C 560XL IS UNABLE TO MAINTAIN ALT WHILE DEVIATING AROUND TSTMS. FLT IS FORCED TO DESCEND PRIOR TO RECEIVING CLRNC IN ORDER TO MAINTAIN AIRSPEED.

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ON IFR FLT PLAN CRUISING AT FL430 BTWN TARGA INTXN AND FLANN INTXN ON HF FREQ 6577. INITIAL OAT (RAT) -45 DEGS C; HAD REQUESTED FROM ATC TO CLB TO FL450. REQUEST DENIED; HAD WX APPROX 60 MI AHEAD; SO ASKED FOR 30 NM (R) OF CRS. WAS IN VMC; WHILE DEVIATING (R) OF LARGEST CELLS; ENTERED IMC AT MACH .73 ONLY OCCASIONAL LIGHT TURB IN SOME FROZEN PRECIP. STARTED TO NOTICE RAPID AIRSPD DECREASE; ADVANCED THRUST TO TKOF THRUST; STILL AIRSPD DECREASING RAPIDLY NOW TO 150 KIAS; REQUESTED LOWER. ATC WAS VERY BUSY; NO REPLY FROM ATC; HAD NO CHOICE BUT TO START DSCNT AS A STALL WAS 'IMMINENT' CO-CAPT WAS ABLE TO NOTIFY ATC WE WERE IN A DSCNT TO FL410; WAS ABLE TO MAINTAIN 150 BUT NOT INCREASING IN DSCNT; HAD ALL (WING; ENG) ANTI-ICE ON PRIOR TO ENTERING IMC; DID NOT HAVE TAIL DE-ICE ON BECAUSE OF BOOT TEMP LIMIT; INSIDE CELL TEMP RAPIDLY CHANGED TO -20 DEGS. AT THAT POINT AT FL410 MANUALLY ACTIVATED TAIL BOOTS; MAINTAINED TKOF THRUST TO LIMIT. SHORTLY AFTER EXITED CELL; AND AIRSPD BEGAN SLOW INCREASE BACK TO MACH .73 AT CLB PWR. 50 OR SO MI MORE W WAS ABLE TO MAINTAIN MACH .73 WITH NORMAL PWR SETTINGS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT AN EMER WAS NOT DECLARED; AND ATC APPROVED FL410 AS CRUISE ALT FOR REMAINDER OF FLT. NO AIRFRAME ICING WAS VISIBLE WHILE THE AIRSPD WAS RAPIDLY DECREASING.