OPERROR OCCURRED WHEN AN HS125 TOOK OFF OVER A C172 ON THE SAME RWY. THE LCL CTLR THOUGHT THE C172 WAS CLR OF THE RWY.

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I HAD JUST RELIEVED THE LCL CTL POS AND AS PART OF MY BRIEFING I WAS TOLD THAT C172 WAS ROLLING OUT ON RWY 31 AND THAT HS125 WAS IN POS FOR TKOF. ALSO WAS TOLD THAT THE TXWY LIGHTS WERE OUT AT THE MIDFIELD INTXN. THE MIDFIELD INTXN COMPRISES THE INTXN OF 4 MAJOR TXWYS: TXWYS A; B; D; AND K. TXWYS A AND K BEING EXITS OFF RWY 31; AND TXWY K BEING THE FURTHEST FROM THE APCH END OF RWY 31; APPROX 4000 FT. AFTER ROLLOUT AND BASED ON PREVIOUS EXPERIENCE; I KNEW C172 WOULD TURN OFF AT TXWY A OR K; I TOLD C172 TO TURN L WHEN ABLE AND CONTACT GND CTL. I OBSERVED C172'S TAIL MOUNTED ANTI-COLLISION LIGHT SWING TO THE L AND PROCEED SW. I THEN CLRED HS125. I THEN HEARD C172 SAY ON MY FREQ THAT HE LOST HIS LNDG LIGHT. I ASKED HIM IF HE COULD STILL TAXI WITHOUT IT. HE REPLIED YES; AS SOON AS HE COULD FIND A TURNOFF. I THEN ASKED HIM IF HE WAS STILL ON THE RWY; AND HE REPLIED IN THE AFFIRMATIVE. AS I OBSERVED HIS LIGHTS PROCEEDING NW; WHEREAS THEY SHOULD HAVE BEEN PROCEEDING EITHER SW OR SE ON THE PARALLEL TXWY B. I GLANCED AT HS125 AND SAW HE WAS GOING TOO FAST (IN MY OPINION) TO ABORT. THE HS125 BECAME AIRBORNE AND PASSED OVER C172 APPROX 5000 FT DOWN THE RWY. I BELIEVE THAT WHAT PREVENTED THIS FROM BECOMING A MAJOR DISASTER WAS THAT THE PLT OF C172 WAS A VETERAN BTR PLT; KNEW WHAT WAS GOING ON AND HUGGED THE L SIDE OF THE RWY. THE PLT OF HS125 WAS ALSO A VETERAN BTR PLT AND STAYED TO THE R SIDE OF THE RWY CTRLINE DURING HIS TKOF. THE CTLR WHOM I RELIEVED WAS STANDING NEXT TO ME AND I ASKED HIM IF C172 LOOKED LIKE HE WAS CLR OF THE RWY. HE REPLIED; 'DEFINITELY.' I BELIEVE THAT THE COMBINATION OF TXWY LIGHTS BEING OTS; WHICH PREVENTED C172 FROM QUICKLY FINDING AN EXIT TXWY; AND C172'S LNDG LIGHT BEING OTS PREVENTED ME FROM DETERMINING IF HE WAS REALLY CLR OF THE RWY. MY ONLY REF WAS HIS TAIL MOUNTED ANTI-COLLISION LIGHT WHICH WAS PROCEEDING SW; BUT TOWARD THE RWY EDGE; AND STILL ON THE RWY; AND NOT ON AN EXIT TXWY A OR K.