A S80 captain reported an altitude bust in the vicinity of las.

Attributes
ACN	490640
Time
Date	200011
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : las.airport
State Reference	NV
Altitude	msl bound lower : 9750 msl bound upper : 10750
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : l30.tracon tower : sfo.tower
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other other vortac
Flight Phase	descent : approach
Route In Use	arrival star : n/s
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	490640
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to assigned altitude
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

First officer flying. During descent to 10000 ft, the autoplt failed to capture the altitude. The altitude had been armed and, at 10750 ft, the altitude alert horn sounded. Just above 10000 ft, I looked at the flight mode annunciator and realized that the altitude was no longer armed. I told the first officer who took immediate steps to correct. The lowest altitude I saw was 9800 ft to 9750 ft. The altitude deviation horn never sounded.

Original NASA ASRS Text

Title: A S80 CAPT RPTED AN ALT BUST IN THE VICINITY OF LAS.

Narrative: FO FLYING. DURING DSCNT TO 10000 FT, THE AUTOPLT FAILED TO CAPTURE THE ALT. THE ALT HAD BEEN ARMED AND, AT 10750 FT, THE ALT ALERT HORN SOUNDED. JUST ABOVE 10000 FT, I LOOKED AT THE FLT MODE ANNUNCIATOR AND REALIZED THAT THE ALT WAS NO LONGER ARMED. I TOLD THE FO WHO TOOK IMMEDIATE STEPS TO CORRECT. THE LOWEST ALT I SAW WAS 9800 FT TO 9750 FT. THE ALTDEV HORN NEVER SOUNDED.

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.