Air carrier equipped with EFIS on approach pdx received indication of deviation from track. Raw data VOR backup was not in agreement with EFIS. Controller queried flight and vectored to back on course. EFIS had shifted with no warning.

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Attributes
ACN	351815
Time
Date	199610
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : pdx
State Reference	OR
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : pdx tower : pit
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 220 flight time total : 10000 flight time type : 5000
ASRS Report	351815
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	aircraft equipment other aircraft equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During descent to pdx, using LNAV (area navigation) on, and with EFIS presentation, the aircraft starting turning left. Stated to captain it didn't seem right but moving map seemed to indicate it was. I had been monitoring VOR as our procedure requires, I stated VOR was not in agreement. At that time approach controller asked our heading. We were rather confused as the map had moved and indicated we were on the correct heading. The controller gave us turn to (immediately) I believe, a 265 degree heading. Our map was then showing 120 degrees and when we started our turn to the right, we crosschecked VOR and compass (real compass) heading and as we were turned, the moving map (both sides) started turning faster than the actual rate of turn and seemed to sort of realign to the correct position. In 7 yrs on this aircraft I've never seen this happen. There was no indication or warning of lost alignment or loss of reference such as IRS failure. We continued via vectors to land at pdx and wrote a maintenance report.

Original NASA ASRS Text

Title: ACR EQUIPPED WITH EFIS ON APCH PDX RECEIVED INDICATION OF DEV FROM TRACK. RAW DATA VOR BACKUP WAS NOT IN AGREEMENT WITH EFIS. CTLR QUERIED FLT AND VECTORED TO BACK ON COURSE. EFIS HAD SHIFTED WITH NO WARNING.

Narrative: DURING DSCNT TO PDX, USING LNAV (AREA NAV) ON, AND WITH EFIS PRESENTATION, THE ACFT STARTING TURNING L. STATED TO CAPTAIN IT DIDN'T SEEM RIGHT BUT MOVING MAP SEEMED TO INDICATE IT WAS. I HAD BEEN MONITORING VOR AS OUR PROC REQUIRES, I STATED VOR WAS NOT IN AGREEMENT. AT THAT TIME APCH CTLR ASKED OUR HDG. WE WERE RATHER CONFUSED AS THE MAP HAD MOVED AND INDICATED WE WERE ON THE CORRECT HDG. THE CTLR GAVE US TURN TO (IMMEDIATELY) I BELIEVE, A 265 DEG HDG. OUR MAP WAS THEN SHOWING 120 DEGS AND WHEN WE STARTED OUR TURN TO THE R, WE XCHKED VOR AND COMPASS (REAL COMPASS) HDG AND AS WE WERE TURNED, THE MOVING MAP (BOTH SIDES) STARTED TURNING FASTER THAN THE ACTUAL RATE OF TURN AND SEEMED TO SORT OF REALIGN TO THE CORRECT POS. IN 7 YRS ON THIS ACFT I'VE NEVER SEEN THIS HAPPEN. THERE WAS NO INDICATION OR WARNING OF LOST ALIGNMENT OR LOSS OF REF SUCH AS IRS FAILURE. WE CONTINUED VIA VECTORS TO LAND AT PDX AND WROTE A MAINT RPT.

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.