SWITCHED TO #1 AND CAPT FLEW APCH TO LNDG. APCH CTL WAS ASKED TO MONITOR OUR COURSE WHICH THEY DID. ON ARR, MAINT REPLACED #2 NAV RECEIVER.

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.