CHARTER PLT FLYING A SENECA AT UNCTLED ARPT MUST BACK TAXI TO TKOF ON AN IFR FLT PLAN. AN ACFT ON APCH MAKES STEEP TURN TO BASE AND RPTR MUST MAKE IMMEDIATE TKOF TO CLR THE AREA FOR LNDG TFC.

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ARPT IS UNCTLED; APCH FACILITY LOCATED AT CLASS C ARPT N; APPROX 15+ MI. IFR CLRNC OBTAINED ON GND VIA APCH CTLR. ARPT HAS NO TXWYS OR TURN-AROUNDS. MUST BACK-TAXI ON RWY SURFACE. I OBTAINED RELEASE FROM CTLR WHO ADVISED AN ACFT JUST ON APCH AND WAS APPROX 4 MI OUT. I ASKED TO RECONFIRM THAT I WAS RELEASED. CTLR AFFIRMED. I ANNOUNCED ON UNICOM 'BACK TAXI RWYS 6 AND 35 FOR IFR DEP.' APCHING END OF DEP RWY I WAS JUST LINING UP FOR TKOF WHEN ARRIVING ACFT ANNOUNCES 'ON BASE.' I LOOKED UP AND SAW ACFT IN STEEP BANK. REALIZING I HAD NOWHERE TO TURN OFF THE RWY I ANNOUNCED DEP AND TOLD ARRIVING ACFT THAT I WOULD BE OUT OF HIS WAY FOR A CLR LNDG. ACCELERATING DOWN THE RWY; THE OTHER ACFT ANNOUNCED HE WAS GOING TO DO A 360 DEG TURN. I THOUGHT NOTHING OF IT SINCE MY OPTIONS WERE LIMITED. I NEEDED TO GET OUT OF HIS WAY AND THE ONLY SOLUTION WAS TO GET OFF THE RWY. SO I DEPARTED. WHEN I ASKED THE CTLR TO VERIFY I WAS RELEASED; EVEN THOUGH THE OTHER ACFT HAD NOT ANNOUNCED ON UNICOM THAT HE WAS IN THE TFC PATTERN; I ASSUMED ENOUGH TIME EXISTED FOR MY BACK TAXI AND DEP AND THAT APCH NEEDED THE AIRSPACE FOR OTHER DEPS/ARRS. I PERSONALLY WAS NOT IN ANY HURRY AND WAS PERFECTLY WILLING TO WAIT FOR LNDG ACFT. I ASSUMED THEY NEEDED ME OFF THE ARPT TO CLR THE AIRSPACE FOR OTHER ACFT. THE CTLR COULD HAVE CLARIFIED HIS NEEDS AND THE ARRIVING ACFT COULD HAVE FOLLOWED RECOMMENDED ARR PROCS NOT EXCEEDING 30 DEG BANK IN THE PATTERN.