Narrative:

Air carrier X was descending to cross lamma intersection on the BITH07 arrival into mlb. The captain (me) was at the controls. I was using VNAV for the descent. The FMS was programmed to cross lamma at 15000 ft and 320 KTS. Approximately 500 ft prior to level off, the speed command bug slowed to 240 KTS, the throttles went to idle, and the sink rate increased to approximately 1800 FPM down. I selected vertical speed on the mode control panel, selected 500 FPM down, and increased the speed bug back to 320 KTS. However, when I did this I inadvertently disarmed the altitude capture. Approximately 100 ft above level off the first officer said 'altitude capture isn't armed.' I attempted to level off using vertical speed, but the aircraft was very slow to respond. Finally, approximately 150 ft below our assigned altitude, I disconnected the autoplt and climbed back to 15000 ft. The first officer and I estimated the aircraft had descended as much as 250-300 ft below 15000 ft during this maneuver. The main factor here was, first, my failure to immediately notice the fact that the altitude capture mode was overridden when I selected vertical speed, and secondly, my less than immediate decision to disconnect the autoplt to return to 15000 ft. ATC made no comment to us, there was no conflict with other aircraft.

Google
 

Original NASA ASRS Text

Title: CAPT OF AN MD88 OVERSHOT DSCNT STAR ARR FIX DUE TO INADVERTENTLY OVERRIDING THE ALT CAPTURE MODE OF THE AUTOPLT WHEN CORRECTING FOR AIRSPD DESIRED.

Narrative: ACR X WAS DSNDING TO CROSS LAMMA INTXN ON THE BITH07 ARR INTO MLB. THE CAPT (ME) WAS AT THE CTLS. I WAS USING VNAV FOR THE DSCNT. THE FMS WAS PROGRAMMED TO CROSS LAMMA AT 15000 FT AND 320 KTS. APPROX 500 FT PRIOR TO LEVEL OFF, THE SPD COMMAND BUG SLOWED TO 240 KTS, THE THROTTLES WENT TO IDLE, AND THE SINK RATE INCREASED TO APPROX 1800 FPM DOWN. I SELECTED VERT SPD ON THE MODE CTL PANEL, SELECTED 500 FPM DOWN, AND INCREASED THE SPD BUG BACK TO 320 KTS. HOWEVER, WHEN I DID THIS I INADVERTENTLY DISARMED THE ALT CAPTURE. APPROX 100 FT ABOVE LEVEL OFF THE FO SAID 'ALT CAPTURE ISN'T ARMED.' I ATTEMPTED TO LEVEL OFF USING VERT SPD, BUT THE ACFT WAS VERY SLOW TO RESPOND. FINALLY, APPROX 150 FT BELOW OUR ASSIGNED ALT, I DISCONNECTED THE AUTOPLT AND CLBED BACK TO 15000 FT. THE FO AND I ESTIMATED THE ACFT HAD DSNDED AS MUCH AS 250-300 FT BELOW 15000 FT DURING THIS MANEUVER. THE MAIN FACTOR HERE WAS, FIRST, MY FAILURE TO IMMEDIATELY NOTICE THE FACT THAT THE ALT CAPTURE MODE WAS OVERRIDDEN WHEN I SELECTED VERT SPD, AND SECONDLY, MY LESS THAN IMMEDIATE DECISION TO DISCONNECT THE AUTOPLT TO RETURN TO 15000 FT. ATC MADE NO COMMENT TO US, THERE WAS NO CONFLICT WITH OTHER ACFT.

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.