Narrative:

From my recollection of the events; we had climbed out on the backside of a line of WX containing large; intense cells. We were IMC this entire time. Our track carried us approximately 80-90 mi north of our filed path. Using the radar; we kept greater than 20 mi from the thunderstorms. Clearance was to FL370; but approaching FL368; we could not sustain a climb or accelerate; so FL350 was requested. After reaching FL350; we were able to remain level at cruise of mach .83. As we approached the north end of the WX; before we turned eastbound; we flew into heavy rain with moderate turbulence at FL350. I commented that this was more than expected considering the altitude and no return on the radar. The rain and turbulence only lasted a couple of mins. After turning eastbound around the north end of the line of WX; it was only a short time before we were in clear air. I believe it to have been about 15 mins after flying in clear air that I heard a rumbling sound that seemed to come from the air conditioning system. I was turned around in my seat discussing this sound with the so when I noticed the high stage bleed light was on. My comment was; 'why is the high bleed on? We are no longer using wing anti-ice?' at that time; the aircraft started to vibrate slightly; then increase. I turned to look at my instruments and noticed my speed decreasing from about 280 KIAS. I disconnected the autoplt and started to lower the nose to regain speed. First officer contacted ATC to inform them that we could not maintain altitude and would be in a descent. I disconnected the autothrottles and pushed them up to approximately +100%. Considering the altitude; I was not aggressive on any control inputs. My pitch attitude was slightly below the horizon; but the speed continued to decrease rapidly to 240 KIAS; before rapidly recovering. We descended to FL250 and maintained about 280 KIAS; to determine what had occurred; and how to respond. At this altitude and speed; no flight control or instrument problems were evident. Pitot heat system was checked and working normally. Everything seemed to be back to normal. Not knowing if the airplane had shed a part that caused the noise and vibration; I decided to declare an emergency and that diverting to the nearest suitable airport was the best course of action. The aircraft was configured early to determine that all controls worked appropriately. An uneventful landing was made. In retrospect; the extremely slow reduction by the autothrottles was the cause of the bleed system going into high. This went unnoticed by all. At last glance; the aircraft was on speed and altitude. No unsafe gear lights on were noticed. The recovery from the slow speed was started before a full stall could occur. My company is now investigation why this aircraft's autothrottles slowly retarded the power to idle while maintaining an indication of staying on speed. The autoplt performed as it should by pitching up to maintain the altitude. Question is: why did the airplane indicate on proper speed; but in reality; slow to stalling speed? From a crew member standpoint; we could have recognized the noise that we were investigating as being caused by the engine bleed system switching to high bleed; which in turn is caused by the reduction in power. Noticing that the power had been reduced would have brought up the red flag that would have led to the prevention of this incident.

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Original NASA ASRS Text

Title: A DC10-10 NEARLY STALLED WHEN THE CREW DID NOT DETECT A SLOW AUTOTHROTTLE REDUCTION. AN EMER WAS DECLARED AND FLT DIVERTED FOR AIRFRAME EXAMINATION.

