ATX ARR AND SMA DEP EXPERIENCED NMAC IN CLASS D AIRSPACE. ATC DID NOT EXCHANGE TFC TO EITHER FLT. SMA DEP WAS ALLOWED TO CHANGE FREQ WHILE IN CLASS D AIRSPACE.

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WHILE WORKING AS A CTLR AT LAFAYETTE TWR; I WAS INFORMED OF A NEAR MIDAIR THAT HAPPENED 3 MI NW OF THE FIELD. IT INVOLVED AN IFR AIR TAXI FROM THE E LNDG RWY 10 AND A VFR CHEROKEE DEPARTING RWY 5 NWBOUND. WHEN THE ATX CALLED HE WAS APPROX 8 MI E OF THE FIELD FOR A L DOWNWIND. AT THE SAME TIME THE CHEROKEE HAD ALREADY LIFTED OFF OF RWY 5 AND WAS IN A TURN W-NW. THE INCIDENT OCCURRED WHEN THE ATX REQUESTED THE RWY 10 LIGHTS TO BE TURNED UP. THE ATX WAS TURNING BASE TO FINAL WHEN HE RPTED A CHEROKEE JUST PASSED OVERHEAD WITHIN 50 FT. LAFAYETTE IS A VFR TWR WITH NO RADAR. IN MY PERCEPTION THE ATX CAME FROM BEHIND THE CHEROKEE AND WAS LOOKING FOR THE RWY INSTEAD OF OUT THE WINDOW. SUPPLEMENTAL INFO FROM ACN 315058: WE WERE ENRTE BTWN MUNCIE AND LAFAYETTE IN GOOD VFR CONDITIONS; AND OPERATING AS ATX Y. THE TWR CTLR ACCEPTED US AND GAVE US A LNDG CLRNC FOR RWY 10. AT NO TIME WERE WE ISSUED ANY TA INFO WHILE WITH THE LAF TWR. JUST PRIOR TO TURNING L BASE THE OTHER ACFT (CHEROKEE) APPEARED IN THE MIDDLE OF THE L WINDSHIELD APPROX 30 FT AWAY. BOTH MYSELF AND THE CAPT PUSHED FULL FORWARD ON CTLS AND THE CHEROKEE PASSED OVER OUR ACFT. THE CAPT AND I FELT A BUMP AS THE CHEROKEE PASSED OVER US AND WE THOUGHT CONTACT MAY HAVE BEEN MADE WITH OUR TAIL. MAINT AND OURSELVES AFTER A THOROUGH INSPECTION OF THE ACFT COULD NOT FIND ANY EVIDENCE OF CONTACT AND DETERMINED THAT THE BUMP WE HEARD WAS DUE TO A COMBINATION OF THE ELEVATOR HITTING ITS STOPS AND THE BAGGAGE FLOATING UP AND HITTING THE BAGGAGE COMPARTMENT FLOOR.