NMAC BTWN A GULFSTREAM; G3; TAKING OFF AND FLYING OVER THE INTXN RWY ON WHICH A C150 WAS ALMOST READY TO TOUCH DOWN FOR LNDG. INCIDENT WAS CAUSED BY RPTR'S NOT HOLDING IN POS ON THE RWY AND NOT ABORTING WHEN TWR INSTRUCTED THEM TO DO SO.

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WHILE TAXIING FOR TKOF FROM SBA RWY 7; WE BACK-TAXIED ON RWY 7 FOR ABOUT 500 FT; AND WERE GIVEN INSTRUCTIONS FROM THE TWR TO TAXI INTO POS AND HOLD. I WAS GOING TO FLY THIS FLT. AS WE TURNED THE AIRPLANE INTO POS FOR TKOF; THE CAPT TOLD ME THAT I HAD THE AIRPLANE. SINCE WE WERE ALREADY TAXIING ON THE ACTIVE RWY; I FELT SURE THAT WE HAD BEEN CLRED FOR TKOF. I SET TKOF PWR AND THE ACFT ACCELERATED TO AT OR NEAR ROTATION SPD. AT APPROX ROTATION SPD; THE CTLR TOLD US TO ABORT. AT THE TIME THE CTLR TOLD ME TO ABORT; IT WAS OBVIOUS TO ME THAT I HAD ADEQUATE SPD AND DISTANCE (FROM THE ACFT XING OUR FLT PATH) TO TKOF AND BE WELL ABOVE THE C150 BY THE TIME WE REACHED THE RWY(S) INTXN. THE C150 WAS LNDG ON RWY 15 (L OR R) AS WE WERE TAKING OFF ON RWY 7. OTHER THAN A MAX EFFORT CLB TO MISS THE CESSNA; NO OTHER EVASIVE ACTION WAS NEEDED AND WE CROSSED BEHIND AND ABOVE THE CESSNA'S FLT PATH ABOUT 200 FT OR SO. THE TWR DIDN'T NOTICE OR ADVISE US TO ABORT UNTIL VERY LATE IN OUR TKOF RUN. SUPPLEMENTAL INFO FROM ACN 379585: I CALLED THE TWR SUPVR ON LNDG AND THEY ASKED FOR EVERYTHING.