FLC OF COMMUTER TURBOPROP LOSE THEIR WAY DURING TAXI AT ISP AND CROSS ACTIVE RWY WHILE A MLG TAKES OFF OVER THEM.

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CALLED FOR TAXI; AND RECEIVED INSTRUCTIONS TO TAXI TO RWY 15R AND CROSS RWY 6. WHILE TAXIING I NOTICED AN MLG JUST BECOMING AIRBORNE IN THE RWY I HAD JUST CROSSED (APPROX 2500 FT OFF MY L). I THEN CHKED MY DIRECTIONAL GYROSCOPE AND REALIZED I WAS TAXIING TOWARDS THE WRONG RWY AND THAT I HAD INADVERTENTLY CROSSED AN ACTIVE RWY. I HAD FLOWN INTO THE ARPT IN QUESTION DOZENS OF TIMES. I FEEL BLAME SHOULD GO TO MY COMPLACENCY AN ARPT DIAGRAM WAS NOT USED. ALSO LACK OF ATTN ON THE TWR/GND CTLR'S PART WAS ALSO A FACTOR. TWR AND GND WERE BEING CTLED BY THE SAME PERSON. THEY WERE UNAWARE OF THE SITUATION UNTIL THE AIRBORNE JET HAD NOTIFIED THEM OF THE MISHAP. SUPPLEMENTAL INFO FROM ACN 204741: I WAS NOT SURE OF THE RTE TO THE RWY BUT THE CAPT SEEMED TO KNOW WHERE HE WAS GOING AND THEN WE DID THE TAXI CHKLIST. AS WE WERE FINISHING THE CHKLIST WE WERE ALSO XING WHAT WE BELIEVED TO BE RWY 6. THE CAPT AND I BOTH LOOKED TO THE L TO SEE THE MLG COMING RIGHT AT US. THE MLG HAD BROKEN GND AND CLRED US BY APPROX 50 FT. THE RWY HAS A SLOPE WITH THE HIGH POINT AT THE MIDDLE SO WE COULD NOT SEE THE MLG AT THE OTHER END. I HAD JUST CLOSED (SIGNED THE FINAL PAPERS FOR PURCHASE) ON A HOUSE THE PREVIOUS DAY AND WAS PREOCCUPIED WITH THE DETAILS INVOLVED WITH THE MOVE AND REDECORATION; ETC. ALTHOUGH THE WX WAS NOT A DIRECT FACTOR IN THE INCIDENT IT WAS ON OUR MINDS AS IT WAS LOW IFR ALL OVER THE NE.