Narrative:

During our climb to cruise altitude, the wind vector computed by the FMC showed the wind to be approximately 295 degrees at 70 KTS. The autoplt as well as the FMS lateral navigation mode were engaged. The aircraft appeared to be tracking the course correctly based on the displayed wind information. In the vicinity of lancaster VOR, the displayed wind vector abruptly shifted to indicate winds from the ssw at 160 KTS. We noticed the shift, but were not unduly concerned because we were expecting strong winds at altitude (we were flight planned for a lower cruise altitude than normal due to forecast strong winds aloft). The autoplt changed aircraft heading approximately 30 degrees left in response to FMC commands (reflecting the new wind information). With the autoplt flying the new heading, the FMS/EFIS showed us tracking the course correctly (J6 from lrp to mrb) albeit with a 20-30 degree left wind correction angle. After some time on this heading, ATC queried us about our course, gave us a heading for direct mrb VOR and asked us to have our navigation equipment checked. For the remainder of the flight we made sure the PNF displayed raw data on his EFIS to xchk FMC operation. We did not notice any additional problems on that flight. We entered the fault in the aircraft maintenance logbook and informed flight control of the situation via ACARS. It is noteworthy that the erroneous heading commanded by the FMC would have been correct if the wind really was ssw 160 KTS. The problem, therefore, seems to have arisen because for some reason the FMC computed incorrect wind information. In future, any abrupt change in lateral/vertical navigation by the autoplt/FMS should be immediately investigated in order to avoid gross navigation altitude errors.

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Original NASA ASRS Text

Title: FLC OF AN MD88 WERE TAKEN OFF COURSE BY AN ERROR IN THE FMS. ARTCC RADAR CTLR INTERVENED AND VECTORED THEM ON A DIRECT COURSE TO THE NEXT VOR.

Narrative: DURING OUR CLB TO CRUISE ALT, THE WIND VECTOR COMPUTED BY THE FMC SHOWED THE WIND TO BE APPROX 295 DEGS AT 70 KTS. THE AUTOPLT AS WELL AS THE FMS LATERAL NAV MODE WERE ENGAGED. THE ACFT APPEARED TO BE TRACKING THE COURSE CORRECTLY BASED ON THE DISPLAYED WIND INFO. IN THE VICINITY OF LANCASTER VOR, THE DISPLAYED WIND VECTOR ABRUPTLY SHIFTED TO INDICATE WINDS FROM THE SSW AT 160 KTS. WE NOTICED THE SHIFT, BUT WERE NOT UNDULY CONCERNED BECAUSE WE WERE EXPECTING STRONG WINDS AT ALT (WE WERE FLT PLANNED FOR A LOWER CRUISE ALT THAN NORMAL DUE TO FORECAST STRONG WINDS ALOFT). THE AUTOPLT CHANGED ACFT HDG APPROX 30 DEGS L IN RESPONSE TO FMC COMMANDS (REFLECTING THE NEW WIND INFO). WITH THE AUTOPLT FLYING THE NEW HDG, THE FMS/EFIS SHOWED US TRACKING THE COURSE CORRECTLY (J6 FROM LRP TO MRB) ALBEIT WITH A 20-30 DEG L WIND CORRECTION ANGLE. AFTER SOME TIME ON THIS HDG, ATC QUERIED US ABOUT OUR COURSE, GAVE US A HDG FOR DIRECT MRB VOR AND ASKED US TO HAVE OUR NAV EQUIP CHKED. FOR THE REMAINDER OF THE FLT WE MADE SURE THE PNF DISPLAYED RAW DATA ON HIS EFIS TO XCHK FMC OP. WE DID NOT NOTICE ANY ADDITIONAL PROBS ON THAT FLT. WE ENTERED THE FAULT IN THE ACFT MAINT LOGBOOK AND INFORMED FLT CTL OF THE SIT VIA ACARS. IT IS NOTEWORTHY THAT THE ERRONEOUS HDG COMMANDED BY THE FMC WOULD HAVE BEEN CORRECT IF THE WIND REALLY WAS SSW 160 KTS. THE PROB, THEREFORE, SEEMS TO HAVE ARISEN BECAUSE FOR SOME REASON THE FMC COMPUTED INCORRECT WIND INFO. IN FUTURE, ANY ABRUPT CHANGE IN LATERAL/VERT NAV BY THE AUTOPLT/FMS SHOULD BE IMMEDIATELY INVESTIGATED IN ORDER TO AVOID GROSS NAV ALT ERRORS.

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.