FLC OF AN LTT CPR JET USED PORTION OF A CLOSED RWY FOR THE TKOF ROLL.

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WE HAD FLOWN INTO STJ AT APPROX XA15Z TO PICK UP 7 PAX. THE ARPT WAS UNFAMILIAR TO BOTH OF US. I HAD USED JET PLAN TO OBTAIN FLT PLANS; WX; AND NOTAMS; AND NOTED THAT THE FIRST 3000 FT OF RWY 17 WAS CLOSED. THIS LEFT 4900 FT FOR LNDG WHICH WAS NO PROB. THE OTHER PLT AND I HAD DECIDED TO USE RWY 13 (6000 FT) FOR DEP SINCE OUR TKOF FIELD LENGTH REQUIREMENT WAS ABOUT 4500 FT. WHEN I GOT THE CLRNC FROM GND I INFORMED THEM THAT WE WOULD BE NEEDING RWY 13 FOR TKOF. THEY INFORMED ME THAT RWY 13 WAS CLOSED AND HAD BEEN FOR APPROX 3 YRS. I THEN ASKED THEM IF WE COULD USE 500 FT OF THE CLOSED PORTION OF RWY 17 (THERE WAS ACTUALLY NOTHING WRONG WITH; OR MEN WORKING ON THE RWY. THEY WERE WORKING ON A PARALLEL TXWY ON THE E SIDE OF THE RWY) AND WAS TOLD WE WOULD HAVE TO CLR IT WITH SECURITY. I THEN TOLD THE OTHER PLT ABOUT THE RWY CLOSURE. HIS RESPONSE WAS 'OH WELL.' AS I WAS TAXIING OUT HE INSISTED THAT I TAXI PAST THE HOLD LINE ONTO THE CLOSED PORTION ANYWAY. WE WERE ABOUT 100 FT INTO IT WHEN THE TWR TOLD US TO TURN AROUND AND THAT THEY HAD NOT RECEIVED PERMISSION FROM SECURITY TO LET US ON THE CLOSED PORTION. I TURNED AROUND AND THE OTHER PLT INSISTED THAT I STOP AND USE ABOUT 200 FT OF THE CLOSED PORTION FOR TKOF. I DID THAT AND THE TKOF WAS UNEVENTFUL. OUR UP-TO- DATE CHARTS AND ARPT DIRECTORY SHOWED 13/31 OPEN. I LATER CALLED THEM TO INFORM THEM ABOUT THE MISTAKE. I HAD PLANNED THE FLT TO USE AND SHOULD HAVE CALLED SECURITY AND GOTTEN PERMISSION TO USE THE CLOSED PORTION OF RWY 17. I ALSO SHOULD NOT HAVE USED THE CLOSED PORTION OF RWY 17 WITHOUT PERMISSION.