Narrative:

While I, the captain, was in bathroom, yeg center advised aircraft was south of course on ncac, 190 NM northeast of ymm. Heading was used to return aircraft to course when ymm VORTAC was receivable. An INS update had not been accomplished for approximately 5 hours. After an update was done southwest of ymm, INS navigation was re-established. After shutdown, #2 and #3 INS units were out of drift error tolerance. An aircraft log entry was made. Supplemental information from acn 818955: place VOR/DME further in northern canada for updating. Supplemental information from acn 518943: all 3 INS's showed directly on centerline, with all coordinates for ysf and ymm agreeing with both flight plan and chart. Lutten ymm VOR came into range, it showed us on the 032 degree radial at 140 DME, instead of the expected 024 degree radial. Checking INS position 'inert compare' page showed all 3 INS's with up to 12 NM off on updt. After receiving VOR stations to recommence updating, #1 INS updt error reduced to less than 1.0, while #2 and #3 initially reduced, then increased. The #2 error was greater than 18 and #3 error was greater than 15.

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Original NASA ASRS Text

Title: A DC10 FLC STRAYS OFF COURSE OVER CANADA WHEN FAILING TO UPDATE THEIR INS FOR OVER 5 HRS ON A FLT FROM EUROPE. DEV NOTED BY CTLR AT CZEG, AB.

Narrative: WHILE I, THE CAPT, WAS IN BATHROOM, YEG CTR ADVISED ACFT WAS S OF COURSE ON NCAC, 190 NM NE OF YMM. HDG WAS USED TO RETURN ACFT TO COURSE WHEN YMM VORTAC WAS RECEIVABLE. AN INS UPDATE HAD NOT BEEN ACCOMPLISHED FOR APPROX 5 HRS. AFTER AN UPDATE WAS DONE SW OF YMM, INS NAV WAS RE-ESTABLISHED. AFTER SHUTDOWN, #2 AND #3 INS UNITS WERE OUT OF DRIFT ERROR TOLERANCE. AN ACFT LOG ENTRY WAS MADE. SUPPLEMENTAL INFO FROM ACN 818955: PLACE VOR/DME FURTHER IN NORTHERN CANADA FOR UPDATING. SUPPLEMENTAL INFO FROM ACN 518943: ALL 3 INS'S SHOWED DIRECTLY ON CTRLINE, WITH ALL COORDINATES FOR YSF AND YMM AGREEING WITH BOTH FLT PLAN AND CHART. LUTTEN YMM VOR CAME INTO RANGE, IT SHOWED US ON THE 032 DEG RADIAL AT 140 DME, INSTEAD OF THE EXPECTED 024 DEG RADIAL. CHKING INS POS 'INERT COMPARE' PAGE SHOWED ALL 3 INS'S WITH UP TO 12 NM OFF ON UPDT. AFTER RECEIVING VOR STATIONS TO RECOMMENCE UPDATING, #1 INS UPDT ERROR REDUCED TO LESS THAN 1.0, WHILE #2 AND #3 INITIALLY REDUCED, THEN INCREASED. THE #2 ERROR WAS GREATER THAN 18 AND #3 ERROR WAS GREATER THAN 15.

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.