An MD80 crew, in descent to sea, discovered their gfms was in error, putting them 5 NM left of cours...

Attributes
ACN	535854
Time
Date	200201
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	intersection : raddy
State Reference	WA
Altitude	msl single value : 16000
Environment
Light	Night
Aircraft 1
Controlling Facilities	tracon : s46.tracon
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	535854
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	535855
Events
Anomaly	aircraft equipment problem : critical other anomaly other other spatial deviation
Independent Detector	atc equipment other atc equipment : radar other controllera other flight crewa other flight crewb
Resolutory Action	controller : issued advisory flight crew : returned to intended or assigned course
Consequence	faa : reviewed incident with flight crew other other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Gfms was properly set up and crosschecked. At this location, gfms showed us centered on course. While xchking with 'rad' data, we discovered aircraft to be 5 mi left of course. After starting our correction, ATC noticed our position and concurred with our correction. Flight continued uneventfully. This brings up a significant concern in regard to our commencement of RNAV approachs with a single gfms in these approachs. Without a second box to xchk and no navaids to back up our position, I feel we could be heading down a questionable path. Supplemental information from acn 535855: 10 DME southeast raddy intersection. At this location, gfms showed us on course. When xchking with VOR, noticed we were left of course. Started correction and ATC confirmed our position to be 5 mi left of course. We flew VOR the rest of the way and landed uneventfully.

Original NASA ASRS Text

Title: AN MD80 CREW, IN DSCNT TO SEA, DISCOVERED THEIR GFMS WAS IN ERROR, PUTTING THEM 5 NM L OF COURSE.

Narrative: GFMS WAS PROPERLY SET UP AND XCHKED. AT THIS LOCATION, GFMS SHOWED US CTRED ON COURSE. WHILE XCHKING WITH 'RAD' DATA, WE DISCOVERED ACFT TO BE 5 MI L OF COURSE. AFTER STARTING OUR CORRECTION, ATC NOTICED OUR POS AND CONCURRED WITH OUR CORRECTION. FLT CONTINUED UNEVENTFULLY. THIS BRINGS UP A SIGNIFICANT CONCERN IN REGARD TO OUR COMMENCEMENT OF RNAV APCHS WITH A SINGLE GFMS IN THESE APCHS. WITHOUT A SECOND BOX TO XCHK AND NO NAVAIDS TO BACK UP OUR POS, I FEEL WE COULD BE HEADING DOWN A QUESTIONABLE PATH. SUPPLEMENTAL INFO FROM ACN 535855: 10 DME SE RADDY INTXN. AT THIS LOCATION, GFMS SHOWED US ON COURSE. WHEN XCHKING WITH VOR, NOTICED WE WERE L OF COURSE. STARTED CORRECTION AND ATC CONFIRMED OUR POS TO BE 5 MI L OF COURSE. WE FLEW VOR THE REST OF THE WAY AND LANDED UNEVENTFULLY.

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.

An MD80 crew, in descent to sea, discovered their gfms was in error, putting them 5 NM left of course.