FLC NOTICES THAT AN INCREASE IN TEMP UNEXPECTEDLY EXCEEDS THE ACFT'S PERFORMANCE LIMITS.

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DURING A PART 135 CHARTER FLT IN ONE OF OUR COMPANY LEAR 35A'S FROM PHL TO DEN WHILE AT FL430 WE ENCOUNTERED A TEMP INVERSION ABOUT 2 HRS INTO THE FLT. PRIOR TO THIS OCCURRENCE WE WERE GIVEN A CLB TO FL430 BY ZAU ABOUT 15 MINS BEFORE ENCOUNTERING THIS INVERSION. WE WERE CRUISING AT FL430 IN STABILIZED FLT AT AN APPROX MACH SPD OF .73 AND ACCELERATING SLOWLY WHICH SEEMED NORMAL AFTER THE CLB. THE TEMP STARTED TO WARM SLOWLY AT FIRST NOT INDICATING ANY ABNORMAL PROBS WITH OUR MACH SPD OR ANGLE OF ATTACK INDICATIONS. THEN I FIRST NOTICED A RAPID DECLINE IN MACH SPD AND INCREASE IN ANGLE OF ATTACK INDICATIONS. THEN I NOTICED THAT THE TEMP HAD STARTED TO WARM RAPIDLY FROM A -40 READING TO ABOUT -30 IN A VERY SHORT PERIOD OF TIME. THE MACH SPD HAD DETERIORATED TO ABOUT AN ESTIMATED .60 OR .58 INDICATING AROUND 160 KTS. I INFORMED ATC THAT WE WERE UNABLE TO MAINTAIN FL430 ANY LONGER AND WOULD NEED AN IMMEDIATE DSCNT TO LOWER ALT AND THAT FL410 WOULD BE ACCEPTABLE. I REQUESTED LOWER 3 TIMES TO ZAU AND INFORMED THE CTLR THAT WE COULD NO LONGER MAINTAIN FL430 DUE TO FLT CONDITIONS. THE CTLR SAID SHE COULD NOT GIVE US LOWER DUE TO CONVERGING TFC AND GAVE US A TURN TO 15 DEGS L OF COURSE. AT ABOUT THAT SAME TIME; I OBSERVED THE TFC IN MY 3 O'CLOCK POS ABOUT 2 MI AT FL410. I CALLED THIS TFC IN SIGHT AND OF NO FACTOR AND NEEDED TO DSND NOW! THE CTLR RESPONDED WITH; 'I CANNOT OFFICIALLY AUTH YOU LOWER AT THIS TIME DUE TO TFC.' DURING THIS TIME WE HAD ATTEMPTED TO TURN THE 15 DEGS L AS REQUESTED; HOWEVER; THIS TURN CAUSED AN ADDITIONAL LOSS IN LIFT WHICH CAUSED OUR ALT TO DETERIORATE TO FL425 ALL THE WHILE TRYING TO MAINTAIN ALT WITH MAX CONTINUOUS PWR. IT SHOULD ALSO BE NOTED THAT DURING THIS TIME THE ANGLE OF ATTACK WAS BORDERING ON THE YELLOW; HOWEVER; AT NO TIME DID THE STICK SHAKER GO OFF. PRIOR TO THIS INCIDENT WE HAD DECIDED TO DEVIATE TO OMA FOR A FUEL STOP DUE TO ENCOUNTERING UNFORECAST HEADWINDS AND 2 EXTRA PAX ON THE FLT AS WELL AS WX DELAYS GETTING OUT OF PHL. WE CONTINUED ON TO OMA WITHOUT FURTHER INCIDENT AND REFUELED AND CONTINUED ON TO DENVER APA. IN CONCLUSION; THIS ENTIRE INCIDENT WOULD HAVE BEEN AVOIDED HAD WE NOT ACCEPTED FL430 FOR THE PURPOSE OF TRYING TO CONSERVE FUEL IN THE HOPES OF POSSIBLY MAKING THE FLT TO DEN NON-STOP. FURTHERMORE; THE CTLRS SHOULD BE MADE AWARE OF THE FLT ENVELOPES AT THIS ALT AND UNDERSTAND THAT IT'S JUST NOT ANOTHER 2000 FT; AND HAD THE CTLR GIVEN US A TURN AND LOWER ALT UPON OUR FIRST REQUEST FOR LOWER; THIS INCIDENT COULD HAVE BEEN AVOIDED.