Narrative: FROM MY RECOLLECTION OF THE EVENTS; WE HAD CLBED OUT ON THE BACKSIDE OF A LINE OF WX CONTAINING LARGE; INTENSE CELLS. WE WERE IMC THIS ENTIRE TIME. OUR TRACK CARRIED US APPROX 80-90 MI N OF OUR FILED PATH. USING THE RADAR; WE KEPT GREATER THAN 20 MI FROM THE TSTMS. CLRNC WAS TO FL370; BUT APCHING FL368; WE COULD NOT SUSTAIN A CLB OR ACCELERATE; SO FL350 WAS REQUESTED. AFTER REACHING FL350; WE WERE ABLE TO REMAIN LEVEL AT CRUISE OF MACH .83. AS WE APCHED THE N END OF THE WX; BEFORE WE TURNED EBOUND; WE FLEW INTO HVY RAIN WITH MODERATE TURB AT FL350. I COMMENTED THAT THIS WAS MORE THAN EXPECTED CONSIDERING THE ALT AND NO RETURN ON THE RADAR. THE RAIN AND TURB ONLY LASTED A COUPLE OF MINS. AFTER TURNING EBOUND AROUND THE N END OF THE LINE OF WX; IT WAS ONLY A SHORT TIME BEFORE WE WERE IN CLR AIR. I BELIEVE IT TO HAVE BEEN ABOUT 15 MINS AFTER FLYING IN CLR AIR THAT I HEARD A RUMBLING SOUND THAT SEEMED TO COME FROM THE AIR CONDITIONING SYS. I WAS TURNED AROUND IN MY SEAT DISCUSSING THIS SOUND WITH THE SO WHEN I NOTICED THE HIGH STAGE BLEED LIGHT WAS ON. MY COMMENT WAS; 'WHY IS THE HIGH BLEED ON? WE ARE NO LONGER USING WING ANTI-ICE?' AT THAT TIME; THE ACFT STARTED TO VIBRATE SLIGHTLY; THEN INCREASE. I TURNED TO LOOK AT MY INSTS AND NOTICED MY SPD DECREASING FROM ABOUT 280 KIAS. I DISCONNECTED THE AUTOPLT AND STARTED TO LOWER THE NOSE TO REGAIN SPD. FO CONTACTED ATC TO INFORM THEM THAT WE COULD NOT MAINTAIN ALT AND WOULD BE IN A DSCNT. I DISCONNECTED THE AUTOTHROTTLES AND PUSHED THEM UP TO APPROX +100%. CONSIDERING THE ALT; I WAS NOT AGGRESSIVE ON ANY CTL INPUTS. MY PITCH ATTITUDE WAS SLIGHTLY BELOW THE HORIZON; BUT THE SPD CONTINUED TO DECREASE RAPIDLY TO 240 KIAS; BEFORE RAPIDLY RECOVERING. WE DSNDED TO FL250 AND MAINTAINED ABOUT 280 KIAS; TO DETERMINE WHAT HAD OCCURRED; AND HOW TO RESPOND. AT THIS ALT AND SPD; NO FLT CTL OR INST PROBS WERE EVIDENT. PITOT HEAT SYS WAS CHKED AND WORKING NORMALLY. EVERYTHING SEEMED TO BE BACK TO NORMAL. NOT KNOWING IF THE AIRPLANE HAD SHED A PART THAT CAUSED THE NOISE AND VIBRATION; I DECIDED TO DECLARE AN EMER AND THAT DIVERTING TO THE NEAREST SUITABLE ARPT WAS THE BEST COURSE OF ACTION. THE ACFT WAS CONFIGURED EARLY TO DETERMINE THAT ALL CTLS WORKED APPROPRIATELY. AN UNEVENTFUL LNDG WAS MADE. IN RETROSPECT; THE EXTREMELY SLOW REDUCTION BY THE AUTOTHROTTLES WAS THE CAUSE OF THE BLEED SYS GOING INTO HIGH. THIS WENT UNNOTICED BY ALL. AT LAST GLANCE; THE ACFT WAS ON SPD AND ALT. NO UNSAFE GEAR LIGHTS ON WERE NOTICED. THE RECOVERY FROM THE SLOW SPD WAS STARTED BEFORE A FULL STALL COULD OCCUR. MY COMPANY IS NOW INVESTIGATION WHY THIS ACFT'S AUTOTHROTTLES SLOWLY RETARDED THE PWR TO IDLE WHILE MAINTAINING AN INDICATION OF STAYING ON SPD. THE AUTOPLT PERFORMED AS IT SHOULD BY PITCHING UP TO MAINTAIN THE ALT. QUESTION IS: WHY DID THE AIRPLANE INDICATE ON PROPER SPD; BUT IN REALITY; SLOW TO STALLING SPD? FROM A CREW MEMBER STANDPOINT; WE COULD HAVE RECOGNIZED THE NOISE THAT WE WERE INVESTIGATING AS BEING CAUSED BY THE ENG BLEED SYS SWITCHING TO HIGH BLEED; WHICH IN TURN IS CAUSED BY THE REDUCTION IN PWR. NOTICING THAT THE PWR HAD BEEN REDUCED WOULD HAVE BROUGHT UP THE RED FLAG THAT WOULD HAVE LED TO THE PREVENTION OF THIS INCIDENT.

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.