Narrative:

We were climbing out of sfo cleared to FL310. Autoplt engaged in center command, LNAV/VNAV captured. FMC was programmed to FL370 on the cruise page and FL310 was entered in the altitude warning indicator (awi) on the MCP. Aircraft was climbing at 1500-2000 FPM. Approaching FL310 autoplt should have altitude captured by FL305. Atl hold was never indicated and the HSI indications did not change to indicate altitude capture until autoplt was disengaged. This was done at FL308 and copilot leveled aircraft manually. Maximum altitude reached was FL313. Once autoplt disconnected flight director operated normally and captured FL310. Aircraft was level at FL310 and center autoplt re-engaged, tried to continue climb. Left and right autoplt were hooked up and both operated normally. Rechked on climb to FL330 and same thing occurred. Remainder of flight continued on right autoplt. Note: system failure was caught 500-700' prior to FL310. Decision to override autoplt was made at 3000', prior to FL310. The autoplt has a variable lead time it uses when capturing an altitude and this lead time is unknown to the pilot. Even while recognizing a problem with the autoplt in advance of the altitude violation the human pilot has to take aggressive action when rates of climb are so great. Human pilots are taught by aim to climb the last 1000' at 500 FPM. Why are our autoplts climbing to an altitude at 2000 FPM and sometimes higher rates the last 1000'?

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Original NASA ASRS Text

Title: ACR LGT OVERSHOT ALT ON CLIMB.

Narrative: WE WERE CLBING OUT OF SFO CLRED TO FL310. AUTOPLT ENGAGED IN CENTER COMMAND, LNAV/VNAV CAPTURED. FMC WAS PROGRAMMED TO FL370 ON THE CRUISE PAGE AND FL310 WAS ENTERED IN THE ALT WARNING INDICATOR (AWI) ON THE MCP. ACFT WAS CLBING AT 1500-2000 FPM. APCHING FL310 AUTOPLT SHOULD HAVE ALT CAPTURED BY FL305. ATL HOLD WAS NEVER INDICATED AND THE HSI INDICATIONS DID NOT CHANGE TO INDICATE ALT CAPTURE UNTIL AUTOPLT WAS DISENGAGED. THIS WAS DONE AT FL308 AND COPLT LEVELED ACFT MANUALLY. MAX ALT REACHED WAS FL313. ONCE AUTOPLT DISCONNECTED FLT DIRECTOR OPERATED NORMALLY AND CAPTURED FL310. ACFT WAS LEVEL AT FL310 AND CENTER AUTOPLT RE-ENGAGED, TRIED TO CONTINUE CLB. LEFT AND RIGHT AUTOPLT WERE HOOKED UP AND BOTH OPERATED NORMALLY. RECHKED ON CLB TO FL330 AND SAME THING OCCURRED. REMAINDER OF FLT CONTINUED ON RIGHT AUTOPLT. NOTE: SYS FAILURE WAS CAUGHT 500-700' PRIOR TO FL310. DECISION TO OVERRIDE AUTOPLT WAS MADE AT 3000', PRIOR TO FL310. THE AUTOPLT HAS A VARIABLE LEAD TIME IT USES WHEN CAPTURING AN ALT AND THIS LEAD TIME IS UNKNOWN TO THE PLT. EVEN WHILE RECOGNIZING A PROB WITH THE AUTOPLT IN ADVANCE OF THE ALT VIOLATION THE HUMAN PLT HAS TO TAKE AGGRESSIVE ACTION WHEN RATES OF CLB ARE SO GREAT. HUMAN PLTS ARE TAUGHT BY AIM TO CLB THE LAST 1000' AT 500 FPM. WHY ARE OUR AUTOPLTS CLBING TO AN ALT AT 2000 FPM AND SOMETIMES HIGHER RATES THE LAST 1000'?

Data retrieved from NASA's ASRS site as of August 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.