A B-747 freighter crew was off track over the pacific.

Attributes
ACN	265260
Time
Date	199403
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : sya
State Reference	AK
Altitude	msl bound lower : 33000 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zan tracon : mia
Operator	common carrier : air carrier
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute : other oceanic enroute : pacific
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : cfi pilot : atp
Experience	flight time last 90 days : 200 flight time total : 29500 flight time type : 1500
ASRS Report	265260
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : critical non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We were advised by zan that their remote radar showed that we were 18 mi north of track while on A-590 which is an INS airway across the north pacific. We were 5 hours into a flight from tpe to anc at FL330 on autoplt coupled to the #1 INS. All 3 INS units were triple-mixed. An INS accuracy check was accomplished at oshima (xa), japan, and indicated that INS units were right on course (2 hours into flight). 3 hours later we were 18 mi left of track. The remainder of the flight was flown in the heading mode taking ADF and VOR bearings. I personally don't believe in triple-mixing INS units just for this reason, although it is our company procedure. This aircraft was recently acquired from air carrier X and the INS program has automatic triple-mix feature so you have to manually separate the units, which we did not do until the error was detected. When the INS units run separately, you can monitor the 3 and, by xrefing, detect one that runs amok. In triple-mix, they all average to select the course, so the bad ones degrade the good one. The ramp position at anc readings were #1 - 23 mi off, #2 - 8 mi off, and #3 - 20 mi off. INS units are delco carousel iv's.

Original NASA ASRS Text

Title: A B-747 FREIGHTER CREW WAS OFF TRACK OVER THE PACIFIC.

Narrative: WE WERE ADVISED BY ZAN THAT THEIR REMOTE RADAR SHOWED THAT WE WERE 18 MI N OF TRACK WHILE ON A-590 WHICH IS AN INS AIRWAY ACROSS THE NORTH PACIFIC. WE WERE 5 HRS INTO A FLT FROM TPE TO ANC AT FL330 ON AUTOPLT COUPLED TO THE #1 INS. ALL 3 INS UNITS WERE TRIPLE-MIXED. AN INS ACCURACY CHK WAS ACCOMPLISHED AT OSHIMA (XA), JAPAN, AND INDICATED THAT INS UNITS WERE RIGHT ON COURSE (2 HRS INTO FLT). 3 HRS LATER WE WERE 18 MI L OF TRACK. THE REMAINDER OF THE FLT WAS FLOWN IN THE HDG MODE TAKING ADF AND VOR BEARINGS. I PERSONALLY DON'T BELIEVE IN TRIPLE-MIXING INS UNITS JUST FOR THIS REASON, ALTHOUGH IT IS OUR COMPANY PROC. THIS ACFT WAS RECENTLY ACQUIRED FROM ACR X AND THE INS PROGRAM HAS AUTOMATIC TRIPLE-MIX FEATURE SO YOU HAVE TO MANUALLY SEPARATE THE UNITS, WHICH WE DID NOT DO UNTIL THE ERROR WAS DETECTED. WHEN THE INS UNITS RUN SEPARATELY, YOU CAN MONITOR THE 3 AND, BY XREFING, DETECT ONE THAT RUNS AMOK. IN TRIPLE-MIX, THEY ALL AVERAGE TO SELECT THE COURSE, SO THE BAD ONES DEGRADE THE GOOD ONE. THE RAMP POS AT ANC READINGS WERE #1 - 23 MI OFF, #2 - 8 MI OFF, AND #3 - 20 MI OFF. INS UNITS ARE DELCO CAROUSEL IV'S.

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.