Narrative:

This narrative is to describe an incident that we, as pilots, are told will not happen: the complete failure of the FMC system and the lack of any indication on the HSI that anything was amiss! Our first indication that we were not on course was a question from ZJX asking us when we planned on turning over a certain intersection on our route. We checked our HSI and informed him that we still had '5.3 mi to go.' his reply was that we had 'passed' the chkpoint 15 mi ago.' we checked our paper maps and manual VOR mode on the HSI and center was indeed correct. We continued to mco with the navigation system in manual without further incident. When we arrived at the gate, set the brakes, and shut off the engines, our FMC was indicating that we were approximately 30 mi west of airport!! We checked our position page 2 in the FMS and found that both right and left FMC's showed a 28 KT drift while the 3 IRS's had normal drift (1-2 KTS). If the reader is not familiar with the FMS system on our widebody transport, the above information will seem important. To those like myself who are everyday users of this system, the above information is, to say the least, shocking! In the 5 plus yrs I've operated this system, nothing remotely resembling the described event has taken place. It is just not supposed to happen! In my mind, it ranks right up there with an engine falling off the wing. If we were not in a radar environment under IFR conditions, it could have been a catastrophe! Callback conversation with reporter revealed the following information: the reporter stated that the 3 IRS's all said that the aircraft was at the gate at orlando while both FMC's showed the aircraft to be 28 mi away. The trip was from ny to orlando. A VOR saturated route, but there was no updating going on the entire way. Company maintenance changed the center IRS and sent the aircraft on its way after being unable to update the FMC's on the ground. The reporter said that he has written an article for his company safety magazine and that he will send ASRS a copy. The manufacturer of the FMC/IRS system claims that this cannot happen and that all hands are scratching their heads.

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Original NASA ASRS Text

Title: AN ACFT WITH ALMOST 'ALL OF THE GOODIES' HAD A NAV PROBLEM THAT THE MANUALS SAY 'CANNOT HAPPEN.' THE IRS SHOWED RIGHT ON COURSE, BUT THE FMC SHOWED 28 MI OFF WITH NO WARNINGS TO THE CREW AND NO UPDATING FROM ANY OF THE VORS ENRTE.

Narrative: THIS NARRATIVE IS TO DESCRIBE AN INCIDENT THAT WE, AS PLTS, ARE TOLD WILL NOT HAPPEN: THE COMPLETE FAILURE OF THE FMC SYS AND THE LACK OF ANY INDICATION ON THE HSI THAT ANYTHING WAS AMISS! OUR FIRST INDICATION THAT WE WERE NOT ON COURSE WAS A QUESTION FROM ZJX ASKING US WHEN WE PLANNED ON TURNING OVER A CERTAIN INTXN ON OUR RTE. WE CHKED OUR HSI AND INFORMED HIM THAT WE STILL HAD '5.3 MI TO GO.' HIS REPLY WAS THAT WE HAD 'PASSED' THE CHKPOINT 15 MI AGO.' WE CHKED OUR PAPER MAPS AND MANUAL VOR MODE ON THE HSI AND CTR WAS INDEED CORRECT. WE CONTINUED TO MCO WITH THE NAV SYS IN MANUAL WITHOUT FURTHER INCIDENT. WHEN WE ARRIVED AT THE GATE, SET THE BRAKES, AND SHUT OFF THE ENGS, OUR FMC WAS INDICATING THAT WE WERE APPROX 30 MI W OF ARPT!! WE CHKED OUR POS PAGE 2 IN THE FMS AND FOUND THAT BOTH R AND L FMC'S SHOWED A 28 KT DRIFT WHILE THE 3 IRS'S HAD NORMAL DRIFT (1-2 KTS). IF THE READER IS NOT FAMILIAR WITH THE FMS SYS ON OUR WDB, THE ABOVE INFO WILL SEEM IMPORTANT. TO THOSE LIKE MYSELF WHO ARE EVERYDAY USERS OF THIS SYS, THE ABOVE INFO IS, TO SAY THE LEAST, SHOCKING! IN THE 5 PLUS YRS I'VE OPERATED THIS SYS, NOTHING REMOTELY RESEMBLING THE DESCRIBED EVENT HAS TAKEN PLACE. IT IS JUST NOT SUPPOSED TO HAPPEN! IN MY MIND, IT RANKS RIGHT UP THERE WITH AN ENG FALLING OFF THE WING. IF WE WERE NOT IN A RADAR ENVIRONMENT UNDER IFR CONDITIONS, IT COULD HAVE BEEN A CATASTROPHE! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE 3 IRS'S ALL SAID THAT THE ACFT WAS AT THE GATE AT ORLANDO WHILE BOTH FMC'S SHOWED THE ACFT TO BE 28 MI AWAY. THE TRIP WAS FROM NY TO ORLANDO. A VOR SATURATED RTE, BUT THERE WAS NO UPDATING GOING ON THE ENTIRE WAY. COMPANY MAINT CHANGED THE CTR IRS AND SENT THE ACFT ON ITS WAY AFTER BEING UNABLE TO UPDATE THE FMC'S ON THE GND. THE RPTR SAID THAT HE HAS WRITTEN AN ARTICLE FOR HIS COMPANY SAFETY MAGAZINE AND THAT HE WILL SEND ASRS A COPY. THE MANUFACTURER OF THE FMC/IRS SYS CLAIMS THAT THIS CANNOT HAPPEN AND THAT ALL HANDS ARE SCRATCHING THEIR HEADS.

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.