Aircraft equipment problem. FMS dropped out of navigation mode to heading mode.

Attributes
ACN	315037
Time
Date	199509
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zzz
State Reference	US
Aircraft 1
Controlling Facilities	artcc : ord
Operator	common carrier : air carrier
Make Model Name	Fokker 100
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	315037
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	other anomaly other 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	other

Narrative:

En route to ord, the navigation mode was being used to navigation and without warning the FMS dropped out of navigation mode and entered the heading hold mode, resulting in a slight navigation deviation off the clearance routing. By the time the crew had noticed the deviation, the aircraft had drifted approximately 4 mi off the airway. Air traffic control was contacted to verify the position of the aircraft with respect to the airway and the crew was told that the aircraft was splitting the airway. We believe this to mean that the aircraft was still within the confines of the airway. We then proceeded directly to the next fix along our route and advised ATC that we were doing so. The flight continued without incident or further discussion of the deviation. The aircraft navigation system was written up in the main log upon arrival into ord. In addition, the outbound crew received a verbal briefing from the inbound crew.

Original NASA ASRS Text

Title: ACFT EQUIP PROB. FMS DROPPED OUT OF NAV MODE TO HDG MODE.

Narrative: ENRTE TO ORD, THE NAV MODE WAS BEING USED TO NAV AND WITHOUT WARNING THE FMS DROPPED OUT OF NAV MODE AND ENTERED THE HDG HOLD MODE, RESULTING IN A SLIGHT NAV DEV OFF THE CLRNC ROUTING. BY THE TIME THE CREW HAD NOTICED THE DEV, THE ACFT HAD DRIFTED APPROX 4 MI OFF THE AIRWAY. AIR TFC CTL WAS CONTACTED TO VERIFY THE POS OF THE ACFT WITH RESPECT TO THE AIRWAY AND THE CREW WAS TOLD THAT THE ACFT WAS SPLITTING THE AIRWAY. WE BELIEVE THIS TO MEAN THAT THE ACFT WAS STILL WITHIN THE CONFINES OF THE AIRWAY. WE THEN PROCEEDED DIRECTLY TO THE NEXT FIX ALONG OUR RTE AND ADVISED ATC THAT WE WERE DOING SO. THE FLT CONTINUED WITHOUT INCIDENT OR FURTHER DISCUSSION OF THE DEV. THE ACFT NAV SYS WAS WRITTEN UP IN THE MAIN LOG UPON ARR INTO ORD. IN ADDITION, THE OUTBOUND CREW RECEIVED A VERBAL BRIEFING FROM THE INBOUND CREW.

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.