Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Training / Qualification; Communication Breakdown; Confusion
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Chart Or Publication; Environment - Non Weather Related; Human Factors
On departure out of ZZZ we were notified our aircraft was off course. ATC then asked us if our clearance was to ZZZZZ we agreed ZZZZZ was our clearance however this was when we noticed the aircraft was improperly navigating on the ZZZZZ1 transition. ATC told us to fly heading 071 and then re-cleared us direct to ZZZZZ.Once ATC notified us of our error; it was immediately clear I had loaded the incorrect transition and we both missed this error during the route review. This is a common 'gotcha' out of ZZZ that I am normally very vigilant of; because both ZZZZZ and ZZZZZ1 have similar names and departure directions. Expectation bias was the casual factor in my case; as I had the clearance in my hand and checked the route points as they were read aloud but as ZZZZZ1 was stated and I crossed checked the printed clearance I now realize that I likely saw and read back ZZZZZ1 on the printout even though ZZZZZ was printed on the actual clearance. Lastly our lack of recency due to the flying draw down was a contributing factor in committing this error. Neither Captain or myself had flown much at all in the last 70+ days. This was also the first east bound flight on the ZZZZZ3 [departure] for both of us in about 2-3 months.While the error clearly rests within the cockpit in this instance; I do believe the similarities in names of both fixes creates an additional challenge. I think all established company procedures are designed to trap this very type of error so nothing else should be added/changed with SOP. We followed company procedure in our route verification however; the execution of that procedure was in error.
At the gate we checked the route and didn't catch the error loading the flight plan. I read the box and the FO checked it against the clearance/flight plan. Similar sounding transitions on the ZZZZZ departure were verified. Total mileage didn't vary enough to trap any errors either. We were cleared the ZZZZZ transition but started to fly the ZZZZZ1 transition. After passing ATC verified our clearance a few miles after ZZZZZ2 and gave us a heading then cleared us to a fix down line. Been off for 2 months I'm sure had a bearing on the event both the FO and I had been off for a long time due to COVID-19 reductions. Verify the clearance. Go slower to make sure all points are verified in the box. Especially after being off for such a lengthy time.
Air carrier flight crew reported flying an incorrect departure transition that had a similar sounding name as the clearance transition. The crew stated that lack of recent flying was a contributing factor.