Narrative:

After departing; I noticed a navigation error when trying to intercept A-699 airway. The aircraft moving map and present position was off by over 1000 mi. I suspected a GPS/inertial reference error. We received vectors inland so we could navigation using vortacs. Once over omn and chs vortacs; I tried to update our 'present position' using a known fix. This did not work. Our map and position kept 'drifting.' ATC also said we were off course. Our aircraft VOR's said we were on course. The WX was clear in the area and marginal in lga. We also contacted our maintenance control via a VHF/phone patch. We did not get any useful information from them. I then decided to divert to chs where the WX was clear and have the system looked at. After that; I hastily made the decision to attempt an in-air reset of the IRS. That reset put the aircraft in a 'degraded' flight mode (no autoplt; flight director; autothrust; and operating on standby instruments). That reset was unrecoverable (which I knew) in-flight. I landed the aircraft uneventfully in chs where maintenance reset; checked; and applied the appropriate MEL to the aircraft. Until the inertial reference system reset the divert was 'normal.' before resetting that system; I should have 'stepped back and slowed down.' that thought process would have allowed me to thoroughly discuss the event with my first officer. It would have also allowed me the time to remember resetting the inertial reference system in-flight is a critical/unrecoverable action. I never regret my decision to divert; just my hastily made decision to attempt an unauthorized reset which made the situation worse.

Google
 

Original NASA ASRS Text

Title: AN A320 NAV SYSTEM DEVELOPED A 1;000 NM ERROR SHORTLY AFTER TKOF.

Narrative: AFTER DEPARTING; I NOTICED A NAV ERROR WHEN TRYING TO INTERCEPT A-699 AIRWAY. THE ACFT MOVING MAP AND PRESENT POS WAS OFF BY OVER 1000 MI. I SUSPECTED A GPS/INERTIAL REF ERROR. WE RECEIVED VECTORS INLAND SO WE COULD NAV USING VORTACS. ONCE OVER OMN AND CHS VORTACS; I TRIED TO UPDATE OUR 'PRESENT POS' USING A KNOWN FIX. THIS DID NOT WORK. OUR MAP AND POS KEPT 'DRIFTING.' ATC ALSO SAID WE WERE OFF COURSE. OUR ACFT VOR'S SAID WE WERE ON COURSE. THE WX WAS CLR IN THE AREA AND MARGINAL IN LGA. WE ALSO CONTACTED OUR MAINT CTL VIA A VHF/PHONE PATCH. WE DID NOT GET ANY USEFUL INFO FROM THEM. I THEN DECIDED TO DIVERT TO CHS WHERE THE WX WAS CLR AND HAVE THE SYS LOOKED AT. AFTER THAT; I HASTILY MADE THE DECISION TO ATTEMPT AN IN-AIR RESET OF THE IRS. THAT RESET PUT THE ACFT IN A 'DEGRADED' FLT MODE (NO AUTOPLT; FLT DIRECTOR; AUTOTHRUST; AND OPERATING ON STANDBY INSTS). THAT RESET WAS UNRECOVERABLE (WHICH I KNEW) INFLT. I LANDED THE ACFT UNEVENTFULLY IN CHS WHERE MAINT RESET; CHKED; AND APPLIED THE APPROPRIATE MEL TO THE ACFT. UNTIL THE INERTIAL REF SYS RESET THE DIVERT WAS 'NORMAL.' BEFORE RESETTING THAT SYS; I SHOULD HAVE 'STEPPED BACK AND SLOWED DOWN.' THAT THOUGHT PROCESS WOULD HAVE ALLOWED ME TO THOROUGHLY DISCUSS THE EVENT WITH MY FO. IT WOULD HAVE ALSO ALLOWED ME THE TIME TO REMEMBER RESETTING THE INERTIAL REF SYS INFLT IS A CRITICAL/UNRECOVERABLE ACTION. I NEVER REGRET MY DECISION TO DIVERT; JUST MY HASTILY MADE DECISION TO ATTEMPT AN UNAUTH RESET WHICH MADE THE SITUATION WORSE.

Data retrieved from NASA's ASRS site as of May 2009 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.