Narrative:

While in cruise at FL370, the captain left the flight station to relieve himself. I was performing some fuel xfeed adjustments at this time and the first officer was looking at the flight plan at the time to the best of my recollection. The aircraft was on autoplt and autothrottle. About 2 mins after the captain left his seat, the first officer suddenly alerted to airspeed deterioration. My attention was immediately directed to the application of power as was the first officer's. I noted airspeed/mach to be at approximately 210 KTS and .650 mach. Immediate application of power arrested airspeed deterioration and airspeed began to increase immediately. However, level flight at FL370 could not be initially sustained without the onset of low speed buffet and it became evident that some altitude would have to be traded for airspeed to immediately return airspeed to desired minimums. By this time, the captain had returned to the flight station. He immediately recognized the situation and checked the 360 degree TCASII display and concurred some descent was required to rapidly regain airspeed. All possible visual clearing was also exercised. This maneuver resulted in the sacrifice of 2000 ft of altitude and climb back to FL370 was begun immediately upon attaining sufficient climb airspeed. Time of altitude was probably about 3 mins. Total time of event is estimated to be 5 or 6 mins. The situation was further complicated by the fact that engine #1 tet (total engine temperature) gauge pointer was noted to be off-scale high at this time and some momentary doubt of engine condition had to be resolved. It was noted that both tet pointer and drum digital display were erroneous and high levels of power were managed with use of EPR and RPM instruments for engine #1. Delayed notice of the failure of the autothrottle system to maintain desired mach and the very slow and insidious retardation of the throttles thus resulting in loss of airspeed, went unnoticed because there was no autothrottle disconnect or other visual or aural warning to alert either remaining crew member to the problem that began while we allowed ourselves to be distraction by other duties which could have been delayed for the brief time the captain was out of his duty station.

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Original NASA ASRS Text

Title: AN L1011-500 IN CRUISE AT FL370 EXPERIENCED A SLOW LOSS OF AIRSPD AND WAS REQUIRED TO DSND 2000 FT TO RECOVER, CAUSED BY AUTOTHROTTLE FAILURE.

Narrative: WHILE IN CRUISE AT FL370, THE CAPT LEFT THE FLT STATION TO RELIEVE HIMSELF. I WAS PERFORMING SOME FUEL XFEED ADJUSTMENTS AT THIS TIME AND THE FO WAS LOOKING AT THE FLT PLAN AT THE TIME TO THE BEST OF MY RECOLLECTION. THE ACFT WAS ON AUTOPLT AND AUTOTHROTTLE. ABOUT 2 MINS AFTER THE CAPT LEFT HIS SEAT, THE FO SUDDENLY ALERTED TO AIRSPD DETERIORATION. MY ATTN WAS IMMEDIATELY DIRECTED TO THE APPLICATION OF PWR AS WAS THE FO'S. I NOTED AIRSPD/MACH TO BE AT APPROX 210 KTS AND .650 MACH. IMMEDIATE APPLICATION OF PWR ARRESTED AIRSPD DETERIORATION AND AIRSPD BEGAN TO INCREASE IMMEDIATELY. HOWEVER, LEVEL FLT AT FL370 COULD NOT BE INITIALLY SUSTAINED WITHOUT THE ONSET OF LOW SPD BUFFET AND IT BECAME EVIDENT THAT SOME ALT WOULD HAVE TO BE TRADED FOR AIRSPD TO IMMEDIATELY RETURN AIRSPD TO DESIRED MINIMUMS. BY THIS TIME, THE CAPT HAD RETURNED TO THE FLT STATION. HE IMMEDIATELY RECOGNIZED THE SIT AND CHKED THE 360 DEG TCASII DISPLAY AND CONCURRED SOME DSCNT WAS REQUIRED TO RAPIDLY REGAIN AIRSPD. ALL POSSIBLE VISUAL CLRING WAS ALSO EXERCISED. THIS MANEUVER RESULTED IN THE SACRIFICE OF 2000 FT OF ALT AND CLB BACK TO FL370 WAS BEGUN IMMEDIATELY UPON ATTAINING SUFFICIENT CLB AIRSPD. TIME OF ALT WAS PROBABLY ABOUT 3 MINS. TOTAL TIME OF EVENT IS ESTIMATED TO BE 5 OR 6 MINS. THE SIT WAS FURTHER COMPLICATED BY THE FACT THAT ENG #1 TET (TOTAL ENG TEMP) GAUGE POINTER WAS NOTED TO BE OFF-SCALE HIGH AT THIS TIME AND SOME MOMENTARY DOUBT OF ENG CONDITION HAD TO BE RESOLVED. IT WAS NOTED THAT BOTH TET POINTER AND DRUM DIGITAL DISPLAY WERE ERRONEOUS AND HIGH LEVELS OF PWR WERE MANAGED WITH USE OF EPR AND RPM INSTS FOR ENG #1. DELAYED NOTICE OF THE FAILURE OF THE AUTOTHROTTLE SYS TO MAINTAIN DESIRED MACH AND THE VERY SLOW AND INSIDIOUS RETARDATION OF THE THROTTLES THUS RESULTING IN LOSS OF AIRSPD, WENT UNNOTICED BECAUSE THERE WAS NO AUTOTHROTTLE DISCONNECT OR OTHER VISUAL OR AURAL WARNING TO ALERT EITHER REMAINING CREW MEMBER TO THE PROB THAT BEGAN WHILE WE ALLOWED OURSELVES TO BE DISTR BY OTHER DUTIES WHICH COULD HAVE BEEN DELAYED FOR THE BRIEF TIME THE CAPT WAS OUT OF HIS DUTY STATION.

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.