Date: 201705
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: A full IRS alignment was completed on the ground prior to flight. There were no position inaccuracies seen on the ground with relation to the runway alignment for takeoff. While climbing through 11;000 ft; we encountered a map shift. LNAV/VNAV were engaged with the center autopilot also on. Aircraft turned about 30 degrees right from previous course to intercept magenta line. Progress page 2 indicated 0.5 NM left of course. Aircraft intercepted course and proceeded on course for remainder of flight with 0 deviation indicated or observed. Later on the arrival and approaching the ZZZZZ intersection; ATC assigned a heading and asked us to verify that we were on the Arrival. We answered in the affirmative and observed 0 deviation from course on Progress page 2 with LNAV engaged; reporting that we showed directly over the ZZZZZ intersection. ATC reported showing us 1 NM west. Both VOR mode selectors were confirmed in the AUTO mode.


Second narrative: Approaching ZZZZZ intersection at 4;000 feet on the Arrival; Approach asked us if we were still on the arrival; to which the captain as PM responded yes. The approach controller responded that he was showing us 1 mile west of course. The captain stated that we were showing on course; and by that point he indicated we were showing directly over ZZZZZ intersection as cleared. The controller thereafter gave us vectors to align with the procedure ground track; however our indications on both sides of the cockpit with zero crosstrack error; as loaded from the database; thereafter showed us offset from our map indications. As a matter of reference; earlier in the arrival the captain and I both noticed what appeared to be a rapid and transient map shift with a cross-track error approximately .5 miles left of course. It was momentary; and the FMC almost immediately began a correction back to the STAR course line and soon thereafter to zero cross-track error. The subsequent comment from the Controller indicated we were essentially paralleling the course line; and affirmed the likelihood that we had an FMS cockpit indication anomaly. Upon landing and block-in; the captain and I briefed the mechanic on what occurred; and made the appropriate entry in the aircraft logbook.


Synopsis: B757 flight crew reported experiencing a map shift of about a half mile during arrival. Later during arrival; ATC informed the crew they were slightly off course and issued vectors.