AN F900 CREW; ENCOUNTERING LIGHT TO OCCASIONAL MODERATE TURB AT FL390; REQUESTED FL430 AND ACCEPTED A 4 MIN RESTR TO GET THERE. ACFT PERFORMANCE DECLINED; LOWER REQUESTED.

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WHILE CRUISING AT FL390; WE WERE EXPERIENCING CONTINUOUS LIGHT; OCCASIONAL; MODERATE TURB. IN ORDER TO ATTEMPT TO FIND SMOOTH AIR; I REQUESTED FL430. ACCORDING TO THE FMS; FOR OUR WT WE COULD CLB AS HIGH AS FL445. CTR ASKED IF WE COULD MAKE THE CLB IN 4 MINS OR LESS. AFTER DISCUSSION (10 SECONDS); I CONFIRMED WE COULD DO IT. I SET AUTOPLT IN VERT SPD MODE AT 1200 FPM CLB. WE PASSED FL410 IN LESS THAN 2 MINS WITH NO PROBS. BY FL415; OUR AIRSPD STARTED TO DECAY RATHER RAPIDLY. I REDUCED VERT SPD TO 700 FPM TO TRY TO STOP AIRSPD DECAY. THE AIRSPD LOSS SLOWED; BUT CONTINUED. UPON REACHING FL430 (IN 4 MINS FLAT); SPD HAD DECAYED TO APPROX .63 MACH. I WAS UNABLE TO GET THE ACFT SPD TO INCREASE AT ALL. IT WAS OBVIOUS THAT I HAD TRIED TO COMPLY WITH AN ATC REQUEST THAT I SHOULD NOT HAVE ACCEPTED. WE WERE EXTREMELY CLOSE TO GETTING THE STICK PUSHER. THE ONLY WAY TO AVOID IT WAS TO START A DSCNT. WE MADE SEVERAL UNANSWERED CALLS TO CTR TO ADVISE OUR SIT. WHEN THEY RESPONDED; THEY WANTED A 50 DEG TURN TO THE R FOR LOWER. I FELT THAT ANY BANK WOULD EXACERBATE AN ALREADY CRITICAL SIT. WE WERE ADVISED OF TFC AT FL410; OPPOSITE DIRECTION. AT THAT POINT WE WERE AT APPROX FL424 AND STILL DSNDING. FINALLY; CTR GOT THAT OTHER ACFT TO TURN. WE WATCHED HIM ON TCASII AND NOTED THE CLOSEST WE CAME WAS 5 - 5 1/2 MI HORIZLY AND OUR ALT DECAYED TO FL418. ONCE WE WERE CLR OF TFC; CTR CLRED US TO FL350. I INCREASED RATE OF DSCNT AND SLOWLY OUR AIRSPD INCREASED TOWARD NORMAL CRUISE SPD. WE EXPLAINED WHAT HAD HAPPENED TO THE CTLR WHO SAID 'NO PROB.' I HAVE BEEN PLTING JET ACFT ALMOST EXCLUSIVELY FOR THE PAST 12 YRS. THE FALCON 900EX IS BY FAR THE MOST IMPRESSIVE PERFORMING ACFT I HAVE EVER FLOWN. ALL OF MY FLT TIME IN THE ACFT IS IN THE LAST 9 MONTHS. I (KNOW NOW) THINK I WAS A LITTLE OVERZEALOUS IN EXPECTING THE ACFT TO BE CAPABLE OF SUSTAINING SUCH A RATE OF CLB AT SUCH A HIGH ALT. I HAD CLBED AND FLOWN THE ACFT AT THE SAME WT TO FL430 WITHOUT A PROB. IT WAS A SERIOUS LAPSE IN JUDGEMENT ON MY PART FOR EXPECTING TOO MUCH PERFORMANCE. IN RETROSPECT; WE NORMALLY TRY TO MAINTAIN A 300-400 FPM RATE OF CLB. MY CONCERN FOR PROVIDING THE PAX A SMOOTH RIDE COMBINED WITH MY DESIRE TO COMPLY WITH AN ATC REQUEST LED US INTO A MARGINAL SIT. THIS IS NOT THE WAY I NORMALLY CONDUCT A FLT. THE FO; A CO-CAPT; SEEMED TO AGREE WITH MY DECISIONS. AFTER DISCUSSING THE OUTCOME; HE ALLOWED THAT WE HAD MADE A WRONG DECISION. I HAD ASKED (NOT TOLD; I GUESS) HIM TO DECLARE AN EMER; BUT HE COMMENTED THAT 'I THINK WE'RE OK.' I TOLD HIM LATER I FELT HE HAD USURPED MY CAPT'S POS WITH THAT OF HIS BEING THE FLT DEPT MGR. AFTER FURTHER DISCUSSION; IT WAS DECIDED THAT FROM HERE FORWARD; WHOEVER IS ACTING AS CAPT; IF HE DECIDES IT IS APPROPRIATE TO DECLARE AN EMER; IT WILL BE DECLARED.