B747 suffered a navigation error due to a malfunctioning INS.

Attributes
ACN	837946
Time
Date	200905
Local Time Of Day	0601-1200
Place
Locale Reference	ZAN.ARTCC
State Reference	AK
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Oceanic
Flight Plan	IFR
Component
Aircraft Component	INS / IRS / IRU
Person 1
Function	First Officer
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Engineer Flight Crew Flight Instructor
Experience	Flight Crew Last 90 Days 80 Flight Crew Total 14000 Flight Crew Type 9000
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Track / Heading All Types

Narrative:

Crossing pacific track eastbound; anc called and asked if we were deviating. We had a navigation problem and were trying to figure out which INS of 3 was correct. Last update was at the sya NDB and all three INS operating independently. Autopilot B was flying aircraft using INS #2 for navigation. After or around amond intersection the INS were updating 'searching' and the aircraft began to turn left. It is normal for these aircraft to turn left; right when updating. The aircraft remained on a north of course heading. #1 INS showed us north of course then ATC called and asked why we were north of course. Contacted sfo by HF and reported navigation failure; determined #2 INS was in error; shut down #2 INS and switched navigation to INS #1. Navigation failure difficult to determine which INS in error with little ground based navaids and no GPS on aircraft.

Original NASA ASRS Text

Title: B747 suffered a navigation error due to a malfunctioning INS.

Narrative: Crossing Pacific track eastbound; ANC called and asked if we were deviating. We had a navigation problem and were trying to figure out which INS of 3 was correct. Last update was at the SYA NDB and all three INS operating independently. Autopilot B was flying aircraft using INS #2 for NAV. After or around AMOND Intersection the INS were updating 'searching' and the aircraft began to turn left. It is normal for these aircraft to turn left; right when updating. The aircraft remained on a north of course heading. #1 INS showed us north of course then ATC called and asked why we were north of course. Contacted SFO by HF and reported NAV failure; determined #2 INS was in error; shut down #2 INS and switched NAV to INS #1. NAV failure difficult to determine which INS in error with little ground based NAVAIDS and no GPS on aircraft.

Data retrieved from NASA's ASRS site as of April 2012 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.