CAPT OF AN LGT ATTEMPTED TO OPERATE AT A HIGHER ALT THAN THE ACFT WT WOULD ALLOW; RESULTING IN A STALL BUFFET DUE TO LOW AIRSPD.

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WE WERE FILED 37000 FT. THE CAPT WANTED TO SAVE GAS SO HE PROCEEDED TO PROGRAM DIFFERENT SCENARIOS INTO THE COMPUTER THAT WOULD HOPEFULLY IMPROVE OUR FUEL ECONOMY. HE APPEARED TO BE VERY EXPERIENCED AT MILKING MORE FUEL OUT OF THE NORMAL PLANNED SIT. I WAS CONVINCED HIS KNOWLEDGE EXCEEDED MY LEVEL OF TRAINING AND EXPERIENCE (I.E.; STICKING WITH THE PLANNED PROGRAM). THUS; HE ASKED FOR 41000 AND RECEIVED CLRNC. AS WE STARTED OUR SLOW CLB TOWARD FL410 THE COMPUTER FLASHED THAT 41000 FT WAS ABOVE OUR MAX ALT BY ABOUT 400 FT (I DON'T RECALL THE EXACT FIGURE). SO I INQUIRED AS TO WHETHER WE SHOULD TRY 41000 FT AT THIS TIME. THE CAPT ASSURED ME WE WOULD BE FINE. I PERSONALLY RECALLED EXCEEDING THE COMPUTER ALT LIMIT ON NUMEROUS OTHER FLTS WITH NO PROBS; ALTHOUGH MAYBE NOT BY 400 FT. THEREFORE; I PUT IT TO REST. WITHIN MY MIND EVERYTHING SEEMED ALL RIGHT. UPON LEVEL OFF AT FL410 (I.E; THE AUTOPLT HELD ALT AND THE AUTOTHROTTLE HELD AIRSPD). IN ACTUALITY; MY HINDSIGHT REVEALED WE WERE IN FACT TOO HIGH FOR OUR GROSS WT (I.E.; WE WERE SORT OF BEHIND THE PWR CURVE). SINCE WE HAD RESOLVED WITHIN OUR MINDS THAT WE COULD FLY AT 41000 FT WE NEVER NOTICED THE AUTOTHROTTLES SLOW CREEP TOWARD MAX EPR (PWR FOR CRUISE). THE CAPT GOT UP TO USE THE BATHROOM. I CHKED AND EVEN AT THAT POINT EVERYTHING APPEARED NORMAL (I.E.; AUTOPLT HOLDING COURSE; ATTITUDE AND ALT AND AUTOTHROTTLE STILL HOLDING TARGET (BUG) AIRSPD. THE CAPT COULDN'T' HAVE BEEN GONE MORE THAN 3 MINS WHEN THE NEXT THING I NOTICED WE WERE DRIFTING INTO A SPD BUFFET. I CHKED MY INSTS TO FIND MY AIRSPD BLEEDING OFF. BY THIS TIME; THE CAPT RETURNS TO THE COCKPIT AND NOTICES ME TRYING TO ADD WHAT LITTLE PWR I HAVE LEFT (THE AUTOTHROTTLE STOPPED AT MAX CRUISE RATHER THAN MAX MAX. AFTER A BRIEF DISCUSSION AS TO WHAT WAS HAPPENING; WE RULED OUT AIRFRAME/ENG ICING AND THE CAPT SAID 'STALL.' SO I DISENGAGED AUTOPLT; EASED THE NOSE DOWN (2000 FPM) AND THE BUFFET STOPS. WHENEVER I TRIED TO RAISE THE NOSE; THE BUFFET RETURNED. THUS; I KEPT THE NOSE DOWN UNTIL I RECOVERED ALL OF MY AIRSPD (LOWEST SPD ABOUT 210 KTS). I WAS ABLE TO LEVEL OFF AT 35000 FT BUFFET FREE. THE REST OF THE FLT PROVED UNEVENTFUL. WE NEVER ACTUALLY GOT CLOSE TO STALL; BUT ONLY LOW SPD BUFFET BECAUSE WE NEVER GOT A 'STICK SHAKER.'