Narrative:

I had climbed to FL470 for wind. The sat was isa -- 10 degrees, mach .70, weight was 19000 pounds per the manual. I requested a FL470-FL490 block to climb and descend back to FL470 to accelerate to cruise. At FL480, a sudden rise in temperature to isa +5 degrees happened, probably due to a front east of florida, causing a warm air influx at altitude. There was nothing in the temperature charts about this. The mach decreased to .68 mach, and the aircraft wouldn't hold altitude. I descended to FL470, and the aircraft would not hold speed. I had no choice except to ask for a descent to FL450. The controller would not give a lower altitude and told me miami would give it to us. I had to descend to keep the speed from decreasing further. The controller finally gave me FL450 after I was at FL465. No problems after that, as the temperature was 15 at -5 at FL450. I have not seen this temperature flux before. But I will pay more attention to fronts and watch the sat more closely.

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Original NASA ASRS Text

Title: A LEAR 45 FLC EXPERIENCES HIGH ALT WINDSHEAR AND IS UNABLE TO MAINTAIN AIRSPD AND ALT.

Narrative: I HAD CLBED TO FL470 FOR WIND. THE SAT WAS ISA -- 10 DEGS, MACH .70, WT WAS 19000 LBS PER THE MANUAL. I REQUESTED A FL470-FL490 BLOCK TO CLB AND DSND BACK TO FL470 TO ACCELERATE TO CRUISE. AT FL480, A SUDDEN RISE IN TEMP TO ISA +5 DEGS HAPPENED, PROBABLY DUE TO A FRONT E OF FLORIDA, CAUSING A WARM AIR INFLUX AT ALT. THERE WAS NOTHING IN THE TEMP CHARTS ABOUT THIS. THE MACH DECREASED TO .68 MACH, AND THE ACFT WOULDN'T HOLD ALT. I DSNDED TO FL470, AND THE ACFT WOULD NOT HOLD SPD. I HAD NO CHOICE EXCEPT TO ASK FOR A DSCNT TO FL450. THE CTLR WOULD NOT GIVE A LOWER ALT AND TOLD ME MIAMI WOULD GIVE IT TO US. I HAD TO DSND TO KEEP THE SPD FROM DECREASING FURTHER. THE CTLR FINALLY GAVE ME FL450 AFTER I WAS AT FL465. NO PROBS AFTER THAT, AS THE TEMP WAS 15 AT -5 AT FL450. I HAVE NOT SEEN THIS TEMP FLUX BEFORE. BUT I WILL PAY MORE ATTN TO FRONTS AND WATCH THE SAT MORE CLOSELY.

Data retrieved from NASA's ASRS site as of July 2007 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.