A CE550 FLC SUFFERS AN ALT EXCURSION; LOSS OF TFC SEPARATION DURING A TEMPORARY LOSS OF ACFT CTL IN APCHING A STALL CONDITION AT FL430; 30 NM FROM SLN; KS.

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CRUISING WBOUND AT FL410; CTR ASKED TO ADVISE THEM WHEN WE WOULD BE ABLE TO CLB TO FL430. THE FO AND I COMPUTED OUR WT AND LOOKED UP THE WT REQUIRED FOR THE CLB IN THE PERFORMANCE MANUAL. FINDING OUR WT 200 POUNDS BELOW THAT REQUIRED FOR ISA CONDITIONS; AND NOTING THAT THE TEMP WAS 2-3 DEGS BELOW ISA; WE DETERMINED THAT OUR WT WAS ADEQUATE FOR THE CLB TO FL430. THIS PROCESS TOOK APPROX 3-4 MINS. CTR AGAIN CALLED AND ASKED ABOUT OUR ABILITY TO CLB TO FL430 AND WE RESPONDED THAT WE COULD. THEY ASKED HOW LONG IT WOULD TAKE US TO REACH FL430 AND I TOLD THEM 4 MINS. CTR REQUESTED THAT WE REACH FL430 IN 5 MINS. THE FO (PF) INITIATED A 500 FPM CLB. REACHING FL430; THE AIRSPD WAS RATHER LOW AND ANGLE OF ATTACK HIGH. BOTH WERE ABOVE STALL VALUES; HOWEVER. THE ANGLE OF ATTACK INDICATOR WAS WITHIN THE GREEN ARC SHOWING .55 ANGLE OF ATTACK (.6 IS USED FOR APCH; AND STALL OCCURS AT .8 ANGLE OF ATTACK). THERE IS NO LOW SPD BUFFET VERSUS ALT INFO PUBLISHED FOR THIS ACFT. WE REMAINED AT FL430 FOR APPROX 5-6 MINS WHEN A STALL BUFFET OCCURRED MOMENTARILY; STOPPED; THEN OCCURRED AGAIN. THE AIRPLANE BEGAN LOSING ALT AND THE FO LOWERED THE NOSE INSTINCTIVELY TO PREVENT A FULL STALL. WE ADVISED CTR THAT WE COULD NOT MAINTAIN ALT. THE ACFT DSNDED RATHER QUICKLY AND SEPARATION WITH TFC BELOW WAS LOST. CTR INSTRUCTED US TO MAINTAIN FL390. UNTIL THIS EVENT; GIVEN THE SAME CIRCUMSTANCES UNDER THE SAME CONDITIONS; I WOULD HAVE MADE THE SAME DECISION TO ACCEPT THE CLB. THE ONLY CONTRIBUTING FACTOR IN OUR INABILITY TO MAINTAIN ALT MAY HAVE BEEN THAT A 500 FPM CLB RATE RESULTED IN A LOWER THAN DESIRED FINAL AIRSPD AT THE TOP OF CLB. I HAVE NO EXPLANATION FOR A STALL OCCURRING AT .55 ANGLE OF ATTACK; BUT WILL NO LONGER CONSIDER THE ANGLE OF ATTACK INDICATIONS VALID AT HIGH ALT. GIVEN THAT THE ACFT MAINTAINED ALT FOR A PERIOD OF SEVERAL MINS; I ALSO SUSPECT THAT WE WERE IN AN AREA OF CHANGING TEMPS. AFTER LEVELING AT FL390 AND ASSESSING THE SIT; WE NOTED THAT THE WIND HAD DECREASED AND THE TEMP WAS SLIGHTLY HIGHER THAN WHAT WE HAD EXPERIENCED AT FL410 BEFORE THE CLB. ALTHOUGH I FELT THAT WE HAD ADEQUATE CONSERVATISM BUILT INTO OUR DECISION (BOTH A WT MARGIN AND A TEMP MARGIN); LOOKING FORWARD; I WILL NO LONGER ACCEPT PERFORMANCE MARGINS THIS CLOSE AS BEING ADEQUATE. SUPPLEMENTAL INFO FROM ACN 843436: AT THAT TIME; ATC ADVISED US THAT WE HAD SET OFF AN ALARM WITH AN ACFT WE HAD SEEN PASS UNDER US BEFORE THE INCIDENT.