AFTER SUFFERING FROM AIRSPD DECAY WITH HIGHER THAN EXPECTED STATIC AIR TEMPS AND WITH LOSS OF ACFT CTL IMMINENT; THE PIC OF A WESTWIND 11 CORP JET LEFT FL430 IN CONFLICT WITH OTHER TFC AT FL410 ON FREQUENCY WITH ZLC; UT.

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WHILE ENRTE FROM BHM TO BFI AT FL410; WE WERE NEAR JACKSON; WY; WHEN WE REQUESTED FL430. WE WERE THEN INSTRUCTED BY ATC TO CLIMB TO FL430. AS THE ACFT CLIMBED TO FL430; WE BEGAN TO ENCOUNTER LIGHT TURB. OVER THE NEXT APPROX FIVE MINS; THE ACFT STARTED TO SLOWLY DECELERATE AND THE TURB CONTINUED TO INCREASE. THE CAPT THEN ADVISED ME TO REQUEST A LOWER ALT; WHICH I DID. WE WERE ADVISED IT WOULD BE FIVE MINS BEFORE WE COULD DESCEND DUE TO CONFLICTING TFC. THE TURB CONTINUED TO GET WORSE AND THE AIRSPD CONTINUED TO DECELERATE. THE CAPT AGAIN INSTRUCTED ME TO ASK FOR A LOWER ALT. I AGAIN REQUESTED A LOWER ALT FROM ATC AND WAS TOLD TO STAND BY; AS THERE WAS CONFLICTING TFC. I THEN OBSERVED THE TFC; AN AIRLINER AT OUR 11 O'CLOCK POSITION; AND ADVISED ATC WE HAD THE TFC IN SIGHT. ATC THEN ADVISED US WE COULD DESCEND WHEN WE HAD 5 MILE SEPARATION. THE CAPT ADVISED ATC WE NEEDED TO DESCEND RIGHT AWAY AND WE COULD ACCEPT A VECTOR IN ANY DIRECTION. THE TURB WAS NOW MODERATE AND THE AIRSPD WAS CONTINUING TO DECREASE. THE CAPT HAD INCREASED PWR TO MAX CONTINUOUS PWR TO MAINTAIN ALT AND WAS HAVING AN INCREASINGLY DIFFICULT TIME MAINTAINING POSITIVE CTL OF THE ACFT. ATC ADVISED US THE ONLY WAY WE COULD DESCEND WAS IF WE DECLARE AN EMER. I STATED; 'OK; WE ARE DESCENDING' AND WE VACATED FL430. ATC INSTRUCTED US TO DESCEND AFTER WE WERE AT FL426. THE CONFLICTING TFC WAS IN SIGHT AND WAS NO FACTOR; AS WE STARTED OUR DESCENT. IT WAS THE CAPT'S OPINION; AND I CONCUR; THAT THE ACFT WAS BECOMING INCREASINGLY HARD TO CTL AND THE CONFLICTING TFC WAS NOT A FACTOR WHEN WE STARTED THE DESCENT. THE CHAIN OF EVENTS CONTRIBUTING TO INFLT EMER WERE: 1) DECAYING AIRSPD 2) INCREASINGLY MODERATE TURB 3) HIGHER THAN NORMAL ISA (INDICATED STATIC AIR TEMP) WHICH DECREASED THE ACFT'S PERFORMANCE. THE HUMAN PERFORMANCE CONSIDERATIONS WERE: 1) WE SHOULD HAVE BEEN MORE INFORMATIVE TO ATC AS TO THE PROBS WE WERE ENCOUNTERING. 2) WE SHOULD HAVE MADE THE DECISION EARLIER TO START THE DESCENT. 3) I SHOULD HAVE BEEN MORE CLR AND CONCISE WITH ATC THAT WE WERE DECLARING AN EMER AS WE STARTED OUR DESCENT.