LEAR 36 FO REPORTS TEMPERATURE INCREASE CLIMBING FROM FL390 TO FL410 THAT ENGINE PERFORMANCE COULD NOT OVERCOME. EMERGENCY IS DECLARED FOR DESCENT TO FL370.

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EBOUND WE JUST LEVELED AT 39000 FT. WE ACCELERATED TO .78 MACH WITH ENG PWR TO SPARE. WE DECIDED TO CLB TO FL410 AND LEVEL OFF. WE THEN NOTICED WE WERE NOT ACCELERATING BEYOND .71 MACH. WE REALIZED THERE HAD BEEN A TEMP INVERSION AND HAD GAINED ABOUT 20 DEG TEMP RAT DURING THE CLB. SPD WAS STABLE AT .71 MACH. WE ASKED CTR FOR A DSCNT BACK TO FL390. THE CTLR ADVISED THERE WOULD BE A 4 MIN DELAY FOR TFC WHICH WE COMPLIED WITH BECAUSE OUR AIRSPD WAS STABLE WITH PWR TO SPARE. ABOUT 3-4 MINS LATER SPD BEGAN TO DETERIORATE RAPIDLY AND WE WERE UNABLE TO REGAIN SPD WITH A PWR ADJUSTMENT. I CALLED CTR REQUESTING A VECTOR FOR AN IMMEDIATE DSCNT TO FL390. HE REPLIED UNABLE AND ASKED IF WE WERE DECLARING AN EMER. I TOLD HIM NO THAT WE ENCOUNTERED AN INVERSION AND WILL BE UNABLE TO MAINTAIN FL410 MUCH LONGER WITHOUT A DSCNT. THE CTLR THEN ASKED ME TO CALL HIM AND BEGAN GIVING ME A NUMBER. AT THAT POINT WE COULD NO LONGER MAINTAIN FL410 WE DECLARED AN EMER AND DSNDED TO FL370; LEVELED OFF; AND ADVISED THE CTLR THE EMER WAS OVER. WE CONTINUED TO OUR DEST WITHOUT INCIDENT.