RAMP EXCURSION AND COLLISION WITH 2 CARS OCCURS WHEN A COLLEGE TRAINEE MAINT CHK PERSON ATTEMPTS A MAGNETO CHK ON A PIPER SENECA WITH THE BRAKE SYS REMOVED FROM THE ACFT AT ZZZ; US.

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ON MAY/WED/01 AT APPROX XA15; I OBSERVED THE MECHS DOING A LEAK CHK TO COMPLETE A 50-HR INSPECTION ON A PIPER SENECA OWNED BY A SCHOOL. ONE OF THE APPRENTICE MECHS TOLD ME THAT THE AIRPLANE HAD JUST COMPLETED A 50-HR MAINT INSPECTION; AND I ASSUMED THAT THE AIRPLANE WAS FULLY OPERATIONAL. THE BRAKES OBVIOUSLY WORKED FINE DUE TO THE FACT THAT THE ENGS WERE BEING RUN UP AND THE AIRPLANE STAYED IN PLACE DURING THE LEAK CHK. I WENT INTO THE BUILDING ADJACENT TO THE HANGAR. AT APPROX XA30; I WALKED OUTSIDE WHERE THE ACFT WAS SITTING. AS I STOOD NEXT TO THE ACFT; SAME MECH APCHED ME AND ASKED IF I WOULD HELP HER START THE ENGS FOR A MAINT MAGNETO CHK. I THOUGHT THAT THE AIRPLANE'S INSPECTION HAD ALREADY BEEN COMPLETED; BUT SHE INFORMED ME THAT THEY ACTUALLY STILL HAD TO DO A MAGNETO CHK AND SOME WORK ON THE BRAKES. I AGREED TO HELP HER START THE ENGS DUE TO THE FACT THAT SHE HAD ONLY DONE IT A COUPLE OF TIMES BEFORE. SHE WAS IN CHARGE OF THE RUN-UP AND MAGNETO CHK; AND I WAS ONLY ASSISTING HER IN THE ENG START PROCS. I WAS NOT INFORMED THAT THEY HAD REMOVED THE AIRPLANE'S BRAKES MINS EARLIER. SHE AND I HAD NO INTENT TO FLY THE ACFT OR EVEN TO MOVE THE ACFT ON THE ARPT SURFACE. I HAD NEVER EXPERIENCED A MAINT MAGNETO CHK; BUT ANTICIPATED THAT SHE WOULD SIMPLY START THE ENGS; CHK THE MAGNETOS; THEN SHUT DOWN THE ENGS. I WAS NEVER INFORMED THAT THE BRAKES HAD BEEN REMOVED. SHE SAT IN THE L SEAT AND SET THE PARKING BRAKE; AND I SAT IN THE R SEAT. WITH BOTH ENGS RUNNING; SHE INCREASED THE THROTTLE TO 1500 RPM; AND WE SOON NOTICED THAT THE AIRPLANE HAD BEGUN TO ROLL FORWARD. SHE IMMEDIATELY BROUGHT THE THROTTLE BACK; AND WE BOTH STEPPED ON OUR TOE BRAKES. THIS HAD NO EFFECT; SO I RELEASED AND PUMPED THE HAND BRAKE; WHICH ALSO DID NOTHING TO STOP THE AIRPLANE. I PRESSED HARD ON THE R PEDAL TO ATTEMPT TO STEER THE AIRPLANE AWAY FROM 2 PARKED VEHICLES; TO MY RECOLLECTION; APPROX 40 FT IN FRONT OF US; AS WE SHUT THE ENGS DOWN. THE PROPS CAME TO A FULL STOP BEFORE THE AIRPLANE ROLLED INTO THE PARKED VEHICLES; DENTING AND BREAKING A WINDOW IN ONE AND SCRATCHING THE OTHER. WHAT I LEARNED FROM THIS INCIDENT IS THAT; REGARDLESS OF WHETHER YOU PLAN ON SIMPLY RUNNING AN ENG AND STAYING IN PLACE; YOU SHOULD ALWAYS DO A PREFLT YOURSELF; EVEN IF SOMEONE WHO YOU THINK IS COMPETENT TELLS YOU EVERYTHING IS OK. YOU SIMPLY HAVE TO CHK IT OUT YOURSELF OR IT CAN BECOME SOMETHING SERIOUS.