A CARJ CREW; UNABLE TO MAINTAIN FL390 DUE TO ACFT PERFORMANCE; REQUESTED AND RECEIVED CLRNC TO FL350.

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THE CAPT WANTED TO CLB FROM FL350 TO FL390. THE FMS SAID THAT WE WOULD BE UNABLE TO ACHIEVE FL390 GIVEN OUR WT AND THE AIR TEMP. THE ACFT PERFORMANCE CHARTS ALSO SAID WE WOULD BE UNABLE TO REACH FL390. I WOULD NOT ASK ATC FOR FL390. THE CAPT PROCEEDED TO CALL ATC AND REQUEST FL390. THE CTLR CLRED US TO FL390. AS SOON AS THE CAPT DIALED IN A 500 FPM CLB; THE AIRSPD BEGAN TO DECAY AT A RATE THAT WAS NOT GOING TO SUPPORT A CLB TO FL390. THE CAPT PUSHED THE THRUST LEVERS UP WELL BEYOND THE RECOMMENDED THRUST SETTINGS IN OUR PERFORMANCE CHARTS; BUT THE AIRSPD CONTINUED TO BLEED OFF. ACCORDING TO OUR RULES; IF YOU CAN'T CLB AT LEAST 300 FPM AT .70 MACH OR BETTER; IT'S TIME TO COME BACK DOWN. HE REFUSED. THE SPD BLED DOWN TO .66 MACH AND ABOUT 20 KTS ABOVE INDICATED STALL AT A 100 FPM SLOW-CLB PASSING FL386; BUT EVEN WHEN HE LEVELED; THE SPD CONTINUED TO BLEED OFF. WITH THE PLANE WALLOWING IN THIS STATE; I FINALLY CONVINCED THE CAPT TO LET ME CALL ATC AND GET US LOWER. ATC APPROVED DSCNT TO FL350. THROUGHOUT THIS EPISODE; ATC WAS VERY PATIENT AND ACCOMMODATING EVEN WHEN WE REQUESTED THE SLOW CLB. THE WHOLE SIT WAS UNCOMFORTABLE AND I DID EVERYTHING WITHIN MY JOB DESCRIPTION AS AN FO TO TRY AND PREVENT; AND LATER REMEDY; THE SIT. THE CAPT SAID IT WAS A GOOD LESSON.