Date: 202101
Local time of day: 1201-1800
Location: APC.Airport, CA
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Light Transport
Operator: Corporate
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class E ZOA
ATC / advisory: Center ZOA; Tower APC
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Corporate
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Situational Awareness; Human-Machine Interface; Distraction
Anomaly: Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft TA
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Software and Automation; Chart Or Publication; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: Flying the NAPAA4.NAPAA departure out of APC airport we loaded the departure into the FMS and verified the routing was correct.As we departed the airport on Runway 19R with a clearance to climb to 3;000 feet we had the autopilot engaged in LNAV and vertical speed. Autothrottles where set at 200 kts.As we started our climb there was a piston single that had departed Runway 19L ahead of us. As we climbed out I had the other aircraft in sight but did get a traffic alert on the TCAS. I think at the time we received the TCAS alert he was already passing off our left wing and 400 feet below us. The autopilot made an initial slight right turn towards the VOR and then sequenced the next leg of our flight plan from the SGD VOR to SAC as was filed on our flight plan.Due to the tight turn that would be required to fly the leg from SGD to SAC I thought it had initiated the turn early in anticipation of our proximity to the VOR and that we were approaching our assigned altitude of 3;000 feet.Between the TCAS alert and seeing that the FMS was sequenced now to fly to the next fix on our flight plan I was focused on ensuring we maintained our separation from the other aircraft and after verifying that the FMS was sequenced to our cleared route I continued in the left turn and established the aircraft on the leg from SGD to SAC.Tower handed us off to Oakland [Center] and it was then that they inquired as to why we had made a left turn after takeoff rather than flying the departure as published. We had a brief exchange with the Controller about how the FMS had sequenced us on the next leg of the flight plan prior to reaching the VOR.Upon landing at our final destination later in the day I placed a call to Oakland Center as well as the Tower at APC. The Oakland Controller that I spoke with did not think there had been any issue with what happened since we had not been given a phone number to call from our Controller. When I called the Tower at APC; I was able to speak with the controller that cleared us for takeoff and handed us off to Oakland when we departed. He stated that they have had a few aircraft making the same left turn after takeoff while flying that same departure. He indicated that they needed to look into it and said that there had not been any conflict with other traffic. He thanked me for getting back to him and talking to him about it and that he would be following up the next day with his colleagues regarding this departure.Our filed and cleared route was APC NAPAA4 SGD SAC ZZZ ZZZ1.




Synopsis: Pilot reported a track heading deviation during departure from APC airport as the aircraft turned early while on the NAAPA4 Departure. Reporter stated ATC made reference to other aircraft incurring similar heading deviations while on the NAAPA4.