B757-200 FLT CREW EXECUTES ETOPS AIR TURNBACK AFTER APU FAILS INFLIGHT; THE APU WAS REQUIRED DUE TO A MAINTENANCE ITEM.

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AIR TURN BACK. UNDER THE MEL THAT WE WERE USING; THE APU HAD TO BE RUN CONTINUOUSLY FOR ETOPS LEGS THAT EXCEEDED 120 MINS. JUST PRIOR TO FOMAS; THE APU QUIT. WE FOLLOWED THE QRH AND GOT AN IMMEDIATE RELIGHT. IT RAN FOR ABOUT ANOTHER 10 MINS OR SO. IT QUIT AGAIN. AS WE WERE NOT UNDER AN EMER CONDITION; WE WERE ABLE TO COORDINATE A TURN BACK WITH ATC AND WERE ABLE TO KEEP OUR ALT AND RTE. WE WERE WELL SHORT OF THE ETP. PRIOR TO TAKING THE ACFT; I BRIEFED THE SITUATION WITH OUR DISPATCHER. THE PLAN WAS THAT SINCE THE APU HAD TO BE IN CONTINUOUS OP FOR ETOPS OPS; ANY FAILURE WOULD REQUIRE A TURN BACK. WE WERE IN AGREEMENT ON THAT POINT. WE CONTACTED DISPATCH WITH THE HF PATCH AS WE WERE HEADED BACK TO THE ISLANDS. HAD WE BEEN EBOUND AT THAT TIME; WE WOULD HAVE BEEN PRETTY CLOSE TO THE ETP; HENCE HIS RPT WOULD SHOW US JUST PRIOR TO THE ETP. WE HAD; IN FACT; TURNED BACK TO THE W AND WERE ESTABLISHED ON A WBOUND COURSE TO HNL WHEN WE'D CONTACTED HIM. THERE HAD ALSO BEEN A SHIFT CHANGE PRIOR TO THIS HAPPENING; SO I'M NOT SURE IF THE NEW DISPATCHER WAS FULLY AWARE OF THE PLAN AGREED UPON PRIOR TO DEP. HNL SEEMED THE BETTER CHOICE OF ARPTS IN THAT WE HAD OUR OWN MAINT SUPVR THERE.