Narrative:

Background: flight from uvf to atl; about xa:20 ete; we're about 3 hours into flight approaching florida from southeast. Trw's along southeast coast of fl and coastal waters; crossing our route. We use PIREPS; ATC; WX radar; FMS; decide to top the WX at FL390; in weak part of line. I slow to turbulent air penetration speed (280 KIAS/.76 imn). Aircraft stable at FL390/.76 M. Sat = 52 degrees C; FMS maximum altitude = 39.9; FMS opt altitude = 37.7; fuel = 7.3/1.0/7.4; gw approximately 136.3. WX at FL390 is very thin cloud; good ride (not much bumping at all). Event: without warning (no obvious changes in flight conditions) airspeed started decaying. Autothrottle responded with maximum current cruise limit. Airspeed decay continued faster; airspeed approached min maneuvering speed. I selected N1 limit of maximum continuous; autothrottle advanced; but insufficient thrust. Airspeed decayed below min maneuvering; autoplt now unable to hold altitude. Autoplt discontinued; EFIS displayed 'buffet alert.' we're well below min maneuvering airspeed; descending; no stick shaker or buffeting. Absolutely no icing on windscreen; wipers; wings is visually detectable. Captain turns on wing and engine anti-ice -- we're out of ideas. Hand flying; unable to contact ATC due to radio traffic; aircraft is in slow descent with engines at N1 limit; airspeed dangerously slow. At 38450 ft; mia ATC acknowledges our request for FL370 and approves it. No other words; no TCAS; no traffic; aircraft accelerates at FL370; flight continues uneventfully. Why? 1) high altitude windshear???? 2) ATC frequency congestion is near dangerous at most times; adequate unless something goes wrong. 3) FMS/EFIS provided no warning of speed decay until after it happened. Callback conversation with reporter revealed the following information: the reporter was at a loss to explain the lack of performance given the weight and balance data provided for the flight. He stated that they climbed to FL390 at the recommended climb speed but then reduced to the published turbulence penetration speed of mach .76 in anticipation of turbulence associated with the area of convective activity. When this reduced mach was obtained the drag associated with the higher angle of attack apparently exceeded the thrust available from the engines even at maximum continuous settings. The subsequent descent was the only avenue available to maintain flight above stall buffet. He stated that the lowest airspeed he saw was almost thirty KTS below minimum maneuver speed for the aircraft's configuration and weight. This is; again; clear evidence of operations on the back side of the power curve from which descent is the only means of recovery. Coupled with the inability to communicate with ATC in a timely fashion; the reporter expressed concern about the captain's reluctance to squawk the emergency transponder code. The captain's decision to initiate anti-ice measures without clear indication of aircraft icing was also of concern in that doing so bled air pressure from the engines and likely exacerbated the thrust decrement.

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Original NASA ASRS Text

Title: ENRTE FROM UVF TO ATL; FLT CREW OF A B737-800 CLB TO FL390 TO AVOID WX. DESPITE SATISFACTORY PERFORMANCE CRITERIA IN THE FMC; THE ACFT IS UNABLE TO SUSTAIN AIRSPD AND IS FORCED TO DSND TO MAINTAIN AIRSPD ABOVE A STALL.

