PLT RPTS 6 ENG FAILURES IN A SMA WITHIN THE LAST YR.

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I AM A FOREIGN PLT WITH USA COMMERCIAL IFR SINGLE ENG LAND AND LEASEE OF ACFT IS BEING USED MAINLY FOR BUSINESS ALL OVER EUROPE AND OCCASIONAL USA/CANADA. WE (MY PARTNER AND *) EXPERIENCED 5 ENG OUTS BTWN MAR/XX/91 AND JUL/XX/92. AFTER THE FIRST 3 OCCURRENCES; WE RETURNED THE ACFT TO THE SELLER IN FL (WHICH HAD INSTALLED A FACTORY NEW ENG PRIOR TO THE DELIVERY). WE THEN FLEW THE ACFT SOME MORE THEN 150 HRS; BEFORE 2 MORE ENG OUTS HAPPENED. ALL PROBLEMS WERE AT ALTS OF 24000 FT OR MORE. THE ABOVE ACFT HAS EXPERIENCED SEVERAL PARTIAL ENG FAILURES. ON JUL/X/92 AND JULY/Y/92; THE ACFT EXPERIENCED 2 PARTIAL ENG FAILURES AT FL240. MR. A&P WAS A SAFETY PLT ON A PLANNED FLT FROM EUROPE TO THE UNITED STATES. MY CONCERN IS HEIGHTENED BY THE FACT THAT THIS SAME ENG AND ACFT EXPERIENCED 3 COMPLETE ENG FAILURES IN 1991. THE FIRST OCCURRED ON MAR/XX/91 AT FL250 NEAR MOBILE; AL. THE ENG FAILED COMPLETELY AND THE ACFT DSNDED TO 17000 FT. AT THAT ALT; THE ENG RE-STARTED. THE AIRPLANE WAS RETURNED TO MAINT CTR AT PUNTA GORDA; FL FOR AN INSPECTION. ON MAR/X/91 AT FL210; THE ACFT EXPERIENCED A SECOND COMPLETE ENG FAILURE. ON THIS FLT; WE WERE NOT ABLE TO RE-START THE ENG UNTIL THE ACFT HAD DSNDED TO 4000 FT. ON MAR/Y/91; THE ACFT EXPERIENCED AN ENG FAILURE THAT WAS ALMOST COMPLETE. THE ENG RAN; BUT DID NOT DEVELOP SUFFICIENT PWR TO KEEP THE ACFT AIRBORNE. THE ENG RE- STARTED AT 6000 FT. IN THE MEANTIME; WE HAD ANOTHER ENG OUT OVER ENGLAND ON SEP/TUE/92 AT FL220 WITH THE SAME SCENARIO; EXCEPT THAT THIS TIME; THE LOW FUEL PUMP DID SOME HELP; HOWEVER; WE HAD TO DSND TO 14000 TO CONTINUE WITH SMOOTH ENG OPS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS ACFT IS FLOWN IMC OVER THE ALPS AND MAKES OCCASIONAL TRIPS TO THE US OVER THE OCEAN. IT IS THEREFORE NECESSARY TO FLY IT ABOVE 18000 FT ROUTINELY. IT HAS EXPERIENCED 6 PWR FAILURES SO FAR; AT LEAST 2 OF THEM COMPLETE FAILURES. THIS ACFT HAD AN ENG CONVERSION PERFORMED BY A MODIFICATION CTR. THEY REMOVED A MODEL 'XA' ENG AND REPLACED IT WITH A FACTORY NEW MODEL 'YA' ENG. THE RPTR THINKS IT POSSIBLE THAT THE OLD FUEL PUMP WAS RE-INSTALLED ON THE NEW ENG AS SERIAL NUMBERS OF THE PUMPS WERE NOT ENTERED IN THE ENG LOGBOOK. AFTER 3 ENG FAILURES; THE ACFT WAS RETURNED TO THE MODIFICATION CTR WHERE THEY SUPPOSEDLY REPLACED THE ENG DRIVEN FUEL PUMP. ONCE AGAIN; NO RECORD OF SERIAL NUMBERS IN THE LOGBOOK. THE ENG FAILS; EITHER COMPLETELY OR PARTIALLY; ONLY AFTER BEING AT ALT FOR 40 TO 60 MINS. THE FUEL FLOW DECREASES. THERE IS NO EVIDENCE OF ICE AND OPENING AND CLOSING THE ALTERNATE AIR DOOR HAS NO EFFECT. RPTR WAS TRYING TO EXPLAIN THE PROBLEM AS A RESULT OF ATMOSPHERIC PRESSURE CHANGES. AFTER SOME CONVERSATION; RPTR AND ANALYST BEGAN TO WONDER IF THE FUEL; WHICH BECAME COLD AFTER 40 TO 60 MINS OF FLT AT TEMPS OF -15 DEGS C TO -30 DEGS C; COULD BE AFFECTING A PRESSURE RELIEF VALVE IN THE ENG DRIVEN FUEL PUMP; THUS REDUCING FUEL FLOW TO THE ENG WHEN THE FUEL WAS BYPASSED BACK TO THE FUEL TANK. ALTHOUGH THE RPTR HAS ONLY 2000 HRS; HIS ATTITUDE ABOUT SAFETY SEEMS BEYOND REPROACH; AND HE SEEMS TO KNOW HOW TO OPERATE THE EQUIP. THERE SEEMS TO BE SOMETHING WRONG WITH THE ENG AND/OR FUEL SYS. THE ENG MANUFACTURER HAS PROMISED THAT THEY WOULD SUPPLY THE RPTR WITH A NEW FUEL PUMP.