EMB ERJ 135 ER/LR
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert
ATC Equipment / Nav Facility / Buildings; Chart Or Publication; Aircraft
During our descent on the FRDMM ONE RNAV Arrival; just past WLLLL; the Controller asked us whether we showed ourselves on course for the arrival. I verified that the autopilot was engaged; it was coupled to the right side; and that the First Officer was navigating in pink needles with the lateral mode set to NAV. Everything was set up correctly and the MFD depicted us as on course. I then responded that we did show ourselves on course. The Controller then informed us that previous company flights; as well as ours; had been flying up to a mile off course and had been rejoining the arrival more precisely at SEPII. He explained that; to him; it looked like our planes were flying the course depicted on the ELDEE 5 RNAV Arrival. I asked the Controller if he could tell me at what point our plane started to fly off course; but he stated he was not sure. He stated that he would try to figure it out with the next inbound company flight. We continued on the arrival with no further incident.
ATC advised an ERJ flight crew descending via the FRDMM RNAV STAR into DCA that their track (as well as that of several previous 'company' flights) between WLLLL and SEPII deviated about a mile before rejoining at SEPII. The flight crew advised their FMS displays showed them on course with the autopilot engaged in the NAV mode.