Date: 201511
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Super King Air 200
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Initial Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Aircraft component: Altimeter
Component problem: Improperly Operated
Reporter organization: Air Taxi
Reporter function: Pilot Flying; Single Pilot; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Confusion; Situational Awareness; Communication Breakdown
Second reporter function: Departure
Anomaly: Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Overcame Equipment Problem
Contributing factors: Procedure; Human Factors
Primary problem: Human Factors

Narrative: Near miss with an altitude deviation of [approximately] 350 feet due to unintended setting of primary altimeter.ATC clearance was 3;000 feet with a heading of 150 degrees. The autopilot was engaged and commanded to climb and hold 3;000 feet; and maintain the 150 degree heading.ATC traffic advisory indicated a Piper Cherokee that would pass overhead at 3;500 feet. When visual contact was made; I called out the traffic to ATC and remarked that it was 'a close one'. No course or altitude deviation seemed necessary at the time; and the aircraft passed each other in a stable and controlled flight manner; albeit close.The Cherokee passed about 100-200 feet over my plane; and with an offset of about 500 feet. Indicated speed at the time was about 250 kts. ATC indicated that my transponder was reporting 3;400 feet; to which I responded that I was in fact indicating 3;000 feet.Several minutes later ATC assigned 12;000 feet. At that point; I noticed that my standby altimeter was indicating 3;400 feet; and was set to the correct 30.29 altimeter out of [the departure airport]. My primary altimeter was indicating 3;000 feet; and the standard Alt of 29.92 was active. The difference here was 370 feet by altimeter difference.Probable cause of the deviation: during climb; I accidentally pushed the Standard 29.92 Altimeter button that comes with the Garmin G1000 instrumentation display unit. Failure to notice the altimeter discrepancy led to the aircraft flying as instructed; but outside of the desired 3;000 feet. The Garmin G1000 does highlight the Standard 29.92 altimeter setting when selected; and I should have picked it up in my instrument scan during climb. Please note that the Piper Cherokee pilot did not have fault in this situation; and even if he had seen me; the closure rate would've been too great to take sufficient evasive action; had my altimeter differential had been even greater.


Second narrative: A BE20 departed IFR; assigned 185 heading to maintain 2;000 feet MSL. A PA28; was receiving VFR service in Class B airspace; and assigned an altitude of 3;500 feet MSL and Mode C altitude was verified. I issued departure traffic information to PA28 about the BE20 as the targets would possibly merge. When the BE20 departed; their Mode C indicated 2;400 feet MSL; but they reported level at 2;000 feet MSL. I instructed [the King Air] to climb to 3;000 feet MSL and issued traffic information about the Piper. I observed [the King Air's] Mode C altitude climb and briefly indicated 3;400 feet MSL shortly after targets passed. Mode C changed to 3;300 feet MSL. I verified with [the King Air] that they were level at 3;000 feet MSL and notified them of the Mode C issue. [The King Air] indicated that separation between the two aircraft had been 'close'. I notified facility management and [the King Air] was asked to contact the facility. I then provided routine service to [the King Air] and instructed them to contact approach. I had no further contact with [the King Air].


Synopsis: A King Air pilot and Air Traffic Controller reported a near-mid-air-collision due to an altimeter error induced by the pilot unintentionally selecting the standard pressure button on the G1000 instrument system.