MD80 FLT CREW HAD A LOW SPD STALL BUFFET AFTER THE CREW CONTINUED TO USE THE AUTOTHROTTLE WITH AN ERRONEOUS ENG EPR INDICATION. THE DISTRACTION OF CREW MEAL SVC WAS ALSO A CONTRIBUTING FACTOR.

Narrative

THE FLT DEPARTED 1 HR 20 MINS LATE FOM DFW DUE TO MAINT. AN EPR GAUGE SPLIT HAD BEEN WRITTEN UP NUMEROUS TIMES. MAINT HAD INSTALLED A NEW COMPUTER AND PERFORMED EXTENSIVE TESTS INCLUDING RUNNING BOTH ENGS PRIOR TO RELEASING THE ACFT FOR FLT. DURING MAX PWR TKOF AND INITIAL CLBOUT; ENG INDICATIONS WERE NORMAL AND ALIGNED. AS THE ACFT CLBED THROUGH 10000 FT; HOWEVER; EPR INDICATIONS BEGAN TO DIVERGE. AS ALT INCREASED; THE DIFFERENCE WIDENED. COMPARISON OF N1; EGT; FUEL FLOW; AS WELL AS REF TO PMS AND THROTTLE ALIGNMENT CONFIRMED THE R EPR TO BE READING HIGH. IN ORDER TO MAINTAIN NORMAL CLB PERFORMANCE; IT WAS NECESSARY TO DISENGAGE THE AUTOTHROTTLES SO THE ERRONEOUSLY HIGH EPR INDICATION WOULD NOT LIMIT CLB PWR. THE FLT INITIALLY LEVELED AT FL270 DUE TO RPTS OF CHOP AT HIGHER ALTS. ONCE LEVEL; AUTOTHROTTLE WAS REENGAGED AND MAINTAINED SPD NORMALLY. APPROX 1 HR 50 MINS INTO THE FLT; A CHK WITH ZID INDICATED ACCEPTABLE RIDES AT FL330. AUTOTHROTTLES WERE AGAIN DISENGAGED FOR THE CLB. I HAVE MADE IT A HABIT TO LEAVE THE AUTOTHROTTLE WARNING LIGHT FLASHING WHEN INTENTIONALLY USING MANUAL THROTTLE IN SUCH SITS. THE CLB TO FL330 IN MACH MODE WAS NORMAL. AS WE APCHED FL330; I RESET MACH TO .76 AND OBSERVED THE ACFT ENTER ALT HOLD. WE ACCELERATED TO .768 WHERE I REENGAGED AUTOTHROTTLE AND OBSERVED THE SYS MAKE TYPICAL ADJUSTMENT TO MAINTAIN .76 MACH. AFTER SEVERAL MINS; WE BEGAN TO ENCOUNTER CHOP AND PUT IN A REQUEST FOR DSCNT BACK TO FL290. SHORTLY THEREAFTER; #1 FLT ATTENDANT RANG THE COCKPIT TO NOTIFY US THAT DINNER WAS READY. THE NEXT 5 MINS OR SO WAS THE USUAL SIT OF TRAYS; PREPARING TO EAT; ETC. WE WERE STILL ENCOUNTERING THE CHOP; SO MY FO PUT IN ANOTHER REQUEST FOR DSCNT. ALMOST AT THE SAME TIME; A LOW LEVEL VIBRATION BEGAN. IT TOOK US A COUPLE OF SECONDS TO RECOGNIZE A LOW-SPD BUFFET. AIRSPD WAS APPROX 200-210 KTS. I IMMEDIATELY DISCONNECTED THE AUTOPLT; AUTOTHROTTLES AND ADVANCED PWR TO MCT. AT THE SAME TIME; ATC CLRED US TO DSND TO FL290 AND I EASED THE ACFT INTO A 1000 FPM DSCNT. WITHIN A FEW SECONDS; WE FLEW OUT OF THE BUFFET AND FLT PARAMETERS RAPIDLY RETURNED TO NORMAL. WE LEVELED AT FL290 AND CRUISE OF .76 MACH WAS ESTABLISHED. AUTOTHROTTLES WERE REENGAGED AND SUBSEQUENTLY CAREFULLY MONITORED. THE REMAINDER OF THE FLT WAS ROUTINE AND UNEVENTFUL. I WOULD EMPHASIZE THAT AT NO TIME WAS THERE A LOSS OF ACFT CTL NOR A VIOLATION OF ANY ATC CLRNC. AS I DISTINCTLY REMEMBER REENGAGING AUTOTHROTTLES AT FL330 AND OBSERVING AT LEAST INITIAL NORMAL OP; I CAN ONLY CONCLUDE THAT THE ALT CHANGE RENDERED THEM UNABLE TO MAINTAIN CRUISE MACH AT THAT ALT WITH THE R EPR GAUGE READING APPROX 0.18 HIGH. AN OBVIOUS CONTRIBUTING FACTOR WAS THE DISTR OF MEAL SVC AT THAT TIME. IN ANALYZING THIS EVENT; I BELIEVE IT COULD HAVE BEEN PREVENTED BY DISCONTINUING THE USE OF AUTOTHROTTLES IN ALL PHASES OF FLT ONCE THE SIGNIFICANT EPR SPLIT WAS EVIDENT. THE 'TRAP' WAS IN USING THEM FOR CRUISE FLT; EVEN THOUGH OP HAD BEEN NORMAL FOR WELL OVER AN HR AT FL270. SETTING CRUISE PWR MANUALLY USING THE ACCURATE ENG PARAMETERS WOULD NOT ONLY HAVE INCREASED OUR ATTN TO THE MACH SPD; BUT WOULD DEFINITELY HAVE PREVENTED SUCH A SIGNIFICANT CHANGE IN SPD DURING THE SHORT TIME WE WERE DISTR. SHOULD I EVER AGAIN ENCOUNTER A SIGNIFICANT 'SPLIT' IN EPR INDICATIONS; I WILL DISCONTINUE ALL USE OF AUTOTHROTTLES IN ALL PHASES OF THAT FLT AND IN FACT; RECOMMEND THAT BE MADE PROC FOR ALL MD80 CREWS. SUPPLEMENTAL INFO FROM ACN 605365: DURING THE CLB ABOVE 10000 FT WE NOTICED THAT THE EPR SPLIT WAS DEVELOPING AGAIN AND THAT THE PREVIOUS WRITE UP MAY NOT HAVE BEEN CORRECTED. WE NOTICED THAT ALL THE ENG PARAMETERS MATCHED UP EXCEPT THE R ENG EPR WHICH INDICATED HIGHER THAN NORMAL. SHORTLY AFTER LEVELING AT FL330 THE #1 FLT ATTENDANT CALLED ON THE PHONE TO TELL ME THAT OUR MEALS WERE READY. I LEFT MY SEAT TO RECEIVE THE MEALS AND WAS JUST BEGINNING TO EAT WHEN ZID NOTIFIED US THAT WE SHOULD CONSIDER CHANGING ALTS DUE TO THE RIDE RPTS AHEAD. I SAID WE WERE UNABLE FL370 AND WOULD LIKE FL290. HE RESPONDED WE COULD EXPECT FL290 IN 3 MINS. A FEW MINS LATER WE FELT A VIBRATION ASSOCIATED WITH LOW SPD BUFFET; I NOTED OUR SPD WAS ABOUT 205 KTS AND I SAID 'WE'RE SLOW' AS I PUSHED THE THROTTLES FORWARD.