An MD90 autoplt failed to capture assigned altitude resulting in an altitude bust of 300 ft.

Attributes
ACN	614098
Time
Date	200404
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : oak.vor
State Reference	CA
Altitude	msl bound lower : 9700 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : nct.tracon
Operator	common carrier : air carrier
Make Model Name	MD-90 Series (DC-9-90) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 200 flight time total : 17500 flight time type : 2700
ASRS Report	614098
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : far
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude flight crew : overcame equipment problem flight crew : overrode automation
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

We were descending to 10000 ft, turning to an assigned heading and slowing to 180 KTS, as cleared by norcal approach on our flight into sfo. As the speed bled off, the flaps were extended to 11 degrees. The autoplt was in the vertical speed mode at the rate of 700 FPM descent. The autoplt did not level the aircraft at the 10000 ft altitude. As we were still descending. I disengaged the autoplt and manually returned to the 10000 ft altitude. The lowest altitude that I noticed was about 9700 ft MSL.

Original NASA ASRS Text

Title: AN MD90 AUTOPLT FAILED TO CAPTURE ASSIGNED ALT RESULTING IN AN ALT BUST OF 300 FT.

Narrative: WE WERE DSNDING TO 10000 FT, TURNING TO AN ASSIGNED HDG AND SLOWING TO 180 KTS, AS CLRED BY NORCAL APCH ON OUR FLT INTO SFO. AS THE SPD BLED OFF, THE FLAPS WERE EXTENDED TO 11 DEGS. THE AUTOPLT WAS IN THE VERT SPD MODE AT THE RATE OF 700 FPM DSCNT. THE AUTOPLT DID NOT LEVEL THE ACFT AT THE 10000 FT ALT. AS WE WERE STILL DSNDING. I DISENGAGED THE AUTOPLT AND MANUALLY RETURNED TO THE 10000 FT ALT. THE LOWEST ALT THAT I NOTICED WAS ABOUT 9700 FT MSL.

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.