Narrative:

Prr flight #1 inertial reference had a delay maintenance status message. Maintenance MEL'd it as an indication problem. Took off and during climb #1 mcdu had a check gross weight message. No problem found in climb noticed captain's heading/trk was 40 degrees off of first officer's and had a headwind of 160 KTS. First officer was 10 KT tailwind. Switched captain's att heading switch to #3 ir all normal. Over las did a navigation position accuracy test. Normal except #1 ir was 13.0 miles off/diff. Notified maintenance. They informed us that they had reports from aircraft's cfds that #1 ir had minor internal problems. Over rwf, ATC asked if on a heading assigned (we were not). ATC said we were 10.13 miles off and going further. Captain and first officer navigation displays showed on course. Tuned to rwf VOR and saw that in fact we were off course. Right before ATC called us, we got a 'navigation accuracy down grade.' we asked for radar vectors to airport. During vectors I checked ir position. #1 ir showed 69.8 miles diff. After landing and taxi to gate, we finally got a #1 ir fault message. Had we been in mountainous non radar environment. This could have been a possible tragic event. I feel maintenance dispatched us with an incorrect MEL. I also feel that the aircraft's sensing/warning/detection system needs to be a lot more sensitive to warn of errors like this. Callback conversation with reporter revealed the following information: the IRU's in the aircraft in essence uses a triple mix position to send to the FMC's. This is then updated in the FMC by the aircraft navigation system. Apparently the single IRU error was enough to throw off the triple mix position, yet not far enough in error to get voted off line or trigger a warning. The moving map does not have the capability to show raw data IRU position on the map. The air carrier only requires the pilot monitor raw data when on approach. The air carrier chief pilot indicated that the air carrier may change SOP to require raw data monitoring beginning at the top of descent. Air carrier X was dscnding on the skeeter 2 arrival for landing at msp. Upon crossing rwf (redwood falls). Minneapolis center notified us that we were over 10 miles off course. Both the captain and first officer flight displays were indicating the aircraft was on course on the arrival. In addition, no warnings or indications were present for navigation faults. The number 1 adiru had a class ii deferral but this was not supposed to affect it's or the other 2 adiru units (inertial, radial navigation system) in anyway and there are no special procedures for operating this condition. The flight continued on vectors and a visual approach at the airport. After the aircraft taxiied off the landing runway, numerous navigation and related warnings appeared on the ECAM. The error recorded at this point was nearly 60 mi from where the aircraft was.

Related reports:

Google
 

Original NASA ASRS Text

Title: A-320 CREW HAD ACFT IRU'S MALFUNCTION PROVIDING FALSE NAVIGATION DATA TO THE ACFT.

Narrative: PRR FLT #1 INERTIAL REFERENCE HAD A DELAY MAINT STATUS MSG. MAINT MEL'D IT AS AN INDICATION PROB. TOOK OFF AND DURING CLB #1 MCDU HAD A CHK GROSS WT MSG. NO PROB FOUND IN CLB NOTICED CAPT'S HDG/TRK WAS 40 DEGREES OFF OF FO'S AND HAD A HEADWIND OF 160 KTS. FO WAS 10 KT TAILWIND. SWITCHED CAPT'S ATT HDG SWITCH TO #3 IR ALL NORMAL. OVER LAS DID A NAV POS ACCURACY TEST. NORMAL EXCEPT #1 IR WAS 13.0 MILES OFF/DIFF. NOTIFIED MAINT. THEY INFORMED US THAT THEY HAD RPTS FROM ACFT'S CFDS THAT #1 IR HAD MINOR INTERNAL PROBS. OVER RWF, ATC ASKED IF ON A HDG ASSIGNED (WE WERE NOT). ATC SAID WE WERE 10.13 MILES OFF AND GOING FURTHER. CAPT AND FO NAV DISPLAYS SHOWED ON COURSE. TUNED TO RWF VOR AND SAW THAT IN FACT WE WERE OFF COURSE. RIGHT BEFORE ATC CALLED US, WE GOT A 'NAV ACCURACY DOWN GRADE.' WE ASKED FOR RADAR VECTORS TO ARPT. DURING VECTORS I CHKED IR POS. #1 IR SHOWED 69.8 MILES DIFF. AFTER LNDG AND TAXI TO GATE, WE FINALLY GOT A #1 IR FAULT MSG. HAD WE BEEN IN MOUNTAINOUS NON RADAR ENVIRONMENT. THIS COULD HAVE BEEN A POSSIBLE TRAGIC EVENT. I FEEL MAINT DISPATCHED US WITH AN INCORRECT MEL. I ALSO FEEL THAT THE ACFT'S SENSING/WARNING/DETECTION SYS NEEDS TO BE A LOT MORE SENSITIVE TO WARN OF ERRORS LIKE THIS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE IRU'S IN THE ACFT IN ESSENCE USES A TRIPLE MIX POSITION TO SEND TO THE FMC'S. THIS IS THEN UPDATED IN THE FMC BY THE ACFT NAV SYS. APPARENTLY THE SINGLE IRU ERROR WAS ENOUGH TO THROW OFF THE TRIPLE MIX POS, YET NOT FAR ENOUGH IN ERROR TO GET VOTED OFF LINE OR TRIGGER A WARNING. THE MOVING MAP DOES NOT HAVE THE CAPABILITY TO SHOW RAW DATA IRU POS ON THE MAP. THE ACR ONLY REQUIRES THE PLT MONITOR RAW DATA WHEN ON APCH. THE ACR CHIEF PLT INDICATED THAT THE ACR MAY CHANGE SOP TO REQUIRE RAW DATA MONITORING BEGINNING AT THE TOP OF DSCNT. ACR X WAS DSCNDING ON THE SKEETER 2 ARR FOR LNDG AT MSP. UPON XING RWF (REDWOOD FALLS). MINNEAPOLIS CENTER NOTIFIED US THAT WE WERE OVER 10 MILES OFF COURSE. BOTH THE CAPT AND FO FLIGHT DISPLAYS WERE INDICATING THE ACFT WAS ON COURSE ON THE ARR. IN ADDITION, NO WARNINGS OR INDICATIONS WERE PRESENT FOR NAVIGATION FAULTS. THE NUMBER 1 ADIRU HAD A CLASS II DEFERRAL BUT THIS WAS NOT SUPPOSED TO AFFECT IT'S OR THE OTHER 2 ADIRU UNITS (INERTIAL, RADIAL NAVIGATION SYSTEM) IN ANYWAY AND THERE ARE NO SPECIAL PROCS FOR OPERATING THIS CONDITION. THE FLT CONTINUED ON VECTORS AND A VISUAL APCH AT THE ARPT. AFTER THE ACFT TAXIIED OFF THE LNDG RWY, NUMEROUS NAVIGATION AND RELATED WARNINGS APPEARED ON THE ECAM. THE ERROR RECORDED AT THIS POINT WAS NEARLY 60 MI FROM WHERE THE ACFT WAS.

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.