Human-Machine Interface; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Human Factors
While in cruise with the autopilot on we were asked if we had the new STAR into COS. We did and ATC told us to expect that soon. We were given a pilots discretion descent from FL340 to FL270. I had not begun my descent. I started briefing the arrival and descent planning. In the middle of this we were cleared direct OZZZY which is on the STAR and that at FL270 to descend via the arrival. We were over LAA when cleared. I selected HDG on the FGCP while I loaded the arrival in the FMS. We were heading 310. I correctly loaded the arrival in the FMS and selected direct to OZZZY. The track to OZZZY was 300. I failed to select LNAV on the FGCP so the aircraft remained on the last heading. I continued with my arrival briefing and about the time I was finished briefing ATC asked if we were direct OZZZY. Realizing what happened we told them we were now turning direct OZZZY. ATC told us to turn to heading 290 immediately for traffic. He also told another aircraft to turn to a heading because of us. When I looked at the TCAS it was set to 20 mile range and I could see an aircraft but it was between 10-20 miles away and only blue in color. Shortly after ATC instructed us to proceed direct OZZZY which we did. The flight ended without further incident. The threat was present when we were given and accepted a new route. ATC asked if we had the new STAR which we did and so he cleared us via that. We anticipated and started briefing the entire procedure but only ended up being cleared to OZZZY and one waypoint after that remained on the arrival. The error present was the lack of verifying the aircraft was navigating according to the new flight plan. I selected HDG while I loaded the new route but did not select NAV after the route was loaded. I verbalized the new route and what to expect for the arrival but I failed to verify the airplane was navigating and the Captain failed to cross check after confirming the new route that we were in fact navigating on that route.
EMB145 First Officer reports a track deviation after being cleared direct to a fix and entering it into the FMC but failing to select NAV on the FGCP. ATC detects the error and issues headings to the reporter and another aircraft to avoid conflict.