Date: 202103
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Training / Qualification; Physiological - Other; Communication Breakdown; Situational Awareness
Second reporter function: Flight Attendant (On Duty)
Anomaly: Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Security
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: General None Reported / Taken
Contributing factors: Environment - Non Weather Related; Human Factors
Primary problem: Human Factors

Narrative: I arrived at the aircraft about an hour prior to departure.  The gate agent checked me in and let me down the jetway.  I began my standard pre-flight flow; loading the flight plan; and setting up my EFB.  Due to COVID; I had not been flying very much (my second turn/trip since 202011) and I was making sure I double checked all data entries and flows/procedures in the cockpit.  After completing the pre-flight; I was again focused on double checking the route and weather.  Unbeknown to me; a deadheading crew stayed on the aircraft after arriving from their inbound/working leg.  The flight departed uneventfully and safely arrived at ZZZ.       Because of COVID and manpower adjustments; I had not been flying recently.  During pre-flight; and while setting up the cockpit; my focus was 100% on completing procedures and properly setting up the FMS.  I had very low situational awareness on what was going on in the back of the aircraft; and any ramifications during the crew swap.During the next recurrent training or quarterly distance learning; the importance of minimum crew should be emphasized.  In addition; it should be emphasized that an off going crew; though trained on the aircraft; are considered 'passengers' if they are not specifically working the flight.


Second narrative: When I scanned my badge at gate; I was advised by Agent that there were deadheading FAs already on-board; because they brought the aircraft in.  I was the last working FA to board aircraft.  When I boarded; I noticed several FAs and a couple pilots already seated. This constituted a minimum crew violation since all working FAs were NOT on-board prior to customer boarding.It could be helpful to make it a bit more clear that; even when FAs bring an aircraft in and are deadheading back on the same aircraft; they must exit the aircraft (take their belongings??); and scan their boarding pass at the gate with the Agent; since they would then be considered a passenger.It could also be helpful to send a reminder to our Agents kindly asking any inbound crew (pilots or FAs) to deplane (with their belongings?) until minimum working FAs are on-board; to comply with FARs.


Synopsis: Air carrier flight crew and a Flight Attendant reported a minimum crew issue prior to boarding of deadheading flight attendants and pilots.