BE90 CREW HAD MULTIPLE RWY INCURSIONS AT ISP.

Narrative

LONG ISLAND GND TOLD THE COPLT AND I TO TAXI TO RWY 33L FROM FBO VIA TXWY S AND TXWY E; HOLD SHORT OF RWY 33L ON TXWY E. WE TAXIED TO THE HOLD SHORT LINE AT TXWY E. I MISREAD THE HOLD SHORT SIGNS; AND I THOUGHT THAT IT WAS THE HOLD SHORT LINE FOR RWY 10/28 ONLY. I DECIDED TO CROSS THE RWY BECAUSE IT WAS NOT THE ACTIVE RWY AND THEN HOLD SHORT OF RWY 33L. UNFORTUNATELY; RWY 10/28 AND RWY 33L APCH ENDS TOUCH AT THAT POINT; AND I ENDED UP XING BOTH RWYS. I REALIZED MY MISTAKE WHEN I SAW THE '33' NUMBERS ON THE CONCRETE. MY COPLT AND I WERE HORRIFIED; CONTACTED THE TWR; AND APOLOGIZED FOR OUR DANGEROUS MISTAKE. MY FIRST MAJOR ERROR WAS MY OVERCONFIDENCE AT AN ARPT WHERE I WAS UNFAMILIAR. I HAD ONLY FLOWN INTO THIS ARPT ONCE BEFORE; AND I HAD NOT USED RWY 33L. MY SECOND ERROR WAS NOT CONSULTING WITH MY COPLT WHO HAD THE ARPT DIAGRAM IN FRONT OF HIM. IN FACT; I SHOULD HAVE LOOKED AT THE ARPT DIAGRAM MYSELF BECAUSE MY COPLT IS RELATIVELY NEW. THIRD; I SHOULD HAVE VERIFIED WITH THE TWR IF I WAS SUPPOSED TO CROSS RWY 10/28. FINALLY; I SHOULD HAVE LOOKED MORE CLOSELY AT THE RWY HOLD SHORT SIGNS. FROM NOW ON; I WILL BE MUCH MORE CAREFUL WHEN APCHING HOLD SHORT LINES. I HAVE BEEN HUMBLED!