Narrative:

On initial descent into tpa, approximately 120 mi north of field, we were restr to .70 mach for traffic. ZJX was stepping us down out of cruise altitude in 2000 ft segments. The captain had to exit the flight deck to go to the restroom. During the confusion, I believe the controller cleared us to descend to FL310. However, I did not change the autoplt altitude window to the new altitude of FL310. Instead, it was left at 33000 ft. On the MD88, during an FMS 'cruise' descent, it requires several inputs. This process is compounded further if you are restr to a speed that is not the default cruise speed. Our aircraft entered the 'altitude capture' mode for 33000 ft. As it did, the speed defaulted to normal cruise speed and began to accelerate. I selected IAS/mach speed on the autoplt panel -- which dropped the 'altitude capture' because of an anomaly in the system. The aircraft descended to FL326 before manual recovery back to FL330. At FL328 the controller issued a continue descent to FL290. I don't believe there was an altitude deviation because we were cleared to FL310. Recommend to change MD88 software so that 'altitude capture' remains 'armed' if you change the descent profile during altitude capture phase. Supplemental information from acn 383085: when I returned, we were descending below FL330 to FL290. The first officer asked me what altitude we had been previously cleared to and I replied that I believed it was FL310. He said that's what he thought also, but that we had not changed the altitude alert window from FL330 and he got a warning at FL327 and climbed back up to FL330 because he was unsure of the actual clearance. 1 of 2 things had happened, either: 1) we forgot to change the altitude alert to FL310 when we were cleared or: 2) we were only cleared to FL330 and erroneously began descent below FL330 because of a 'glitch' that exists in the FMS software regarding the altitude capture feature.

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Original NASA ASRS Text

Title: AN MD88 DSNDING INTO TPA, FL, OVERSHOOTS CLRED ALT BECAUSE OF FMC SETTINGS AND MISUNDERSTANDING OF CLRED ALT.

Narrative: ON INITIAL DSCNT INTO TPA, APPROX 120 MI N OF FIELD, WE WERE RESTR TO .70 MACH FOR TFC. ZJX WAS STEPPING US DOWN OUT OF CRUISE ALT IN 2000 FT SEGMENTS. THE CAPT HAD TO EXIT THE FLT DECK TO GO TO THE RESTROOM. DURING THE CONFUSION, I BELIEVE THE CTLR CLRED US TO DSND TO FL310. HOWEVER, I DID NOT CHANGE THE AUTOPLT ALT WINDOW TO THE NEW ALT OF FL310. INSTEAD, IT WAS LEFT AT 33000 FT. ON THE MD88, DURING AN FMS 'CRUISE' DSCNT, IT REQUIRES SEVERAL INPUTS. THIS PROCESS IS COMPOUNDED FURTHER IF YOU ARE RESTR TO A SPD THAT IS NOT THE DEFAULT CRUISE SPD. OUR ACFT ENTERED THE 'ALT CAPTURE' MODE FOR 33000 FT. AS IT DID, THE SPD DEFAULTED TO NORMAL CRUISE SPD AND BEGAN TO ACCELERATE. I SELECTED IAS/MACH SPD ON THE AUTOPLT PANEL -- WHICH DROPPED THE 'ALT CAPTURE' BECAUSE OF AN ANOMALY IN THE SYS. THE ACFT DSNDED TO FL326 BEFORE MANUAL RECOVERY BACK TO FL330. AT FL328 THE CTLR ISSUED A CONTINUE DSCNT TO FL290. I DON'T BELIEVE THERE WAS AN ALTDEV BECAUSE WE WERE CLRED TO FL310. RECOMMEND TO CHANGE MD88 SOFTWARE SO THAT 'ALT CAPTURE' REMAINS 'ARMED' IF YOU CHANGE THE DSCNT PROFILE DURING ALT CAPTURE PHASE. SUPPLEMENTAL INFO FROM ACN 383085: WHEN I RETURNED, WE WERE DSNDING BELOW FL330 TO FL290. THE FO ASKED ME WHAT ALT WE HAD BEEN PREVIOUSLY CLRED TO AND I REPLIED THAT I BELIEVED IT WAS FL310. HE SAID THAT'S WHAT HE THOUGHT ALSO, BUT THAT WE HAD NOT CHANGED THE ALT ALERT WINDOW FROM FL330 AND HE GOT A WARNING AT FL327 AND CLBED BACK UP TO FL330 BECAUSE HE WAS UNSURE OF THE ACTUAL CLRNC. 1 OF 2 THINGS HAD HAPPENED, EITHER: 1) WE FORGOT TO CHANGE THE ALT ALERT TO FL310 WHEN WE WERE CLRED OR: 2) WE WERE ONLY CLRED TO FL330 AND ERRONEOUSLY BEGAN DSCNT BELOW FL330 BECAUSE OF A 'GLITCH' THAT EXISTS IN THE FMS SOFTWARE REGARDING THE ALT CAPTURE FEATURE.

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.