MD81 crew has a track deviation after the FMS began a shallow turn in ZJX class east airspace.

Attributes
ACN	636566
Time
Date	200411
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	intersection : leese
State Reference	FL
Altitude	msl single value : 10000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx.artcc
Operator	common carrier : air carrier
Make Model Name	MD-81
Operating Under FAR Part	Part 121
Navigation In Use	other other vortac
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	636566
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	non adherence : clearance other spatial deviation
Independent Detector	atc equipment other atc equipment : radar other controllera other flight crewa other flight crewb
Resolutory Action	controller : issued alert controller : issued new clearance
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

FMS initiated turn without known cause. On descent into mco, we were cleared direct to leese intersection while descending. We input the direct course into the FMS, concurred that it looked correct and edited the route of flight. A short time late the FMS initiated a shallow (10 degree) left bank turn. We were both a little confused about why it might be turning but after a few seconds the captain initiated a turn back toward leese. Just after we started the turn back, ZJX questioned us about our heading. I let him know that the FMS initiated a turn and we were returning to course. At that point he cleared us direct to the orlando VOR. There were no known conflicts. The FMS showed our route as direct to the fix during the incident, I don't know why it turned.

Original NASA ASRS Text

Title: MD81 CREW HAS A TRACK DEV AFTER THE FMS BEGAN A SHALLOW TURN IN ZJX CLASS E AIRSPACE.

Narrative: FMS INITIATED TURN WITHOUT KNOWN CAUSE. ON DSCNT INTO MCO, WE WERE CLRED DIRECT TO LEESE INTXN WHILE DSNDING. WE INPUT THE DIRECT COURSE INTO THE FMS, CONCURRED THAT IT LOOKED CORRECT AND EDITED THE RTE OF FLT. A SHORT TIME LATE THE FMS INITIATED A SHALLOW (10 DEG) L BANK TURN. WE WERE BOTH A LITTLE CONFUSED ABOUT WHY IT MIGHT BE TURNING BUT AFTER A FEW SECONDS THE CAPT INITIATED A TURN BACK TOWARD LEESE. JUST AFTER WE STARTED THE TURN BACK, ZJX QUESTIONED US ABOUT OUR HDG. I LET HIM KNOW THAT THE FMS INITIATED A TURN AND WE WERE RETURNING TO COURSE. AT THAT POINT HE CLRED US DIRECT TO THE ORLANDO VOR. THERE WERE NO KNOWN CONFLICTS. THE FMS SHOWED OUR RTE AS DIRECT TO THE FIX DURING THE INCIDENT, I DON'T KNOW WHY IT TURNED.

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.