A B777-200 flight crew deviated from their assigned offset course over the north pacific.

Attributes
ACN	796356
Time
Date	200807
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzzz.airport
State Reference	FO
Altitude	msl single value : 34000
Environment
Flight Conditions	Marginal
Aircraft 1
Controlling Facilities	artcc : zzzz.artcc
Operator	common carrier : air carrier
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Route In Use	enroute : pacific
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 250 flight time total : 20000 flight time type : 3500
ASRS Report	796356
Events
Anomaly	aircraft equipment problem : less severe other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended or assigned course
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

We had deleted a non reporting fix just prior and needed to re-establish our one NM slop. For the sake of smoothness; I re-entered R1 on the route page approaching fix; executed it just past. In LNAV the aircraft made a much more aggressive turn to the right than normal (25 degree bank); and flew through the offset course on a 45 degree angle; and continued to turn right; but slowly now. I went to heading select to turn back to the desired track. Shortly thereafter; the magenta line disappeared from the map (off scale; right) and the first officer stated that the next waypoint was gone. The FMC we noticed was showing the course for the next leg. This was about 40 mi to our right. We rebuilt the original course and re-intercepted: back to normal. We reported this wandering to ATC via cpdlc; estimating that we got 8-10 mi right of course while maneuvering back on course. They rogered. Since we were watching; I feel the deviation was minimized; once we realized our course line was gone. Additionally; nothing reported here absolutely eliminates human error; but I cannot think of what it could be. I believe there was no traffic conflict as well. My question is: why would the aircraft just blow right through the R1 offset? There must be a logical explanation; but I cannot think of one.

Original NASA ASRS Text

Title: A B777-200 FLT CREW DEVIATED FROM THEIR ASSIGNED OFFSET COURSE OVER THE NORTH PACIFIC.

Narrative: WE HAD DELETED A NON RPTING FIX JUST PRIOR AND NEEDED TO RE-ESTABLISH OUR ONE NM SLOP. FOR THE SAKE OF SMOOTHNESS; I RE-ENTERED R1 ON THE RTE PAGE APCHING FIX; EXECUTED IT JUST PAST. IN LNAV THE ACFT MADE A MUCH MORE AGGRESSIVE TURN TO THE R THAN NORMAL (25 DEG BANK); AND FLEW THROUGH THE OFFSET COURSE ON A 45 DEG ANGLE; AND CONTINUED TO TURN R; BUT SLOWLY NOW. I WENT TO HDG SELECT TO TURN BACK TO THE DESIRED TRACK. SHORTLY THEREAFTER; THE MAGENTA LINE DISAPPEARED FROM THE MAP (OFF SCALE; R) AND THE FO STATED THAT THE NEXT WAYPOINT WAS GONE. THE FMC WE NOTICED WAS SHOWING THE COURSE FOR THE NEXT LEG. THIS WAS ABOUT 40 MI TO OUR R. WE REBUILT THE ORIGINAL COURSE AND RE-INTERCEPTED: BACK TO NORMAL. WE RPTED THIS WANDERING TO ATC VIA CPDLC; ESTIMATING THAT WE GOT 8-10 MI R OF COURSE WHILE MANEUVERING BACK ON COURSE. THEY ROGERED. SINCE WE WERE WATCHING; I FEEL THE DEV WAS MINIMIZED; ONCE WE REALIZED OUR COURSE LINE WAS GONE. ADDITIONALLY; NOTHING RPTED HERE ABSOLUTELY ELIMINATES HUMAN ERROR; BUT I CANNOT THINK OF WHAT IT COULD BE. I BELIEVE THERE WAS NO TFC CONFLICT AS WELL. MY QUESTION IS: WHY WOULD THE ACFT JUST BLOW RIGHT THROUGH THE R1 OFFSET? THERE MUST BE A LOGICAL EXPLANATION; BUT I CANNOT THINK OF ONE.

Data retrieved from NASA's ASRS site as of May 2009 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.