Narrative:

En route from smf to lax an medium large transport was cleared to climb to FL370. As FL350 came and went, it became evident that the aircraft was nearing its ceiling so both the captain and I looked at the performance numbers which stated the optimum ceiling was FL360 and maximum ceiling FL370. So we continued climb to FL370 without incident. After level off, the autoplt was placed on and shortly thereafter the clearance to a new heading 30 degrees to right was given. With a bank angle of 15 degrees selected the autoplt began the turn. Both pilots observed this then set about our other duties. A few seconds later both pilots looked up to observe the altitude slowly descending through FL36.8. At that point I disconnected autoplt and tried to regain the altitude to no avail. The aircraft continued to sink on the edge of a stall buffet, unable to hold altitude. After first being denied a lower altitude, the captain again stated that we were unable to maintain FL370 and a clearance to FL330 was received. We believe that either the aircraft was heavier than indicated (i.e., heavier cargo) and our numbers were incorrect. Next time we will both be more aware of the aircraft's warning signs.

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Original NASA ASRS Text

Title: ACR FLC CLBED MLG ABOVE OPTIMUM CEILING. COULD NOT MAINTAIN ALT.

Narrative: ENRTE FROM SMF TO LAX AN MLG WAS CLRED TO CLB TO FL370. AS FL350 CAME AND WENT, IT BECAME EVIDENT THAT THE ACFT WAS NEARING ITS CEILING SO BOTH THE CAPT AND I LOOKED AT THE PERFORMANCE NUMBERS WHICH STATED THE OPTIMUM CEILING WAS FL360 AND MAX CEILING FL370. SO WE CONTINUED CLB TO FL370 WITHOUT INCIDENT. AFTER LEVEL OFF, THE AUTOPLT WAS PLACED ON AND SHORTLY THEREAFTER THE CLRNC TO A NEW HDG 30 DEGS TO R WAS GIVEN. WITH A BANK ANGLE OF 15 DEGS SELECTED THE AUTOPLT BEGAN THE TURN. BOTH PLTS OBSERVED THIS THEN SET ABOUT OUR OTHER DUTIES. A FEW SECONDS LATER BOTH PLTS LOOKED UP TO OBSERVE THE ALT SLOWLY DSNDING THROUGH FL36.8. AT THAT POINT I DISCONNECTED AUTOPLT AND TRIED TO REGAIN THE ALT TO NO AVAIL. THE ACFT CONTINUED TO SINK ON THE EDGE OF A STALL BUFFET, UNABLE TO HOLD ALT. AFTER FIRST BEING DENIED A LOWER ALT, THE CAPT AGAIN STATED THAT WE WERE UNABLE TO MAINTAIN FL370 AND A CLRNC TO FL330 WAS RECEIVED. WE BELIEVE THAT EITHER THE ACFT WAS HEAVIER THAN INDICATED (I.E., HEAVIER CARGO) AND OUR NUMBERS WERE INCORRECT. NEXT TIME WE WILL BOTH BE MORE AWARE OF THE ACFT'S WARNING SIGNS.

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.