Narrative:

Taking off on runway 18L we checked runway and apex of the aircraft symbol and all were aligned on he map; also the appropriate RNAV departure; SLOTT2 was in the FMC. On the takeoff roll and after lift off the aircraft was tracking fine. We armed LNAV at 500 ft and engaged the autopilot at about 800 ft. Again everything was normal just as we were making the right hand turn at bpart ATC gave us a call and asked our position; just that moment we had a map shift to our left and this was both on the first officer's and captain's side. ATC said that we were a mile north of course and we concurred; but we showed .6 miles north of course. There was no conflict with other aircraft and we were given vectors to slott intersection. Our concern here is that we did everything correct; but can't figure out what could have caused the map shift. We did have several mels on our plane; one being the center autopilot being inoperative which required a maintenance inspection prior to every flight; but we don't see where that would give us a map shift. At the end of our flight in sfo all of our IRU's were well within their tolerances. One other note; our aircraft was a non pegasus FMC equipped aircraft; no GPS.callback conversation with reporter revealed the following information: the reporter stated that the crew followed all applicable preflight system accuracy checks before takeoff and were very surprised at the track deviation. A 0.6 NM FMS cross track error was seen by the crew while ATC reported a 1 NM error. Attempting to reconstruct the deviation; the reporter talked with his chief pilot who stated that apparently the 80 KT FMS update did not occur during the takeoff roll. Because the frequency of this error type is increasing; the reporter's chief pilot said that the air carrier is considering accelerating the pegasus upgrade schedules. The crew flew an FMS arrival with this same aircraft on this flight with near perfect performance and no tracking errors.

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Original NASA ASRS Text

Title: A non Pegasus (non GPS) equipped B757 experienced a map shift and track deviation after takeoff even though all applicable preflight procedures were followed intended to ensure FMS performance accuracy.

Narrative: Taking off on Runway 18L we checked runway and apex of the aircraft symbol and all were aligned on he map; also the appropriate RNAV departure; SLOTT2 was in the FMC. On the takeoff roll and after lift off the aircraft was tracking fine. We armed LNAV at 500 FT and engaged the autopilot at about 800 FT. Again everything was normal just as we were making the right hand turn at BPART ATC gave us a call and asked our position; just that moment we had a map shift to our left and this was both on the First Officer's and Captain's side. ATC said that we were a mile north of course and we concurred; but we showed .6 miles north of course. There was no conflict with other aircraft and we were given vectors to SLOTT intersection. Our concern here is that we did everything correct; but can't figure out what could have caused the map shift. We did have several MELS on our plane; one being the center autopilot being inoperative which required a maintenance inspection prior to every flight; but we don't see where that would give us a map shift. At the end of our flight in SFO all of our IRU's were well within their tolerances. One other note; our aircraft was a non Pegasus FMC equipped aircraft; no GPS.Callback conversation with reporter revealed the following information: The reporter stated that the crew followed all applicable preflight system accuracy checks before takeoff and were very surprised at the track deviation. A 0.6 NM FMS cross track error was seen by the crew while ATC reported a 1 NM error. Attempting to reconstruct the deviation; the reporter talked with his chief pilot who stated that apparently the 80 KT FMS update did not occur during the takeoff roll. Because the frequency of this error type is increasing; the reporter's chief pilot said that the air carrier is considering accelerating the Pegasus upgrade schedules. The crew flew an FMS arrival with this same aircraft on this flight with near perfect performance and no tracking errors.

Data retrieved from NASA's ASRS site as of April 2012 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.