Narrative:

We were cruising at FL330 just passing fam in light enough cirrus that we could see the cloud deck below us. We were at mach pt 76, engine and airfoil anti-ice off. Our first indication of something wrong was the feel of developing moderate chop which became more uniform. The captain, who was flying the aircraft, then noticed our airspeed had decreased to approximately 220 KIAS. He immediately disconnected the autothrottle and autoplt, pushed up both throttles and initiated a very slight descent. Immediately after he mentioned our airspeed, I placed both the engine and airfoil anti-ice system on and got a clearance from center to descend to FL290. Center approved the descent with no delay and asked us if all was ok and I responded in the affirmative. That was the extent of center's inquiry. Everything was back to normal well before leveling at FL290. At that point the captain made a PA to passenger and asked the flight attendants what they had felt in the back of the airplane. The flight attendants said they felt no compressor stall, but on deplaning, a deadheading pilot said he felt a slight compressor stall in one of the engines. Therefore, the captain wrote it up and called maintenance to investigate. Supplemental information from acn 582149: we felt what we first thought was light to moderate chop developing. It was different from most chop in that it was more uniform in strength. Feeling that I immediately looked at the airspeed indicator. Airspeed had unknowingly decayed to approximately 220 KIAS. Right engine N1 was fluctuating about +/-6 RPM's and egt was not elevated. I immediately perceived that we were possibly approaching a low speed buffet. He then cleared us to FL290 and asked if we needed further assistance. By that time the buffet had stopped and airspeed had increased. The buffeting had stopped at approximately FL326. The only conclusion I can come up with is that we must have been in actual icing conditions and I misperceived it, and although the autothrottles did not disengage I should have noticed the uncommanded power reduction while we were conversing.

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Original NASA ASRS Text

Title: AN MD80 IN CRUISE AT FL330 EXPERIENCED AN UNCOMMANDED PWR REDUCTION. BELIEVED BY CREW AND DEADHEADING PLT TO HAVE BEEN A COMPRESSOR STALL.

Narrative: WE WERE CRUISING AT FL330 JUST PASSING FAM IN LIGHT ENOUGH CIRRUS THAT WE COULD SEE THE CLOUD DECK BELOW US. WE WERE AT MACH PT 76, ENG AND AIRFOIL ANTI-ICE OFF. OUR FIRST INDICATION OF SOMETHING WRONG WAS THE FEEL OF DEVELOPING MODERATE CHOP WHICH BECAME MORE UNIFORM. THE CAPT, WHO WAS FLYING THE ACFT, THEN NOTICED OUR AIRSPD HAD DECREASED TO APPROX 220 KIAS. HE IMMEDIATELY DISCONNECTED THE AUTOTHROTTLE AND AUTOPLT, PUSHED UP BOTH THROTTLES AND INITIATED A VERY SLIGHT DSCNT. IMMEDIATELY AFTER HE MENTIONED OUR AIRSPD, I PLACED BOTH THE ENG AND AIRFOIL ANTI-ICE SYS ON AND GOT A CLRNC FROM CTR TO DSND TO FL290. CTR APPROVED THE DSCNT WITH NO DELAY AND ASKED US IF ALL WAS OK AND I RESPONDED IN THE AFFIRMATIVE. THAT WAS THE EXTENT OF CTR'S INQUIRY. EVERYTHING WAS BACK TO NORMAL WELL BEFORE LEVELING AT FL290. AT THAT POINT THE CAPT MADE A PA TO PAX AND ASKED THE FLT ATTENDANTS WHAT THEY HAD FELT IN THE BACK OF THE AIRPLANE. THE FLT ATTENDANTS SAID THEY FELT NO COMPRESSOR STALL, BUT ON DEPLANING, A DEADHEADING PLT SAID HE FELT A SLIGHT COMPRESSOR STALL IN ONE OF THE ENGS. THEREFORE, THE CAPT WROTE IT UP AND CALLED MAINT TO INVESTIGATE. SUPPLEMENTAL INFO FROM ACN 582149: WE FELT WHAT WE FIRST THOUGHT WAS LIGHT TO MODERATE CHOP DEVELOPING. IT WAS DIFFERENT FROM MOST CHOP IN THAT IT WAS MORE UNIFORM IN STRENGTH. FEELING THAT I IMMEDIATELY LOOKED AT THE AIRSPD INDICATOR. AIRSPD HAD UNKNOWINGLY DECAYED TO APPROX 220 KIAS. R ENG N1 WAS FLUCTUATING ABOUT +/-6 RPM'S AND EGT WAS NOT ELEVATED. I IMMEDIATELY PERCEIVED THAT WE WERE POSSIBLY APCHING A LOW SPD BUFFET. HE THEN CLRED US TO FL290 AND ASKED IF WE NEEDED FURTHER ASSISTANCE. BY THAT TIME THE BUFFET HAD STOPPED AND AIRSPD HAD INCREASED. THE BUFFETING HAD STOPPED AT APPROX FL326. THE ONLY CONCLUSION I CAN COME UP WITH IS THAT WE MUST HAVE BEEN IN ACTUAL ICING CONDITIONS AND I MISPERCEIVED IT, AND ALTHOUGH THE AUTOTHROTTLES DID NOT DISENGAGE I SHOULD HAVE NOTICED THE UNCOMMANDED PWR REDUCTION WHILE WE WERE CONVERSING.

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.