A B757 flight crew experiences navigation instrument discrepancy during an approach.

Attributes
ACN	521260
Time
Date	200108
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : bos.airport
State Reference	MA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : multi engine pilot : commercial pilot : instrument pilot : flight engineer
ASRS Report	521260
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other anomaly other
Resolutory Action	aircraft : equipment problem dissipated controller : issued advisory controller : issued new clearance
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft ATC Human Performance
Primary Problem	Aircraft

Narrative:

A VMC day with hazy conditions. We were cleared to intercept localizer runway 4R. First officer was at the controls with autoplt engaged. Aircraft captured localizer, but flew through the course and turned back to the north to get on course. During this time the CDI was indicating normal (ie, showing us to the right off course with correction to center). The first officer had LNAV selected on his HSI and I had localizer selected on my HSI display. Localizer course is 036 degrees when I looked back at the HSI a short time later, the aircraft was still maintaining the heading to intercept (I think about 015 degrees or so), but my localizer CDI was to the right about 1 DOT off center. I questioned out loud as to what 'this thing is doing.' the first officer checked his HSI LNAV display and it too indicated we were left off track. Heading mode was selected and a turn was made to the right (about 070 degrees) in an effort to correct to course. The controller then asked if we were established and I told him we were about 1 DOT off. After a min or so we were still indicating left off course on both localizer and LNAV, with the localizer not moving toward center. Again we crosschecked LNAV and localizer and both indicated left off course. Around this time the controller said he showed us 8 NM right off course and had us break off approach and reenter. (We were now about 15 NM from bos.) the second approach was flown normally.

Original NASA ASRS Text

Title: A B757 FLT CREW EXPERIENCES NAV INST DISCREPANCY DURING AN APCH.

Narrative: A VMC DAY WITH HAZY CONDITIONS. WE WERE CLRED TO INTERCEPT LOC RWY 4R. FO WAS AT THE CTLS WITH AUTOPLT ENGAGED. ACFT CAPTURED LOC, BUT FLEW THROUGH THE COURSE AND TURNED BACK TO THE N TO GET ON COURSE. DURING THIS TIME THE CDI WAS INDICATING NORMAL (IE, SHOWING US TO THE R OFF COURSE WITH CORRECTION TO CTR). THE FO HAD LNAV SELECTED ON HIS HSI AND I HAD LOC SELECTED ON MY HSI DISPLAY. LOC COURSE IS 036 DEGS WHEN I LOOKED BACK AT THE HSI A SHORT TIME LATER, THE ACFT WAS STILL MAINTAINING THE HEADING TO INTERCEPT (I THINK ABOUT 015 DEGS OR SO), BUT MY LOC CDI WAS TO THE R ABOUT 1 DOT OFF CTR. I QUESTIONED OUT LOUD AS TO WHAT 'THIS THING IS DOING.' THE FO CHKED HIS HSI LNAV DISPLAY AND IT TOO INDICATED WE WERE L OFF TRACK. HEADING MODE WAS SELECTED AND A TURN WAS MADE TO THE R (ABOUT 070 DEGS) IN AN EFFORT TO CORRECT TO COURSE. THE CTLR THEN ASKED IF WE WERE ESTABLISHED AND I TOLD HIM WE WERE ABOUT 1 DOT OFF. AFTER A MIN OR SO WE WERE STILL INDICATING L OFF COURSE ON BOTH LOC AND LNAV, WITH THE LOC NOT MOVING TOWARD CTR. AGAIN WE XCHKED LNAV AND LOC AND BOTH INDICATED L OFF COURSE. AROUND THIS TIME THE CTLR SAID HE SHOWED US 8 NM R OFF COURSE AND HAD US BREAK OFF APCH AND REENTER. (WE WERE NOW ABOUT 15 NM FROM BOS.) THE SECOND APCH WAS FLOWN NORMALLY.

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.