ALTDEV UNABLE TO HOLD ALT DUE TO TEMP INVERSION.

Narrative

THE ACFT WAS ON A FLT FROM CHA TO MKE AND HAD BEEN CLRED BY ARTCC TO CLB AND MAINTAIN FL430. THE FLT WAS A SECOND FLT OF THE DAY (THE FIRST BEING FROM CHA TO SGF TO CHA); AND WE HAD OBSERVED THAT THE OUTSIDE AIR TEMP WAS SOME 10-12 DEGS WARMER THAN USUAL ON THE FIRST FLT AT FL430. ALTHOUGH WE ENCOUNTERED THE WARMER THAN USUAL AIR TEMP DURING THE FIRST FLT AT FL430 (WHICH IS USUAL FOR AIR BEHIND A STRONG COLD FRONTAL BOUNDARY); WE HAD NO PROBS IN A CRUISE CLB TO AND THEN MAINTAINING THE FLT LEVEL (FL430). OUR NUMBER OF PAX WAS 2 ON THE FIRST FLT AND 3 ON THE SECOND FLT. IT WAS DURING THE SECOND TRIP THAT WE ENCOUNTERED THE SEVERE AREA OF TEMP INVERSION (SOME 30 PLUS DEGS WARMER THAN ISA). WE HAD JUST LEVELED THE ACFT WHEN WE EXPERIENCED THE EXTREMELY WARM AIR; AND WE IMMEDIATELY CONTACTED THE ARTCC CTLR AND REQUESTED FL390 AS THE ACFT WOULD NOT ACCELERATE TO ITS STANDARD MACH SPD: RATHER THE ACFT WAS LOSING SPD IN THE THIN AIR AND BEGAN TO SETTLE SLOWLY AS WE WERE CONVERSING WITH THE CTLR. THERE WAS OPPOSING TFC AT FL410 WITHIN 5 MI OF OUR POS AND THE ARTCC CTLR ADVISED THAT HE COULD NOT GIVE US A DSCNT CLRNC UNTIL WE HAD CLRED THE TFC. WHEN THE OPPOSING TFC PASSED US; WE HAD SETTLED APPROX 250 FT BELOW FL430 AS WE COULD NOT DO ANYTHING MORE THAN HOLD ATTITUDE WHILE IN THE INVERSION. EVEN WITH FULL CLB PWR AVAILABLE TO US AT FL430; WE WERE CONTINUING TO SETTLE VERY SLOWLY AS WE AWAITED CLRNC TO FL390: AND WHEN THE CTLR ADVISED THAT WE COULD DSND TO AND MAINTAIN FL390; THE ACFT HAD SETTLED APPROX 450 FT BELOW FL430. WE THEN DSNDED TO FL390 AND COMPLETED THE FLT TO MKE WITH NO FURTHER PROBS. OUR AIRSPD CAME UP TO NORMAL CRUISE (.79 MACH) AND WE NOTICED THAT THE OUTSIDE AIR TEMP HAD DECREASED SOME 10 DEGS. DURING OUR RETURN TRIP TO CHA AT FL410; WE AGAIN ENCOUNTERED WARM AIR; BUT NOTHING SIMILAR TO THE TYPE OF INVERSION THAT WE ENCOUNTERED AS DESCRIBED HEREIN ABOVE.