Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 120; Flight Crew Total 11350; Flight Crew Type 2700
Confusion; Situational Awareness; Troubleshooting
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Procedure; Human Factors
On departure from KATL were cleared for takeoff on 08R to fly the VARNM2 departure RNAV to RONII; RESPE transition. We deviated approximately 0.4 miles south of the 082 course to RONII due to the fact that we were in HDG and not NAV on the FMA (Flight Mode Annunciator). Upon realizing that the aircraft was not in NAV we immediately corrected using HDG SEL to rejoin the course. It was at about that same time Atlanta Tower queried us about our track I advised we were turning to rejoin the course to RONII. The controller asked what the heading we were turning thru and I advised 080. We continued and rejoined the departure in NAV mode. No further comments were made by ATC. My FO (First Officer) and I had flown this departure earlier and recognized the fact that we needed to pay very close attention to the RNAV departure and that we were very tired since fatigue was now a bigger factor. I am sure that NAV was selected and armed on the briefing before taxi out. I think what may have happened is one of us during the subsequent taxi out and briefing pushed the NAV select tile on the MCP (Mode Control Panel) unintentionally causing it to be deselected and then neither of us noticed that NAV was not armed on the FMA. We briefed the threats on departure including the LIFR weather; complicated taxi out; T/O alternate; RNAV SID; to watch each other for fatigue and to be on alert. Even though we missed the initial error we captured our mistake at autopilot engagement altitude (1000 ft AGL) during the FMA scan engaging CMD2 we noticed we were in HDG and began our correction. We learned once again how fatigue can sneak up on you and that FMA awareness is extremely important not only to avoid errors but to trap them.
Air carrier Captain reported deviating off course shortly after takeoff.