B767ER suffers degraded FMS navigation positioning guidance following atlantic crossing.

Attributes
ACN	746048
Time
Date	200702
Local Time Of Day	0601 To 1200
Place
Locale Reference	intersection : lakes
State Reference	FO
Altitude	msl single value : 32000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : czul.artcc tower : ord.tower
Operator	common carrier : air carrier
Make Model Name	B767-300 and 300 ER
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Route In Use	enroute : atlantic
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 161 flight time total : 14250 flight time type : 5815
ASRS Report	746048
Person 2
Affiliation	government : foreign
Function	controller : radar
Events
Anomaly	aircraft equipment problem : critical non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : provided flight assist controller : issued advisory
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft Airspace Structure
Primary Problem	Aircraft

Narrative:

Upon exiting the north atlantic track system and coming into radar and VHF contact with montreal center; we were asked to verify position. We were approximately 10 NM east of lakes intersection. Montreal had our position further south. Their position did not agree with either our FMC or IRS position. Radar vectors were provided until we were able to receive land based radio NAVAID signals. Center was advised that our RNAV capability was unreliable and the remaining portion of the flight was flown using manual radio NAVAID tuning. The aircraft position was initialized and verified at the gate in frankfurt and an RNAV departure was flown. At no time did any controling agency question our position. Prior to entering the tracks a navigation accuracy check was performed and raw data and FMC data were within tolerance. There was also another aircraft entering the tracks at the same time and position 1000 ft below us. Neither our position nor his was questioned by shannon control. Once across the tracks and in range of radio navaids it seemed as if the FMC was not updating its position as frequent navigation accuracy checks showed raw data versus FMC to be out of limits. The FMC system was written up in the maintenance log as being unreliable.

Original NASA ASRS Text

Title: B767ER SUFFERS DEGRADED FMS NAV POSITIONING GUIDANCE FOLLOWING ATLANTIC CROSSING.

Narrative: UPON EXITING THE NORTH ATLANTIC TRACK SYS AND COMING INTO RADAR AND VHF CONTACT WITH MONTREAL CTR; WE WERE ASKED TO VERIFY POS. WE WERE APPROX 10 NM E OF LAKES INTXN. MONTREAL HAD OUR POS FURTHER S. THEIR POS DID NOT AGREE WITH EITHER OUR FMC OR IRS POS. RADAR VECTORS WERE PROVIDED UNTIL WE WERE ABLE TO RECEIVE LAND BASED RADIO NAVAID SIGNALS. CTR WAS ADVISED THAT OUR RNAV CAPABILITY WAS UNRELIABLE AND THE REMAINING PORTION OF THE FLT WAS FLOWN USING MANUAL RADIO NAVAID TUNING. THE ACFT POS WAS INITIALIZED AND VERIFIED AT THE GATE IN FRANKFURT AND AN RNAV DEP WAS FLOWN. AT NO TIME DID ANY CTLING AGENCY QUESTION OUR POS. PRIOR TO ENTERING THE TRACKS A NAV ACCURACY CHK WAS PERFORMED AND RAW DATA AND FMC DATA WERE WITHIN TOLERANCE. THERE WAS ALSO ANOTHER ACFT ENTERING THE TRACKS AT THE SAME TIME AND POS 1000 FT BELOW US. NEITHER OUR POS NOR HIS WAS QUESTIONED BY SHANNON CTL. ONCE ACROSS THE TRACKS AND IN RANGE OF RADIO NAVAIDS IT SEEMED AS IF THE FMC WAS NOT UPDATING ITS POS AS FREQUENT NAV ACCURACY CHKS SHOWED RAW DATA VERSUS FMC TO BE OUT OF LIMITS. THE FMC SYS WAS WRITTEN UP IN THE MAINT LOG AS BEING UNRELIABLE.

Data retrieved from NASA's ASRS site as of January 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.