A B727 freighter leaves its assigned altitude when the autoplt altitude hold malfunctions.

Attributes
ACN	417180
Time
Date	199806
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : shv
State Reference	LA
Altitude	msl bound lower : 12800 msl bound upper : 13000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zfw
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : second officer
Qualification	pilot : private pilot : instrument pilot : flight engineer pilot : atp pilot : commercial
Experience	flight time last 90 days : 300 flight time total : 3500 flight time type : 1200
ASRS Report	417180
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Cleared to descend to 13000 ft. At 13000 ft, PF (captain) clicked on altitude hold on autoplt and continued his scan. At 12800 ft, the first officer called out 'below altitude,' and captain immediately corrected to 13000 ft. On ground, captain wrote up autoplt where maintenance replaced altitude hold function.

Original NASA ASRS Text

Title: A B727 FREIGHTER LEAVES ITS ASSIGNED ALT WHEN THE AUTOPLT ALT HOLD MALFUNCTIONS.

Narrative: CLRED TO DSND TO 13000 FT. AT 13000 FT, PF (CAPT) CLICKED ON ALT HOLD ON AUTOPLT AND CONTINUED HIS SCAN. AT 12800 FT, THE FO CALLED OUT 'BELOW ALT,' AND CAPT IMMEDIATELY CORRECTED TO 13000 FT. ON GND, CAPT WROTE UP AUTOPLT WHERE MAINT REPLACED ALT HOLD FUNCTION.

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.