Narrative:

Proceeding direct cartr while descending on the arrival to slc in B767-200, first officer flying with autoplt on, LNAV and VNAV engaged. Just felt FMS was not giving us correct position from cartr. Crosschecked on fix page and manual VOR -- ogd radial. All checked ok on map. When called approach to verify, no answer. Made 3 calls, still no answer. Finally switched to tower and instructed to return to frequency we were originally on. Approach answered we had passed cartr by 20 mi (west). Informed approach FMS had become inaccurate -- change type aircraft -- needed vectors. On final to runway 16R, I extended runway on FMS and at touchdown we showed 62 mi from runway. Discovered all 3 inertial reference units over 1 degree east of correct position. Crosschecked gate position at boston (our starting point) -- its latitude/longitude was correct. I also checked aircraft position on map as we turned to active runway -- showed us on runway 22R (bos). This turned out fine because we visually felt uncomfortable. (By looking at the various valleys we thought cartr was in.) if IMC, would have driven extra 25 mi to cartr fat, dumb, and blind. Still don't understand what happened to approach control on the radio. Must have been in blind spot? We trust the magic so much -- we had no clues. One pilot could have been in manual to verify crossing radials from ogden but who thought all 3 inertial reference units could get lost on longitude only, latitude was correct. No FMC message. It's an eye opener!

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Original NASA ASRS Text

Title: B767-200 ACFT ON STAR ARR HAD COM PROBS CONTACTING APCH CTLR AND WHEN FINALLY ESTABLISHING COM THE FLC WAS INFORMED THEY WERE OFF COURSE. AFTER CHKING NAV ACCURACY THEY FOUND ALL 3 INERTIAL REF UNITS OUT OF TOLERANCE (60 MI OFF). ATC VECTORED ACFT FOR APCH AND LNDG.

Narrative: PROCEEDING DIRECT CARTR WHILE DSNDING ON THE ARR TO SLC IN B767-200, FO FLYING WITH AUTOPLT ON, LNAV AND VNAV ENGAGED. JUST FELT FMS WAS NOT GIVING US CORRECT POS FROM CARTR. XCHKED ON FIX PAGE AND MANUAL VOR -- OGD RADIAL. ALL CHKED OK ON MAP. WHEN CALLED APCH TO VERIFY, NO ANSWER. MADE 3 CALLS, STILL NO ANSWER. FINALLY SWITCHED TO TWR AND INSTRUCTED TO RETURN TO FREQ WE WERE ORIGINALLY ON. APCH ANSWERED WE HAD PASSED CARTR BY 20 MI (W). INFORMED APCH FMS HAD BECOME INACCURATE -- CHANGE TYPE ACFT -- NEEDED VECTORS. ON FINAL TO RWY 16R, I EXTENDED RWY ON FMS AND AT TOUCHDOWN WE SHOWED 62 MI FROM RWY. DISCOVERED ALL 3 INERTIAL REF UNITS OVER 1 DEG E OF CORRECT POS. XCHKED GATE POS AT BOSTON (OUR STARTING POINT) -- ITS LATITUDE/LONGITUDE WAS CORRECT. I ALSO CHKED ACFT POS ON MAP AS WE TURNED TO ACTIVE RWY -- SHOWED US ON RWY 22R (BOS). THIS TURNED OUT FINE BECAUSE WE VISUALLY FELT UNCOMFORTABLE. (BY LOOKING AT THE VARIOUS VALLEYS WE THOUGHT CARTR WAS IN.) IF IMC, WOULD HAVE DRIVEN EXTRA 25 MI TO CARTR FAT, DUMB, AND BLIND. STILL DON'T UNDERSTAND WHAT HAPPENED TO APCH CTL ON THE RADIO. MUST HAVE BEEN IN BLIND SPOT? WE TRUST THE MAGIC SO MUCH -- WE HAD NO CLUES. ONE PLT COULD HAVE BEEN IN MANUAL TO VERIFY XING RADIALS FROM OGDEN BUT WHO THOUGHT ALL 3 INERTIAL REF UNITS COULD GET LOST ON LONGITUDE ONLY, LATITUDE WAS CORRECT. NO FMC MESSAGE. IT'S AN EYE OPENER!

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.