FMS problem causes aircraft to turn 8 miles off course.

Attributes
ACN	87184
Time
Date	198805
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : gve
State Reference	VA
Altitude	msl bound lower : 28000 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute airway : j75
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	87184
Person 2
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Using the internal (LNAV) navigation system. Departed gordonsville VOR on J75 on course. Approximately 20 mi southwest of the gordonsville VOR the aircraft started a slow left turn. The FMS computer was supposed to be heading toward draik intersection. ATC and the pilots noticed the course error as the aircraft approached 8 mi off course. An immediate turn to 270 degree was initiated and the aircraft returned to jet-75 centerline. The flight was continued without further incident using raw (VOR) data. Maintenance at destination could find no fault with the navigation computer. Navigation data that was being generated by the FMS computer showed on centerline at all times. Recommend that all FMS generated data be backed up by VOR data.

Original NASA ASRS Text

Title: FMS PROBLEM CAUSES ACFT TO TURN 8 MILES OFF COURSE.

Narrative: USING THE INTERNAL (LNAV) NAVIGATION SYSTEM. DEPARTED GORDONSVILLE VOR ON J75 ON COURSE. APPROX 20 MI SW OF THE GORDONSVILLE VOR THE ACFT STARTED A SLOW LEFT TURN. THE FMS COMPUTER WAS SUPPOSED TO BE HEADING TOWARD DRAIK INTXN. ATC AND THE PLTS NOTICED THE COURSE ERROR AS THE ACFT APCHED 8 MI OFF COURSE. AN IMMEDIATE TURN TO 270 DEG WAS INITIATED AND THE ACFT RETURNED TO JET-75 CENTERLINE. THE FLT WAS CONTINUED WITHOUT FURTHER INCIDENT USING RAW (VOR) DATA. MAINT AT DEST COULD FIND NO FAULT WITH THE NAVIGATION COMPUTER. NAV DATA THAT WAS BEING GENERATED BY THE FMS COMPUTER SHOWED ON CENTERLINE AT ALL TIMES. RECOMMEND THAT ALL FMS GENERATED DATA BE BACKED UP BY VOR DATA.

Data retrieved from NASA's ASRS site as of August 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.