Narrative:

On IFR flight plan cruising at FL430 between targa intersection and flann intersection on HF frequency 6577. Initial OAT (RAT) -45 degrees C; had requested from ATC to climb to FL450. Request denied; had WX approximately 60 mi ahead; so asked for 30 NM (right) of crs. Was in VMC; while deviating (right) of largest cells; entered IMC at mach .73 only occasional light turbulence in some frozen precipitation. Started to notice rapid airspeed decrease; advanced thrust to takeoff thrust; still airspeed decreasing rapidly now to 150 KIAS; requested lower. ATC was very busy; no reply from ATC; had no choice but to start descent as a stall was 'imminent' co-captain was able to notify ATC we were in a descent to FL410; was able to maintain 150 but not increasing in descent; had all (wing; engine) anti-ice on prior to entering IMC; did not have tail de-ice on because of boot temperature limit; inside cell temperature rapidly changed to -20 degrees. At that point at FL410 manually activated tail boots; maintained takeoff thrust to limit. Shortly after exited cell; and airspeed began slow increase back to mach .73 at climb power. 50 or so mi more west was able to maintain mach .73 with normal power settings. Callback conversation with reporter revealed the following information: reporter stated that an emergency was not declared; and ATC approved FL410 as cruise altitude for remainder of flight. No airframe icing was visible while the airspeed was rapidly decreasing.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.