While on the ord bullz 5 arrival; an emb-145 FMS without knowledge from the flight crew; transitione...

Narrative:

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 navigation. 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 navigation. 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1117434
Time
Date	201309
Local Time Of Day	1201-1800
Place
Locale Reference	ZAU.ARTCC
State Reference	IL
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	EMB ERJ 145 ER/LR
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	STAR BULLZ 5
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	First Officer Pilot Flying
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Deviation - Track / Heading All Types

While on the ord bullz 5 arrival; an emb-145 FMS without knowledge from the flight crew; transitioned from the navigation to the rol mode which caused a track deviation but no apparent conflict.