Narrative:

Altitude deviation of about 350 ft. We had been cleared from 8000 ft to 6000 ft. First officer flying, autoplt engaged. We all observed altitude capture on FMA at about 6400 ft, however, a few seconds later the altitude was observed at 5700 ft with aircraft still in a shallow descent and autoplt in vertical speed mode. An immediate climb was initiated back to 6000 ft. We don't know whether first officer accidentally bumped vertical speed knob after altitude capture, or if autoplt lost capture due to turbulence. Reviewed proper autoplt technique with first officer and cautioned him to keep hand away from vertical speed knob after altitude capture. Supplemental information from acn 368990: we were in moderate turbulence due to associated buildups of thunderstorms at the time.

Related reports:

Google
 

Original NASA ASRS Text

Title: DC10 DSNDING TO 6000 FT. FO, PF, INADVERTENTLY GOT INTO VERT SPD MODE ON THE AUTOPLT RESULTING IN OVERSHOOTING ALT 300 FT.

Narrative: ALTDEV OF ABOUT 350 FT. WE HAD BEEN CLRED FROM 8000 FT TO 6000 FT. FO FLYING, AUTOPLT ENGAGED. WE ALL OBSERVED ALT CAPTURE ON FMA AT ABOUT 6400 FT, HOWEVER, A FEW SECONDS LATER THE ALT WAS OBSERVED AT 5700 FT WITH ACFT STILL IN A SHALLOW DSCNT AND AUTOPLT IN VERT SPD MODE. AN IMMEDIATE CLB WAS INITIATED BACK TO 6000 FT. WE DON'T KNOW WHETHER FO ACCIDENTALLY BUMPED VERT SPD KNOB AFTER ALT CAPTURE, OR IF AUTOPLT LOST CAPTURE DUE TO TURB. REVIEWED PROPER AUTOPLT TECHNIQUE WITH FO AND CAUTIONED HIM TO KEEP HAND AWAY FROM VERT SPD KNOB AFTER ALT CAPTURE. SUPPLEMENTAL INFO FROM ACN 368990: WE WERE IN MODERATE TURB DUE TO ASSOCIATED BUILDUPS OF TSTMS AT THE TIME.

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.