Commercial Fixed Wing
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human-Machine Interface; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Environment - Non Weather Related; Human Factors
Upon reaching ZZZZZ on the ZZZZZ1X arrival; approach control issued clearance to descend to 6;000 feet. As PF; I selected 6;000 on the MCP and pressed level change to initiate descent. However; as I was referencing the iPad I noticed the aircraft banking left and immediately checked the MCP and realized I had accidentally pressed the heading select switch next to level change; with the bug displaced to the left; causing the aircraft to initiate a left turn. I immediately adjusted the bug to correct heading and maintain course to the next waypoint; however approach already noticed the slight deviation and issued a corrective heading. The time between our initial turn and corrective action was less than 5 seconds and the deviation was minimal; however we were in terminal environment with approach coordinating multiple arrivals and departures so it was not surprising that they immediately noticed.Proximity of heading and level change buttons require extra attention to ensure aircraft is correctly maneuvered.Normally I verify the selection of the mode control panel and verbalize the type descent as I check the FMA. Not exactly sure why that didn't happen this time. Perhaps a little rusty with the decreased flying over the last three months. In retrospect; I think disengagement of the autopilot as soon as I noticed would made it a quicker course correction with less deviation.
Air carrier Captain reported a track heading deviation after accidentally pressing the heading select switch.