CL600 FLT CREW HAS AN ALTDEV WHILE IN CRUISE.

Narrative

WE WERE CLRED TO FL380 AND CLRED DIRECT TO NIBOG INTXN. AT LEVELOFF THE TEMP WAS ISA +2 DEGS AND WE HAD ISA +4 DEGS PROGRAMMED INTO THE FMS. FOR OUR WT; ALT; AND TEMP THERE WAS NO WARNING FROM THE FMS STATING 'UNABLE CRUISE ALT.' AT THE TOP OF CLB THE MACH FELL TO .71 AT ISA +2 DEGS. THE FMS WAS PROGRAMMED FOR EACH LEG WITH BOTH TEMP AND WIND FOR EACH INDIVIDUAL LEG AND THE FMS DID NOT GIVE A WARNING OF 'UNABLE CRUISE ALT' FOR THE WHOLE FLT AT FL380. WITHIN A VERY SHORT PERIOD OF TIME THE TEMP WENT TO ISA +8 DEGS AND MACH FELL TO .66. WE TWICE ASKED SCOTTISH CTL FOR LOWER (FL370) AND DID NOT GET A CLRNC FOR LOWER OR EVEN A RESPONSE. SCOTTISH CTL THEN HANDED US OFF TO SHANNON; THE MACH WAS STILL DROPPING AND THE ACFT STARTED A VERY LIGHT BUFFET. WE TOLD SHANNON UNABLE TO MAINTAIN FL380 AND NEEDED IMMEDIATE DSCNT CLRNC TO FL370. WITH ACFT STILL IN A LIGHT BUFFET WE TOLD SHANNON WE WERE UNABLE TO MAINTAIN FL380 AND DSNDING TO FL370. AT ABOUT FL374 SHANNON CLRED US TO FL370 AND ADVISED US OF AN ALT BUST. WE TOLD THE CTLR THAT WE WERE UNABLE TO MAINTAIN ALT DUE TO RAPID TEMP RISE. FORTUNATELY THERE WERE 'NO' OTHER ACFT DISPLAYED ON TCAS. THE TEMP RISE WAS FROM ISA +2 DEGS TO ISA +8 DEGS. WE WERE RECLRED FOR OCEANIC XING AT FL370 AND THE REMAINDER OF THE FLT WAS UNEVENTFUL. TO AVOID THIS TYPE OF SITUATION IN THE FUTURE ATC SHOULD ACCEPT THE FILED ALT AND CLR ACFT AS SUCH NOT TRY TO GET HIGHER ALT OUT OF CREW. SUPPLEMENTAL INFO FROM ACN 702140: WE HAD MADE OUR DECISION TO ACCEPT FL380 BASED ON THE FORECAST TEMP; OUR WT AT CRUISE AND THE IDEA OF OPENING UP LOWER ALTS FOR OTHER ACFT. THE PERFORMANCE NUMBERS WERE CLOSE BUT WE FELT THERE WAS ENOUGH MARGIN AND THE FMS TO XCHK THE DECISION. IN RETROSPECT; IF I CANNOT FLY THE ALT AT ISA +10 DEGS I WILL NOT GO THERE WITH THIS ACFT. SECOND; WE SHOULD HAVE DECLARED AN EMER AFTER OUR SECOND REQUEST FOR A LOWER ALT GOT NO RELIEF BECAUSE SHORTLY AFTER THAT WE HAD NO CHOICE BUT DSND. FOR WHO KNOWS WHAT REASON WE SEEM TO BE RELUCTANT TO DO THAT UNLESS WE ARE ON FIRE OR OUT OF GAS.