Narrative:

The B737-300 has 2 autoplt system -- the 'a' system normally used by the captain and the 'B' system used by the first officer. The flight originated out of sac to bur with my first officer flying the first leg. The aircraft had no write-ups and the first flight was normal. The second leg was from bur to phx flown by myself. On climb out, we were assigned 16000 ft by departure. As we were climbing through 15000 ft, the 'a' autoplt went into the altitude acquire mode and appeared to be working properly. We were given a heading and radial to intercept at about the same time. After setting in the radial and heading, I noticed that we were still climbing and approaching 17000 ft. Obviously the automatic leveloff function of the 'a' autoplt had failed. I started a quick descent back to 16000 ft. That was correctly set into the altitude window. The autoplt again went into the altitude acquire mode but did not capture the altitude or leveloff, which I had to do manually. Nothing was said by departure about our altitude deviation and we were cleared over to ZLA. When we checked in with center he questioned our assigned altitude, which we stated was 16000 ft. We were then cleared to FL180 and I switched to the 'B' autoplt system, which leveled off and functioned normally for the remainder of the flight. In phx we had maintenance fix the 'a' autoplt and it checked out ok to our next flight to stl. Having flown the 300 series for over 10 yrs, this was the first time I've had a failure of the automatic level system. The system is so reliable and we depend and trust it so much that when it fails it can really catch you off guard. I feel that if the autoplt goes into the altitude acquire mode but fails to level off at the correct altitude, a more positive warning than just a horn should be given, ie, a verbal 'altitude, altitude!' would be a far better system.

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Original NASA ASRS Text

Title: B737-300 CLBING TO 16000 FT. 'A' AUTOPLT INDICATED ALT ACQUIRE AT 15000 FT. GIVEN VECTOR HDG AND IN SETTING HDG NOTICED AUTOPLT DID NOT CAPTURE AND ACFT WAS NEARING 17000 FT. REGAINED 16000 FT AND CHANGED TO 'B' AUTOPLT. NO COMMENT FROM CTLR.

Narrative: THE B737-300 HAS 2 AUTOPLT SYS -- THE 'A' SYS NORMALLY USED BY THE CAPT AND THE 'B' SYS USED BY THE FO. THE FLT ORIGINATED OUT OF SAC TO BUR WITH MY FO FLYING THE FIRST LEG. THE ACFT HAD NO WRITE-UPS AND THE FIRST FLT WAS NORMAL. THE SECOND LEG WAS FROM BUR TO PHX FLOWN BY MYSELF. ON CLBOUT, WE WERE ASSIGNED 16000 FT BY DEP. AS WE WERE CLBING THROUGH 15000 FT, THE 'A' AUTOPLT WENT INTO THE ALT ACQUIRE MODE AND APPEARED TO BE WORKING PROPERLY. WE WERE GIVEN A HDG AND RADIAL TO INTERCEPT AT ABOUT THE SAME TIME. AFTER SETTING IN THE RADIAL AND HDG, I NOTICED THAT WE WERE STILL CLBING AND APCHING 17000 FT. OBVIOUSLY THE AUTOMATIC LEVELOFF FUNCTION OF THE 'A' AUTOPLT HAD FAILED. I STARTED A QUICK DSCNT BACK TO 16000 FT. THAT WAS CORRECTLY SET INTO THE ALT WINDOW. THE AUTOPLT AGAIN WENT INTO THE ALT ACQUIRE MODE BUT DID NOT CAPTURE THE ALT OR LEVELOFF, WHICH I HAD TO DO MANUALLY. NOTHING WAS SAID BY DEP ABOUT OUR ALTDEV AND WE WERE CLRED OVER TO ZLA. WHEN WE CHKED IN WITH CTR HE QUESTIONED OUR ASSIGNED ALT, WHICH WE STATED WAS 16000 FT. WE WERE THEN CLRED TO FL180 AND I SWITCHED TO THE 'B' AUTOPLT SYS, WHICH LEVELED OFF AND FUNCTIONED NORMALLY FOR THE REMAINDER OF THE FLT. IN PHX WE HAD MAINT FIX THE 'A' AUTOPLT AND IT CHKED OUT OK TO OUR NEXT FLT TO STL. HAVING FLOWN THE 300 SERIES FOR OVER 10 YRS, THIS WAS THE FIRST TIME I'VE HAD A FAILURE OF THE AUTO LEVEL SYS. THE SYS IS SO RELIABLE AND WE DEPEND AND TRUST IT SO MUCH THAT WHEN IT FAILS IT CAN REALLY CATCH YOU OFF GUARD. I FEEL THAT IF THE AUTOPLT GOES INTO THE ALT ACQUIRE MODE BUT FAILS TO LEVEL OFF AT THE CORRECT ALT, A MORE POSITIVE WARNING THAN JUST A HORN SHOULD BE GIVEN, IE, A VERBAL 'ALT, ALT!' WOULD BE A FAR BETTER SYS.

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.