Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 189; Flight Crew Total 6399; Flight Crew Type 2900
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew FLC Overrode Automation
Aircraft
We departed SAN runway 27. Normal take off to a heading of 290 for noise abatement. Upon reaching 7000 ft MSL; we received the instructions to fly direct to PGY VOR via the left turn (almost a 180 degree turn). We set PGY VOR on top in the FMC and upon reaching 90 degree intercept executed and confirmed that NAV function is 'tracking' inbound direct to PGY VOR; in both NAV and VOR function. At approximately 2.9 DME to PGY VOR; aircraft made a slight left turn at about 10 degrees off course indicated; which from what we were seeing on our MFD's didn't make any sense. At the same time the controller said 'I see you in a left turn direct to PGY VOR'. This caused confusion on the flight deck; as well as prompted the controller to ask us if our equipment is operational. There was no abnormal or caution messages displayed. As soon as we situated ourselves; we started troubleshooting and couldn't find the issue. We also checked with center controller and as it was a clear night confirmed we are on the cleared route. We continued flying now in VOR mode as when the problem occurred it happened in NAV.Upon descent to [our destination] our FMS displayed 'FMS NAV DR' 'NAV BUS FAIL' and 'XFEED INOP'.Upon landing we recycled power to the FMC and the fault messages cleared. I made an entry in the logbook describing the erroneous indication as well as the annunciators observed. I believe that we have experienced NAV failure without any indications which caused us to deviate slightly right of the PGY VOR. Either way DME indicated 2.9 NM TO PGY VOR when the turn started. Paying closer attention and flying in both NAV and VOR modes simultaneously monitoring both 'pictures' --not only selecting it--would of prevented the initial confusion and possible deviation.
An MD-83 flight crew got confusing NAV information between their FMS and VOR based systems. They opted to continue to destination utilizing the VOR. Approaching their destination they received CAS messages reflecting FMS and NAV buss problems. Upon recycling power to the FMC post landing the fault messages cleared. They wrote up the event in the logbook.