Narrative: BACKGROUND: FLT FROM UVF TO ATL; ABOUT XA:20 ETE; WE'RE ABOUT 3 HRS INTO FLT APCHING FLORIDA FROM SE. TRW'S ALONG SE COAST OF FL AND COASTAL WATERS; CROSSING OUR RTE. WE USE PIREPS; ATC; WX RADAR; FMS; DECIDE TO TOP THE WX AT FL390; IN WEAK PART OF LINE. I SLOW TO TURBULENT AIR PENETRATION SPD (280 KIAS/.76 IMN). ACFT STABLE AT FL390/.76 M. SAT = 52 DEGS C; FMS MAX ALT = 39.9; FMS OPT ALT = 37.7; FUEL = 7.3/1.0/7.4; GW APPROX 136.3. WX AT FL390 IS VERY THIN CLOUD; GOOD RIDE (NOT MUCH BUMPING AT ALL). EVENT: WITHOUT WARNING (NO OBVIOUS CHANGES IN FLT CONDITIONS) AIRSPD STARTED DECAYING. AUTOTHROTTLE RESPONDED WITH MAX CURRENT CRUISE LIMIT. AIRSPEED DECAY CONTINUED FASTER; AIRSPEED APCHED MIN MANEUVERING SPD. I SELECTED N1 LIMIT OF MAX CONTINUOUS; AUTOTHROTTLE ADVANCED; BUT INSUFFICIENT THRUST. AIRSPEED DECAYED BELOW MIN MANEUVERING; AUTOPLT NOW UNABLE TO HOLD ALT. AUTOPLT DISCONTINUED; EFIS DISPLAYED 'BUFFET ALERT.' WE'RE WELL BELOW MIN MANEUVERING AIRSPEED; DSNDING; NO STICK SHAKER OR BUFFETING. ABSOLUTELY NO ICING ON WINDSCREEN; WIPERS; WINGS IS VISUALLY DETECTABLE. CAPT TURNS ON WING AND ENG ANTI-ICE -- WE'RE OUT OF IDEAS. HAND FLYING; UNABLE TO CONTACT ATC DUE TO RADIO TFC; ACFT IS IN SLOW DSCNT WITH ENGS AT N1 LIMIT; AIRSPEED DANGEROUSLY SLOW. AT 38450 FT; MIA ATC ACKNOWLEDGES OUR REQUEST FOR FL370 AND APPROVES IT. NO OTHER WORDS; NO TCAS; NO TFC; ACFT ACCELERATES AT FL370; FLT CONTINUES UNEVENTFULLY. WHY? 1) HIGH ALT WINDSHEAR???? 2) ATC FREQ CONGESTION IS NEAR DANGEROUS AT MOST TIMES; ADEQUATE UNLESS SOMETHING GOES WRONG. 3) FMS/EFIS PROVIDED NO WARNING OF SPD DECAY UNTIL AFTER IT HAPPENED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS AT A LOSS TO EXPLAIN THE LACK OF PERFORMANCE GIVEN THE WT AND BAL DATA PROVIDED FOR THE FLT. HE STATED THAT THEY CLBED TO FL390 AT THE RECOMMENDED CLB SPD BUT THEN REDUCED TO THE PUBLISHED TURB PENETRATION SPD OF MACH .76 IN ANTICIPATION OF TURB ASSOCIATED WITH THE AREA OF CONVECTIVE ACTIVITY. WHEN THIS REDUCED MACH WAS OBTAINED THE DRAG ASSOCIATED WITH THE HIGHER ANGLE OF ATTACK APPARENTLY EXCEEDED THE THRUST AVAILABLE FROM THE ENGS EVEN AT MAXIMUM CONTINUOUS SETTINGS. THE SUBSEQUENT DSCNT WAS THE ONLY AVENUE AVAILABLE TO MAINTAIN FLT ABOVE STALL BUFFET. HE STATED THAT THE LOWEST AIRSPD HE SAW WAS ALMOST THIRTY KTS BELOW MINIMUM MANEUVER SPD FOR THE ACFT'S CONFIGURATION AND WT. THIS IS; AGAIN; CLEAR EVIDENCE OF OPS ON THE BACK SIDE OF THE POWER CURVE FROM WHICH DSCNT IS THE ONLY MEANS OF RECOVERY. COUPLED WITH THE INABILITY TO COMMUNICATE WITH ATC IN A TIMELY FASHION; THE RPTR EXPRESSED CONCERN ABOUT THE CAPT'S RELUCTANCE TO SQUAWK THE EMER XPONDER CODE. THE CAPT'S DECISION TO INITIATE ANTI-ICE MEASURES WITHOUT CLEAR INDICATION OF ACFT ICING WAS ALSO OF CONCERN IN THAT DOING SO BLED AIR PRESSURE FROM THE ENGS AND LIKELY EXACERBATED THE THRUST DECREMENT.

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.