Confusion; Training / Qualification
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; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Airspace Structure; Procedure
I was the pilot flying. Received a clearance shortly after level off to precede direct PGS. I entered it in the FMC; verified with non pilot flying (NFP) and executed. As workload permitted I added the fixes from the flight plan for abeam point monitoring; again verifying with NFP. (I used the plan mode on Nav screen and saw no anomalies. (Abeam fixes were KD51U and KD42Q).Over an hour later; ZDV complained that it appeared we were about 5 degrees off course to the north and asked us to verify. I re-entered PGS direct and found that the Controller was correct. The abeam points were not in a straight line with the direct course. To my knowledge we followed correct procedures; however the abeam points were a few degrees off. The Controller did not seem upset; nor were we aware of a conflict.Since a long distance was involved; a few degrees of course change between points on the FMC did not alarm me. I thought the abeam points were using great circle course.
A flight crew on a NRS (Navigational Reference System) flight plan was issued a clearance direct to a distant NAVAID then entered NRS abeam points which caused a five degree track deviation.