PIPER PA28-201 PLT CROSSES RWY AT CHS; SC; WITHOUT PROPER CLRNC.

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I HAD JUST FINISHED A 4 HR IFR FLT FROM POMPANO BEACH; FL; TO CHARLESTON; SC. THE FLT HAD BEEN EXTREMELY DIFFICULT AS IT WAS TURBULENT; RAINY AND DONE MOSTLY IN IMC. I WAS VERY TIRED AND VERY HAPPY TO BE SAFELY ON THE GND. I TURNED OFF AT THE FIRST TXWY AS INSTRUCTED; AND CONTACTED GND. I RECEIVED NO ANSWER AND AFTER SEVERAL TRIES WITH NO ANSWER; WAS BEGINNING TO FEEL ANXIOUS. WHEN THE CTLR FINALLY ANSWERED; HE SOUNDED DISTR; EXPLAINING HE HAD BEEN ON ANOTHER LINE RPTING A FIRE AND THEN; BOTH MY PAX AND I WERE SURE WE HEARD HIM SAY; '...CROSS RWY 33 FOR NOW.' BECAUSE I WAS TIRED AND DISTR MYSELF; I DIDN'T REPEAT BACK HIS INSTRUCTIONS; BUT PROCEEDED TO CROSS RWY 33. AFTER I CROSSED THE RWY; I WAS TOLD THAT I DID NOT HAVE CLRNC TO DO SO. HOWEVER; IT SHOULD BE NOTED THAT WHILE I WAS TAXIING TOWARD RWY 33 AND ACROSS IT; THE CTLR DID NOT CONTACT ME SO I WAS VERY SURPRISED WHEN I HEARD FROM HIM AFTER THE XING. I VERY RARELY; IF EVER; DO NOT READ BACK INSTRUCTIONS BOTH IN THE AIR AND ON THE GND; BUT DID NOT DO SO THIS TIME. I BELIEVE THIS LAPSE WAS CAUSED BY A COMBINATION OF TIREDNESS AND RELIEF THAT I WAS FINALLY ON THE GND AND RELEASED FROM THE PRESSURE OF THE IMC ENVIRONMENT. I WILL CERTAINLY BE PUTTING A PLACARD ON THE PANEL; IN PLAIN SIGHT; TO REMEMBER TO READ BACK ALL INSTRUCTIONS SO I WON'T MAKE THE SAME MISTAKE IN THE FUTURE.