OCEANIC MIL FLT IN VERY COLD TEMPS; FUEL TEMP WAS GETTING CLOSE TO CRITICAL (FREEZING); BUT DUE TO CONFLICTING TFC AT LOWER ALT; ATC WAS UNABLE TO CLR RPTR ACFT TO LOWER (WARMER) ALT. BECAUSE OF CRITICAL TEMP; FLC DIVERTED 30 MI OFF COURSE AND DSNDED WITHOUT CLRNC.

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AT APPROX THE TIME AND LOCATION LISTED ABOVE; A DSCNT FROM FL390 TO FL330 WAS INITIATED DUE TO A STEADILY DECLINING FUEL TEMP WHICH WAS APCHING THE LIMITS LISTED IN THE C-141B OPS SPECS. GANDER ADVISED OF TFC EBOUND AT FL370 AND FL350; AND REPEATEDLY DENIED OUR DSCNT REQUESTS. AS THE FUEL TEMP DROPPED TO A CRITICALLY LOW LEVEL; WE OFFSET 30 NM N OF TRACK AND BEGAN A DSCNT TO FL330 WITHOUT CLRNC FROM GANDER TO AVOID A POTENTIALLY HAZARDOUS SIT. WE RELAYED OUR PROGRESS THROUGHOUT THE OFFSET; DSCNT; LEVEL OFF; AND RETURN TO COURSE TO GANDER. GANDER HAD NO RPTED TFC AT FL330. SHORTLY AFTER LEVEL OFF; RADAR CONTACT WAS ESTABLISHED WITH REYKJAVIK CTL. THE NEED FOR A DSCNT FOR OPERATIONAL NECESSITY WAS DISCUSSED WITH GANDER FOR AT LEAST 20 MINS PRIOR TO THE INITIATION OF OUR DSCNT. WHEN THE FUEL TEMP FELL TO -40 DEGS C; WE AS A CREW FELT IT NECESSARY TO IMMEDIATELY BEGIN A DSCNT TO WARMER TEMP IN THE INTEREST OF SAFETY. AFTER LEVEL OFF AT FL330; IT WAS DETERMINED THAT THE TEMP IN THE #1 MAIN TANK WAS -45 DEGS C AT THE TIME OF DSCNT. THE FREEZE POINT OF JP5 IS -46 DEGS C. LATER; UNDER RADAR CTL AND WITH CLRNC; ANOTHER DSCNT TO FL290 HAD TO BE INITIATED TO CTL THE FUEL TEMP.