DC10-40 flew 12 mi off track when the autoplt made an uncommanded 20 degree left turn.

Attributes
ACN	356330
Time
Date	199612
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : yay
State Reference	NF
Altitude	msl bound lower : 31000 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : czqx
Operator	common carrier : air carrier
Make Model Name	DC-10 40
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute airway : czqx
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : atp pilot : instrument pilot : flight engineer
Experience	flight time last 90 days : 150 flight time total : 8500 flight time type : 950
ASRS Report	356330
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
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	Pilot Deviation

Narrative:

We told them that our slight left turn off course was due to the autoplt turning to left by itself. Our FMA showed still on navigation track, but autoplt turned about 20 degrees off course uncommanded by us. We got off track about 12 NM, then turned back immediately. No problems encountered and we don't know why the autoplt turned the wrong way.

Original NASA ASRS Text

Title: DC10-40 FLEW 12 MI OFF TRACK WHEN THE AUTOPLT MADE AN UNCOMMANDED 20 DEG L TURN.

Narrative: WE TOLD THEM THAT OUR SLIGHT L TURN OFF COURSE WAS DUE TO THE AUTOPLT TURNING TO L BY ITSELF. OUR FMA SHOWED STILL ON NAV TRACK, BUT AUTOPLT TURNED ABOUT 20 DEGS OFF COURSE UNCOMMANDED BY US. WE GOT OFF TRACK ABOUT 12 NM, THEN TURNED BACK IMMEDIATELY. NO PROBS ENCOUNTERED AND WE DON'T KNOW WHY THE AUTOPLT TURNED THE WRONG WAY.

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.