DA90; CREW ENRTE OCEANIC ACCEPT FL390 AND ARE FORCED TO MAINTAIN MAX THRUST DUE TO TFC CONGESTION AT LOWER ALTS.

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DEPARTED BIKF AT AB00Z PM; WITH FULL LOAD OF FUEL. FLT PLAN CALLED FOR INITIAL CLB TO FL350 WITH CLB TO FL390 NEAR COAST IN POINT ENRTE TO TEB. AT APPROX 10-15 MINS AFTER TKOF; KEFLIVIK DEP ASKED US IF WE COULD CLB TO FL390 BEFORE REACHING OUR THIRD FIX. PIC DETERMINED THAT WE COULD; AND I STATED SAME TO KEFLIVIK. UPON APCHING FIX; WE NOTED OUR CLB PERFORMANCE WAS DETERIORATING RAPIDLY. WE NOTED SAT TEMPS WERE WARMER THAN FORECAST. WE FINALLY CREPT UP TO FL390 BUT WERE UNABLE TO MAINTAIN OUR REQUIRED CRUISE MACH. WE WERE THEN TALKING TO ICELAND RADIO AND REQUESTED FL350. HE STATED FL350 WAS NOT AVAILABLE. SEEING THAT THE TEMP WAS STILL RISING AND MACH DECREASING; WE IMMEDIATELY INSISTED WE HAD TO DSND TO ANY LOWER ALT. WHILE WAITING FOR THE RADIO OPERATOR TO CONTACT CTL; WE BEGAN TO GET WING BUFFETING AND HAD NO OTHER CHOICE BUT TO DSND OR WE WOULD NEXT BE HEARING THE STALL WARNING. WHEN THE OPERATOR CAME BACK; WE STATED WE WERE UNABLE TO MAINTAIN ALT ANY MORE. THE OPERATOR ASKED OUR EXACT ALT AND I STATED FL386. THE NEXT XMISSION FROM THE OPERATOR I COULD NOT UNDERSTAND BECAUSE OF HIS HVY ACCENT. THE PIC SAID HE UNDERSTOOD HIM TO SAY THE ONLY THING HE COULD OFFER US WAS TO TURN BACK TO RADAR CTLED AIRSPACE. A TURN AT THIS POINT WOULD NOT HAVE BEEN FEASIBLE SINCE IN LEVEL FLT WE WERE ALREADY NEARING A STALL. ALSO; BY THIS TIME AT OUR SLOW SPD WE WERE PROBABLY AT LEAST 45 MINS AWAY FROM ICELAND RADAR CTL AT WHICH WE WOULD HAVE BEEN REQUIRED TO MAINTAIN OUR ALT. THE ONLY OPTION THAT MAY HAVE SOLVED OUR DILEMMA WOULD HAVE BEEN TO FLY A PARALLEL TRACK OR OFF TRACK RTE SO AS TO NOT CONFLICT WITH OTHER TFC. AT THIS POINT; THE PIC DECIDED TO CONTINUE ON COURSE AND FLY AT FULL THROTTLES; SLOWLY CLBING TO FL390. ONCE LEVEL AT FL390; I REQUESTED TO BE CLRED TO CRUISE AT OUR PRESENT MACH (JUST ABOVE STALL WARNING) AND WAS GRANTED OUR REQUEST. THIS SIT CONTINUED FOR ABOUT 1 HR UNTIL ENOUGH FUEL WAS BURNED OFF TO ACCELERATE TO NORMAL CRUISE MACH. REST OF FLT WAS UNEVENTFUL. ACCORDING TO OUR PERFORMANCE DATA AND PAST OPERATING EXPERIENCE; THE AIRPLANE SHOULD HAVE BEEN ABLE TO CLB TO FL390 AT OUR WT AND INDICATED TEMP. IN RETROSPECT; THE DECISION TO ACCEPT THE CLRNC TO CLB TO FL390 SHOULD HAVE BEEN DECLINED; EVEN IF IT MEANT STAYING AT OUR THEN PRESENT FL280. FOR OCEANIC XINGS; IT WOULD BE ADVISABLE TO HAVE A LOW ALT FLT PLAN AVAILABLE SO YOU CAN INSTANTLY KNOW IF YOU CAN ACCEPT A LOWER FLT LEVEL FOR THE ENTIRE XING. ALSO; A SIGNIFICANT CONTRIBUTING FACTOR WAS CREW FATIGUE CAUSED BY MANY DEP DELAYS FROM OUR PAX; NOT TO MENTION WE WERE FLYING OUTSIDE OUR CIRCADIAN RHYTHM. THESE TYPES OF OPS SHOW GOOD CAUSE FOR PART 91 DUTY AND FLT TIME REGS. AT THE COMPLETION OF OUR FINAL LEG; I WAS AWAKE FOR OVER 27 HRS. ROUTINELY; OUR CREWS ARE AWAKE OVER 30 HRS AFTER COMPLETION OF THE LAST LEG OF A DUTY PERIOD.