During arrival, aircraft drifted off of assigned altitude.

Attributes
ACN	491220
Time
Date	200011
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sfo.airport
State Reference	CA
Altitude	msl bound lower : 10500 msl bound upper : 11000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : o90.tracon tower : bos.tower
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 4 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Route In Use	approach : visual arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : relief pilot
Qualification	pilot : atp
Experience	flight time type : 307
ASRS Report	491220
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : company policies non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Inbound to sfo, copilot turned autoflt off. Was assigned to hold 11000 ft and turn to a new heading. While in the turn, he was slowing and inadvertently started to descend without clearance. We went to 10500 ft. The captain and I (in the jumpseat) caught the deviation and told the copilot to climb. This occurred at 10600 ft, but the descent stopped at 10500 ft.

Original NASA ASRS Text

Title: DURING ARR, ACFT DRIFTED OFF OF ASSIGNED ALT.

Narrative: INBOUND TO SFO, COPLT TURNED AUTOFLT OFF. WAS ASSIGNED TO HOLD 11000 FT AND TURN TO A NEW HDG. WHILE IN THE TURN, HE WAS SLOWING AND INADVERTENTLY STARTED TO DSND WITHOUT CLRNC. WE WENT TO 10500 FT. THE CAPT AND I (IN THE JUMPSEAT) CAUGHT THE DEV AND TOLD THE COPLT TO CLB. THIS OCCURRED AT 10600 FT, BUT THE DSCNT STOPPED AT 10500 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.