Aircraft equipment problem induced by a 'map shift' creates a heading track deviation.

Attributes
ACN	284412
Time
Date	199409
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : las
State Reference	NV
Altitude	msl bound lower : 28000 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude
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 total : 8000
ASRS Report	284412
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 166 flight time total : 12000 flight time type : 6000
ASRS Report	284327
Events
Anomaly	aircraft equipment problem : critical non adherence : published procedure non adherence : clearance other spatial deviation
Independent Detector	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	Pilot Deviation

Narrative:

Departed las, runway 25. VFR climb out approved via las R-251 to join the bty R-126, remainder of oasis 7 departure. R-nav indicated intersection of bty R-126, began turn to intercept. ZLA vectored us back to the left, away from an inbound flight to las. ATC indicated our turn was about 20 mi early, conflicting with the cockpit indications. Remainder of flight to sfo uneventful. Computers crosschecked with raw data, suspect a map shift/erroneous indications. Supplemental information from acn 284327: all was normal on the screen. All of our indications were on course. The FMC had shown to be accurate on the DME xchk while still tuned to las. Have been flying this aircraft for 5 yrs and this is the 3RD time I have seen an occurrence of this nature on the FMC which can never be reduplicated at the time nor explained.

Original NASA ASRS Text

Title: ACFT EQUIP PROB INDUCED BY A 'MAP SHIFT' CREATES A HDG TRACK DEV.

Narrative: DEPARTED LAS, RWY 25. VFR CLBOUT APPROVED VIA LAS R-251 TO JOIN THE BTY R-126, REMAINDER OF OASIS 7 DEP. R-NAV INDICATED INTXN OF BTY R-126, BEGAN TURN TO INTERCEPT. ZLA VECTORED US BACK TO THE L, AWAY FROM AN INBOUND FLT TO LAS. ATC INDICATED OUR TURN WAS ABOUT 20 MI EARLY, CONFLICTING WITH THE COCKPIT INDICATIONS. REMAINDER OF FLT TO SFO UNEVENTFUL. COMPUTERS XCHKED WITH RAW DATA, SUSPECT A MAP SHIFT/ERRONEOUS INDICATIONS. SUPPLEMENTAL INFO FROM ACN 284327: ALL WAS NORMAL ON THE SCREEN. ALL OF OUR INDICATIONS WERE ON COURSE. THE FMC HAD SHOWN TO BE ACCURATE ON THE DME XCHK WHILE STILL TUNED TO LAS. HAVE BEEN FLYING THIS ACFT FOR 5 YRS AND THIS IS THE 3RD TIME I HAVE SEEN AN OCCURRENCE OF THIS NATURE ON THE FMC WHICH CAN NEVER BE REDUPLICATED AT THE TIME NOR EXPLAINED.

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.