ACFT ENG STARTED WITHOUT PARKING BRAKE SET. RPTR PLT DISTR HELPING PAX CLOSE HIS DOOR; ACFT CREPT FORWARD AND HIT ANOTHER PARKED ACFT BEFORE RPTR COULD STOP.

Narrative

AFTER STARTING THE ENG; WHILE PARKED IN THE TIE-DOWN SPACE; I WAS DISTR BY THE PAX DOOR NOT BEING CLOSED AND ASSISTED MY PAX IN LATCHING AND LOCKING IT. I THEN IMMEDIATELY TURNED TOWARD THE INST PANEL AND CHKED THE GAUGES. I HAD FAILED TO ENGAGE THE PARKING BRAKE AND THE PLANE CREPT FORWARD WITHOUT EITHER OF US NOTICING IT. WITHIN A FEW FT OF AN ACFT ACROSS THE PARKING APRON (PERHAPS 75 FT); MY PAX NOTICED WE WERE ON A COLLISION COURSE AND TRIED TO WARN ME. WITH HEADSET ON; I DIDN'T HEAR HIM UNTIL TOO LATE. MY PROP DAMAGED THE TAIL OF THE OTHER SINGLE ENG ACFT AND THE PROP AND SPINNER OF MY ACFT WAS DAMAGED. ENG ALSO HAS TO BE BROKEN DOWN. THE LESSON TO BE LEARNED HERE IS TO ALWAYS SET THE PARKING BRAKE UNTIL YOU ARE READY TO TAXI. I NEVER REALIZED I WAS MOVING UNTIL HITTING THE OTHER ACFT. I WAS ALSO TOO BUSY EXPLAINING EVERY PROC TO MY INTERESTED PAX TO GET MY HEAD OUT OF THE COCKPIT. THIS ACCIDENT HAS BEEN RPTED TO SNA ARPT STAFF; ORANGE COUNTY SHERIFF'S DEPT ON THE FIELD; INSURANCE COMPANY AND THE FAA. THE ACCIDENT DID NOT OCCUR ON THE 'ACTIVE SIDE' OF THE DOUBLE YELLOW LINE; IE; THE TXWY. I AM RPTING IT TO HELP OUT OTHER PLTS WHO MAY HAVE A SIMILAR MENTAL LAPSE. SETTING THE PARKING BRAKE IS NOW UNDERLINED ON MY CHKLIST.