EMB ERJ 145 ER/LR
Confusion; Distraction; Troubleshooting; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft Automation Overrode Flight Crew; Flight Crew Became Reoriented; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft
We were descending on the BULLZ 5 arrival into ORD when the aircraft laterally deviated from the FMS course missing the turn from COGGS to the SOOLU Intersection.  For reasons that neither me nor the Captain can explain; the aircraft was in ROL mode instead of NAV.  The Captain identified the error and I corrected the aircraft back on course.  ATC questioned whether we were on the BULLZ 5.  The Captain said we had missed the turn and were already in the turn back on course.  No further action was needed and we continued to ORD. We are unsure as to why the aircraft was in ROL mode instead of NAV.  Neither of us recalls hearing an audible ding notifying us of the change to the FMA nor remembers making any unfavorable selections.  We believe that the aircraft must have entered into the undesired state by accident and without a clear explanation. It just goes to prove that vigilance in the cockpit is vital for a safe flight.  I am glad we caught the error when we did.
While on the ORD BULLZ 5 arrival; an EMB-145 FMS without knowledge from the flight crew; transitioned from the NAV to the ROL Mode which caused a track deviation but no apparent conflict.