ATP FLYING A PA23-250 EXPERIENCED PWR LOSS AFTER XFEEDING FUEL; CAUSING HIM TO DSND. THE PLT WAS INFORMED BY ATC THAT A CONFLICT WITH ANOTHER ACFT HAD OCCURRED.

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PLT WAS FLYING A PIPER AZTEC ON AN IFR FLT PLAN AT A CRUISING ALT OF 14000 FT MSL IN VMC ABOVE A CLOUD LAYER. PLT WAS XFEEDING FUEL FROM THE OUTBOARD L TANK TO CORRECT A FUEL IMBAL. WHEN COMING OUT OF XFEED; THE CHKLIST WAS USED TO SELECT THE R OUTBOARD TANK; THE R BOOST PUMP ON; AND AFTER A SLIGHT HESITATION OF A FEW SECONDS; THE XFEED WAS TURNED OFF. SEVERAL SECONDS LATER THERE WAS A PWR LOSS DUE TO A FUEL FLOW INTERRUPTION. THE PLT VERIFIED THE FUEL SELECTORS WERE IN THE DETENT OF THE OUTBOARD TANKS; THE BOOST PUMPS WERE ON; AND THE MIXTURE WAS ENRICHED. THE PWR LOSS LASTED FOR SEVERAL SECONDS BEFORE THE FUEL FLOW AND PWR WERE RESTORED. PLT NOTICED AN ALT LOSS OF 400-500 FT AT ABOUT THE SAME TIME THE ZID CTLR REQUESTED THE ACFT'S ALT. PLT STATED THE ACFT ALT TO BE 13500 FT AND REQUESTED AN ALT ASSIGNMENT OF 13000 FT. THE CTLR INFORMED THE PLT THAT THE LOSS OF ALT CAUSED A TFC CONFLICT AND 13000 FT WAS UNAVAILABLE. THE PLT RETURNED TO 14000 FT AND WAS GIVEN A TELEPHONE NUMBER TO CALL WHEN ON THE GND AND WAS XFERRED TO ANOTHER CTLR. THE FLT CONTINUED WITHOUT FURTHER INCIDENT. THE FUEL FLOW INTERRUPTION COULD HAVE POSSIBLY BEEN PREVENTED BY SELECTING THE INBOARD TANK COMING OUT OF XFEED INSTEAD OF THE OUTBOARD TANK SINCE THE DISTANCE FROM THE ENG IS LESS AND WOULD TAKE LESS TIME AND FUEL TO REPLENISH THE EMPTY LINES. THE BOOST PUMPS MAY BE LESS EFFICIENT AT THE HIGHER ALT AND REQUIRE A LONGER TIME TO PRESSURIZE/FILL THE FUEL LINE. THE PLT MAY HAVE UNDERESTIMATED THE AMOUNT OF FUEL ACTUALLY IN THE LINE FROM THE R OUTBOARD FUEL TANK TO THE ENG PRIOR TO ENDING XFEED (POSSIBLE ZERO). THE PLT CONSIDERS THE ONE PROC THAT COULD HAVE PREVENTED THE INCIDENT WAS TO HAVE LEFT THE FUEL XFEED SWITCH IN THE XFEED POS FOR AT LEAST 2 MINS AFTER SELECTING THE R OUTBOARD TANK AND BOOST PUMP BEFORE TURNING THE XFEED SELECT OFF.