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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.