AN MD80 WEIGHING 137000 LBS ATTEMPTED TO CLB TO FL340 BUT HAD TO DSND AFTER THE ACFT ENTERED A LIGHT STALL BUFFET AT 200 KTS.

Narrative

WE DEPARTED ZZZ FOR ZZZ1. IT WAS THE CAPT'S LEG. THE AUTOTHROTTLES WERE INOP. WE LEVELED AT FL330; NEAR ZZZ2. WE CHKED OUR WT; 137000 LBS; AND CHANGED THE SPD BOOKLET TO THAT WT. WE THEN BUGGED 10 KTS. ABOVE CLEAN SPD; 250 KTS. SET THE THROTTLES TO 1.68 EPR. THIS WAS HOLDING .75 MACH. ATC CALLED AND ASKED IF WE COULD ACCEPT FL340. WE BOTH CHKED THE FMS AND VERIFIED THAT WE COULD CLB TO FL340. THE AUTOPLT WAS ON; SO WE SELECTED 800 FPM AND ADVANCED THE THROTTLES TO CLB PWR. IT WAS AT THIS POINT THAT WE BOTH NOTICED THAT OUR AIRSPD WAS 225 KTS. IT STAYED AT 225 KTS UNTIL WE REACHED FL335; EVEN THOUGH I HAD REDUCED OUR VERT SPD TO 300 FPM. AT THIS POINT THE AIRSPD DECAYED TO 200 KTS; I STOPPED THE CLB AND REQUESTED A DSCNT BACK DOWN TO FL320. ATC SAID THAT THEY COULDN'T LET US DSND. STARTED TO FEEL LIGHT BUFFET. INFORMED ATC THAT WE WERE BEGINNING A DSCNT. SHORTLY THEREAFTER ATC CLRED US TO DSND TO FL300. AS WE DSNDED WE WERE ABLE TO ACCELERATE TO 260 KTS. ATC MADE NO MENTION OF ANY KIND OF CONFLICT WITH ANY OTHER ACFT. THE EVENT WAS CAUSED BY 2 THINGS. FIRST; THE AUTOTHROTTLES WERE INOP. SECOND; I WAS NOT IN THE HABIT OF CROSSCHKING AIRSPD AS OFTEN AS I SHOULD HAVE WITHOUT AUTOTHROTTLES BEING AVAILABLE (HABIT PATTERN). FINALLY; THE COMPANY USES A COST INDEX OF 11 IN THE FMS. THIS KEEPS THE ACFT TOO CLOSE TO THE BOTTOM MARGIN. WITHOUT AUTOTHROTTLES I SHOULD HAVE USED A HIGHER COST INDEX.