A PVT PLT OBSERVED AN SMT TWIN MAKE A BASE LEG TURN ON APCH TO LNDG AT AN UNCTLED ARPT DANGEROUSLY CLOSE TO PWR LINES AND TREES AT NIGHT.

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THE ACFT WAS APCHING AT THE 11 O'CLOCK POS; FROM WHERE I WAS STANDING. THE ACFT MADE A L TURN; FROM DOWNWIND TO SHORT BASE LEG; L TURN TO FINAL. I WAS APPROX 800 FT FROM HIS FLT PATH - BASE LEG. HE WAS JUST ABOVE THE TREES. 80 FT TREES. THIS TOOK PLACE AT XA00 PM. IT WAS DARK; 5 MI VISIBILITY. OFF TO MY L AT 9 O'CLOCK POS; WAS A XMISSION PWR LINE; HT 100 FT APPROX. BACKGRND LIGHTING FOR TWR. MY MAIN CONCERN IS THAT HE WASN'T AWARE HOW CLOSE HE WAS TO THE PWR LINES. AS THE ACFT CONTINUED TO THE ARPT; I LOOKED AT HIS APPROX FLT PATH OVER TREES; AND AGAIN TO THE TWR TO MY L. MY ESTIMATE IS THAT HE JUST CLRED THE PWR LINES. THIS ACFT CAUGHT MY ATENTION AS I WAS COMING OUT OF APARTMENT TO GO TO BATAVIA. HIS LNDG LIGHTS WERE ON. SOUNDED LIKE A TWIN ENG. I VERY SELDOM SEE AN ACFT USING THE L SIDE OF THE ARPT. MOST ACFT I SEE USE THE R SIDE. LOCATION OF APARTMENT COMPLEX. HE WAS USING L BASE FOR RWY 4. THE L TURN WAS 2 NM FROM THE ARPT. CLERMONT COUNTY ARPT 848 FT PLUS 1000 FT EQUALS 1848 FT. I THINK THE PLT TRAVELED TOO FAR FROM THE END OF THE RWY. MAYBE WAS LOOKING FOR A LONG FINAL. ALLOWED THE DECISION HT TO GET TOO LOW. HE WAS JUST ABOVE THE TOPS OF THE TALL TREES. 100 FT AND 2 NM FROM END OF RWY. TO PREVENT A RECURRENCE IS TO WARN THE PLT OF HIS GND TRACK AND HT ABOVE GND. MORE TIME SPENT ON ARPT FAMILIARITY. ALSO DECISION HT NEEDS ATTENTION. ADVISORY SVE AT ARPT WARNING OF XMISSION PWR LINES FOR L BASE RWY4.