Air carrier descends below assigned altitude when autoplt does not capture descent altitude.

Attributes
ACN	287965
Time
Date	199411
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sjc
State Reference	CA
Altitude	msl bound lower : 3600 msl bound upper : 4000
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : oak tower : lns
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	287965
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : atp
Events
Anomaly	altitude deviation : overshoot non adherence : clearance other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	none taken : insufficient time
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Flight descended into sjc airspace in inclement WX following passage of a pacific storm. An icing SIGMET was in effect and we experienced moderate bumps during the descent through rain showers. We were given numerous vectors for the unusual south landing at sjc. We had been on duty for over 10 hours at the time. We were cleared to 4000 ft and both confirmed that and acknowledged that the flight director indicated 4000 ft and armed. First officer was flying and still had the autoplt engaged. We were at 250 KTS -- 12 mi west of sjc, and descending through 5000 ft when approach control directed us to slow to 180 KTS. This was done by slat extension and changing vertical speed to 300 ft or so down. We were involved in numerous radio xmissions. Concern for terrain west of sjc and airspeed control, and did not notice descent through 4000 ft until the alert chime sounded. We stopped the descent at 3600 ft and were almost back to 4000 ft when subsequently cleared to 3000 ft. We have no idea why the autoplt did not capture at 4000 ft.

Original NASA ASRS Text

Title: ACR DSNDS BELOW ASSIGNED ALT WHEN AUTOPLT DOES NOT CAPTURE DSCNT ALT.

Narrative: FLT DSNDED INTO SJC AIRSPACE IN INCLEMENT WX FOLLOWING PASSAGE OF A PACIFIC STORM. AN ICING SIGMET WAS IN EFFECT AND WE EXPERIENCED MODERATE BUMPS DURING THE DSCNT THROUGH RAIN SHOWERS. WE WERE GIVEN NUMEROUS VECTORS FOR THE UNUSUAL S LNDG AT SJC. WE HAD BEEN ON DUTY FOR OVER 10 HRS AT THE TIME. WE WERE CLRED TO 4000 FT AND BOTH CONFIRMED THAT AND ACKNOWLEDGED THAT THE FLT DIRECTOR INDICATED 4000 FT AND ARMED. FO WAS FLYING AND STILL HAD THE AUTOPLT ENGAGED. WE WERE AT 250 KTS -- 12 MI W OF SJC, AND DSNDING THROUGH 5000 FT WHEN APCH CTL DIRECTED US TO SLOW TO 180 KTS. THIS WAS DONE BY SLAT EXTENSION AND CHANGING VERT SPD TO 300 FT OR SO DOWN. WE WERE INVOLVED IN NUMEROUS RADIO XMISSIONS. CONCERN FOR TERRAIN W OF SJC AND AIRSPD CTL, AND DID NOT NOTICE DSCNT THROUGH 4000 FT UNTIL THE ALERT CHIME SOUNDED. WE STOPPED THE DSCNT AT 3600 FT AND WERE ALMOST BACK TO 4000 FT WHEN SUBSEQUENTLY CLRED TO 3000 FT. WE HAVE NO IDEA WHY THE AUTOPLT DID NOT CAPTURE AT 4000 FT.

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.