Regional Jet 900 (CRJ900)
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Distraction; Human-Machine Interface
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Human Factors
While in cruise at FL340; ARTCC advised us that we were 5-6 miles off course. Upon hearing this; I noticed the FMA showing we were in HDG mode. ARTCC cleared us to turn back left and rejoin QXXX and we did. No other issues were noted. I was Pilot Monitoring for the flight and allowed myself to not be watching the FMA for a bit of time after level off at cruise altitude. I was looking at fuel calculations since ZZZ had weather all morning and was expecting to be slowed and possibly hold in route. I was noticing some discrepancy in my calculations based on fuel flow numbers and was looking into that. My MFD was on MFD advance (not FMS plan map) and my attention was diverted from monitoring the aircraft. I had chosen to do this once cruise flight was reached. The autopilot was on; and FMS was the active lateral mode. At some point; HDG mode became active. I hadn't noticed this selected and there was no reason for it to be selected as we had been on our FMS course and were not assigned a heading. I honestly do not know how HDG mode became the active mode. Alas; I was too distracted with fuel calculations and should have been watching the FMA closer.Even when busy with other items; in the future I will not turn my attention away from the FMA for so long.
CRJ-900 Captain reported becoming distracted while performing a fuel calculation; resulting in a navigation error.