Narrative:

Several mins after flying through an overhang for about 3 mins in relative smooth air, in cruise, VFR, at FL310, airspeed was slowing. All other engine indication normal. No indication of possible ice on windshield or wipers or engine readings. Did not notice RAT temperature. Pitot heat on and normal. Speed was now down to about 235 mph and aircraft started to buffet. Autoplt and autothrottle still on. Turned on engine and airfoil anti-ice. Speed deteriorated further. Disengaged autoplt and autothrottle. Lowered nose to avoid buffeting or possible stall. As soon as center frequency was clear we advised center of our situation (unable to hold altitude and in a slow descend). Center asked if we could level off at FL290 and to advise when and if we could climb back up to FL310. We answered affirmatively. It took 5-7 mins before speed started slowly to increase. Eventually we climbed back to FL310 at a rate of 200-300 FPM. At altitude (FL310) all anti-ice system off. Autoplt and throttle engaged. Speed normal, all indication normal. Continued flight without further problems. No traffic conflict during this situation. Again, the only indication of possible ice was loss of airspeed.

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Original NASA ASRS Text

Title: AN ACR LGT IS UNABLE TO MAINTAIN CRUISE SPD AND ALT AT FL310 FOR NO APPARENT REASON. PIC HAS TO DSND TO PREVENT STALL BUFFET.

Narrative: SEVERAL MINS AFTER FLYING THROUGH AN OVERHANG FOR ABOUT 3 MINS IN RELATIVE SMOOTH AIR, IN CRUISE, VFR, AT FL310, AIRSPD WAS SLOWING. ALL OTHER ENG INDICATION NORMAL. NO INDICATION OF POSSIBLE ICE ON WINDSHIELD OR WIPERS OR ENG READINGS. DID NOT NOTICE RAT TEMP. PITOT HEAT ON AND NORMAL. SPD WAS NOW DOWN TO ABOUT 235 MPH AND ACFT STARTED TO BUFFET. AUTOPLT AND AUTOTHROTTLE STILL ON. TURNED ON ENG AND AIRFOIL ANTI-ICE. SPD DETERIORATED FURTHER. DISENGAGED AUTOPLT AND AUTOTHROTTLE. LOWERED NOSE TO AVOID BUFFETING OR POSSIBLE STALL. AS SOON AS CTR FREQ WAS CLR WE ADVISED CTR OF OUR SIT (UNABLE TO HOLD ALT AND IN A SLOW DSND). CTR ASKED IF WE COULD LEVEL OFF AT FL290 AND TO ADVISE WHEN AND IF WE COULD CLB BACK UP TO FL310. WE ANSWERED AFFIRMATIVELY. IT TOOK 5-7 MINS BEFORE SPD STARTED SLOWLY TO INCREASE. EVENTUALLY WE CLBED BACK TO FL310 AT A RATE OF 200-300 FPM. AT ALT (FL310) ALL ANTI-ICE SYS OFF. AUTOPLT AND THROTTLE ENGAGED. SPD NORMAL, ALL INDICATION NORMAL. CONTINUED FLT WITHOUT FURTHER PROBS. NO TFC CONFLICT DURING THIS SIT. AGAIN, THE ONLY INDICATION OF POSSIBLE ICE WAS LOSS OF AIRSPD.

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.