A LOW FLYING PA31-350 PLT GETS A LETTER FROM THE FAA REGARDING HIS LOW PASS OVER RWY 17 AT TIW ARPT WHILE IN PROX OF A MANEUVERING HELI.

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I WAS RETURNING TO MY HOME ARPT AT WHICH I'VE WORKED FOR THE LAST 2 YRS (A NON PAX STATION BASE) FOR THE PARENT COMPANY OF THE COMMUTER AIRLINE FOR WHICH I RECENTLY BEGAN WORKING. I HAD JUST COMPLETED IOE. AFTER BEING HANDED OFF TO THE TWR FROM APCH (VFR FLT FOLLOWING) I REQUESTED A LOW APCH FOLLOWED BY R CLOSED TFC TO A FULL STOP LNDG ON RWY 17. (I WAS THE SOLE OCCUPANT OF THE ACFT ON A PART 91.) I WAS CLRED FOR LOW APCH AND ADVISED OF HELI TFC OPERATING IN THE GRASS W OF THE RWY. I REQUESTED CLARIFICATION THAT THE HELI WOULD NOT CROSS THE RWY. I WAS INFORMED BY THE TWR THAT THE HELI WOULD REMAIN W OF THE RWY. I NEVER RPTED SIGHTING THE HELI. I FLEW MY ACFT AT APPROX 30 FT AGL OVER THE RWY (SLIGHTLY E OF CTRLINE TO GIVE THE HELI MORE ROOM) AT AN IAS OF 150-160 KTS WITH THE GEAR IN THE RETRACTED POS. AT THE END OF THE RWY I DID A NORMAL PULL-UP AND RESUMED A NORMAL TFC PATTERN. AFTER RECEIVING CLRNC; I LANDED ON RWY 17 AND TAXIED TO THE RAMP WITHOUT INCIDENT. THE NEXT DAY I WAS ADVISED AN FAA INSPECTOR WITNESSED THE EVENT AND WAS PURSUING AN INVESTIGATION. AT THE TIME I SUBMITTED MY FIRST NASA FORM THE INSPECTOR HAD YET TO CONTACT ME. I'VE SINCE RECEIVED A LETTER OF INVESTIGATION. I DO NOT BELIEVE I VIOLATED FARS DURING THIS PHASE OF FLT.