A SMALL AIRCRAFT TAXIED ACROSS AN ACTIVE RUNWAY WITHOUT CLEARANCE.

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ACFT CLRED FROM RAMP TO TAXI TO RWY XX AT TXWY D. AS THE PLT OPERATING THE RADIO; I MISTAKENLY COPIED RWY XY AT TXWY D; AND ACKNOWLEDGED THE TAXI CLRNC. I'M NOT SURE WHETHER I READ BACK RWY XX OR RWY XY. PF HAS APPROX 200 HRS TOTAL TIME; PLT OPERATING THE RADIO HAS APPROX 1200 HRS AND OUTRANKS THE PF. ACFT WAS TAXIED TO RWY XY; XING ACTIVE RWY XX IN THE PROCESS. CTLR NOTIFIED ACFT THAT IT HAD BEEN INSTRUCTED TO TAXI TO RWY XX AT TXWY D; AND HAD CROSSED THE ACTIVE RWY WITHOUT PERMISSION. FORTUNATELY; NO TFC WAS ON OR APCHING THE RWY. THE PROB AROSE WITH THE MISUNDERSTOOD TAXI CLRNC; AND WAS COMPOUNDED BY THE PF NOT QUESTIONING THE CLRNC (ALTHOUGH HE ADMITTED HE THOUGHT HE HEARD RWY XX) AND THE FAILURE OF THE CTLR TO HALT OUR INCORRECT TAXI PATH UNTIL AFTER WE HAD CROSSED THE ACTIVE RWY. WE ESPOUSE CRM; AND STRESS FLT SAFETY. NEVERTHELESS; THE PF DID NOT FEEL COMFORTABLE QUESTIONING THE TAXI CLRNC. RE-EMPHASIS ON CRM PROCS; AND THE DUTY TO CLARIFY UNCLR OR UNCERTAIN OR INCORRECT DIRECTIONS NEED TO BE STRESSED PRIOR TO EACH FLT.