Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Aircraft; Human Factors
After completing the climb checklist; the First Officer (Pilot Monitoring) pointed out that the two FMS's were not aligned and that hers showed us left of course. Given that information; I failed to tell TRACON that we were unsure of the accuracy of our navigation instruments. First Officer monitored the #2 FMS and emphasized that it was continuing to show off course; with the course deviating to the east. Just at the point that I was going to heading mode to turn in a more westerly direction and let ATC know that we were concerned about our navigation instruments; the Controller gave us a turn to the west and told us we had been .10 off course and then drifted to approximately .20 off course to the east. The responsibility for this deviation rests solely with me. I was given excellent information by the First Officer and failed to act upon it when it was first offered. I do not know why the course diverged from that of the NELYN 3 departure course. Q parameters were met and both FMS's were GPS. If there is a .1 variation in course deviation on an RNAV departure (or arrival) immediately ask ATC for heading information as navigation capabilities are suspect.
The reporter could offer no explanation for the discrepancy between the two FMS's but indicates that it does occasionally happen in this aircraft.
Embraer Captain experiences a track deviation departing DFW on the NELYN 3 departure. A slight discrepancy existed between the Captain's and First Officer's display with the First Officer's apparently correct; according to ATC.