Date: 201711
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Aircraft: Citation Latitude (C680A)
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Flight Director
Component problem: Improperly Operated
Reporter organization: Fractional
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance; Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: While in cruise flight; I turned the controls over to the Captain; and I left the cockpit to check on the operation of the iPads in the cabin as they were written up for not being able to connect to the inflight entertainment system.While I was gone; the aircraft (with AP engaged); failed to make a course turn with a dog-leg at ZZZZZ intersection. The controller queried the Captain; and the Captain then noticed the deviation; and that the FMS/Auto Pilot had failed to make the turn at ZZZZZ despite being in FMS mode. The Captain then re-sequenced the FMS to make the next stop leg the active way point; and the aircraft turned back onto the course. Upon my return to the flight deck; the Captain described what had happened; and stated that he had probably flown no more than 3 miles past ZZZZZ before correcting back to the proper course.Upon hearing the description; it sounded like the active leg had been suspended similar to when one would engage OBS mode to change the active course using the course selector knob. I brought up the Garmin 5000 manuals on my iPad; and read the description of what the OBS knob does; and surmised what might have happened. Earlier; and prior to leveling off at FL400; I had used my PFD GTC (Primary Flight Display Garmin Touchscreen Controller) to select MONITOR FO COMM 2; in order to listen to guard frequency. I recall that we hit some descent turbulence just as I was trying to press the small 'MON' button; and that I had to hit it again as the first touch landed lower on the screen.Well; the OBS button is directly underneath the audio selector; and all we could figure is that I must have engaged OBS mode; thus suspending the sequencing of the FMS way points; and when the Captain forced the next leg (after ZZZZZ intersection) to be the active way point; OBS mode was then deselected. This fit with the description of OBS mode exactly. It would be better to have the OBS mode button lower on the GTC; maybe where the timer button is; that way; if one accidentally touches the button while trying to select an audio MON (a commonly used function); then all that would happen is that you would bring up the timer panel. Likewise; it would be helpful to have a white CAS message pop up after a certain amount of time in OBS mode; as one would generally only select OBS to manually make a course change; which would be a quick operation. Likewise; it would be helpful to have another alert (white CAS message; or general message) if one were in OBS mode while approaching a course way point; as the FMS is basically suspended while in OBS mode.




Synopsis: CE-680 First Officer reported that the aircraft failed to make a course turn while the autopilot was engaged.