Narrative:

Aircraft was reported by ATC as 4 mi north of course. IRS navigation showed on course. When we switched to VOR's to verify course, we found ATC was correct. Shortly thereafter, we got FMC message, 'verify position.' somehow an error had entered the IRS navigation system and the message was not displayed on the FMC before ATC had noted the 4-5 mi error. VOR/ILS was used for the rest of the flight.

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Original NASA ASRS Text

Title: ACR MLG TRACK DEVIATION USING IRS NAVIGATION SYSTEM.

Narrative: ACFT WAS RPTED BY ATC AS 4 MI N OF COURSE. IRS NAV SHOWED ON COURSE. WHEN WE SWITCHED TO VOR'S TO VERIFY COURSE, WE FOUND ATC WAS CORRECT. SHORTLY THEREAFTER, WE GOT FMC MESSAGE, 'VERIFY POS.' SOMEHOW AN ERROR HAD ENTERED THE IRS NAV SYS AND THE MESSAGE WAS NOT DISPLAYED ON THE FMC BEFORE ATC HAD NOTED THE 4-5 MI ERROR. VOR/ILS WAS USED FOR THE REST OF THE FLT.

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.