A B777 crew continues on a pacific route when their EICAS reports 'navigation unable required naviga...

Attributes
ACN	569469
Time
Date	200212
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zzz.artcc
State Reference	US
Altitude	msl single value : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zzz.artcc
Operator	common carrier : air carrier
Make Model Name	B777 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 40 flight time total : 14000 flight time type : 1400
ASRS Report	569469
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	aircraft equipment other aircraft equipment : eicas other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	other other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

EICAS message 'navigation unable required navigation performance.' investigation showed left GPS to be 57 mi off course. No other EICAS or status messages from aircraft. Required navigation performance valve was 2.0. Actual navigation performance was 4.14. Consulted with airline maintenance control center and B777 fleet manager by satcom. Switched off GPS (both) system and continued in 'inertial' navigation. 3 hours later, EICAS message 'navigation unable required navigation performance' reappeared. Satcom to company and GPS turned back on. Required navigation performance was 4.0, actual navigation performance was 4.17. The left GPS system slowly corrected to the identical position on the right GPS. At shutdown at the gate, the navigation system 'drift' was 0.1. Fleet manager is investigating the problem. At no time during the flight did the actual aircraft position appear to be 'off course.' (ZZZ center found us on course at first radar contact.) callback conversation with reporter revealed the following information: maintenance reported after investigation that the failure in the system was the GPS antenna. It was found to have moisture in the antenna area. It was found that a single failure of the system is not alerted to the crew by any message and a dual failure is required before a message is displayed. Further company investigation revealed that the system operated as it was designed when erroneous information was being received. The bad information was biased out and backup information from IRS and radio navigation updating resulting in an accurate track being flown.

Original NASA ASRS Text

Title: A B777 CREW CONTINUES ON A PACIFIC RTE WHEN THEIR EICAS RPTS 'NAV UNABLE REQUIRED NAV PERFORMANCE.' AFTER CONFERRING WITH THE CARRIER'S MAINT DEPT GPS UPDATING OF THE FMS WAS DISABLED. NO APPARENT DEVS FROM TRACK WERE NOTED BY THE CREW OR ATC.

Narrative: EICAS MESSAGE 'NAV UNABLE REQUIRED NAV PERFORMANCE.' INVESTIGATION SHOWED L GPS TO BE 57 MI OFF COURSE. NO OTHER EICAS OR STATUS MESSAGES FROM ACFT. REQUIRED NAV PERFORMANCE VALVE WAS 2.0. ACTUAL NAV PERFORMANCE WAS 4.14. CONSULTED WITH AIRLINE MAINT CTL CTR AND B777 FLEET MGR BY SATCOM. SWITCHED OFF GPS (BOTH) SYS AND CONTINUED IN 'INERTIAL' NAV. 3 HRS LATER, EICAS MESSAGE 'NAV UNABLE REQUIRED NAV PERFORMANCE' REAPPEARED. SATCOM TO COMPANY AND GPS TURNED BACK ON. REQUIRED NAV PERFORMANCE WAS 4.0, ACTUAL NAV PERFORMANCE WAS 4.17. THE L GPS SYS SLOWLY CORRECTED TO THE IDENTICAL POS ON THE R GPS. AT SHUTDOWN AT THE GATE, THE NAV SYS 'DRIFT' WAS 0.1. FLEET MGR IS INVESTIGATING THE PROB. AT NO TIME DURING THE FLT DID THE ACTUAL ACFT POS APPEAR TO BE 'OFF COURSE.' (ZZZ CTR FOUND US ON COURSE AT FIRST RADAR CONTACT.) CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: MAINT RPTED AFTER INVESTIGATION THAT THE FAILURE IN THE SYS WAS THE GPS ANTENNA. IT WAS FOUND TO HAVE MOISTURE IN THE ANTENNA AREA. IT WAS FOUND THAT A SINGLE FAILURE OF THE SYS IS NOT ALERTED TO THE CREW BY ANY MESSAGE AND A DUAL FAILURE IS REQUIRED BEFORE A MESSAGE IS DISPLAYED. FURTHER COMPANY INVESTIGATION REVEALED THAT THE SYS OPERATED AS IT WAS DESIGNED WHEN ERRONEOUS INFO WAS BEING RECEIVED. THE BAD INFO WAS BIASED OUT AND BACKUP INFO FROM IRS AND RADIO NAV UPDATING RESULTING IN AN ACCURATE TRACK BEING FLOWN.

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.

A B777 crew continues on a pacific route when their EICAS reports 'navigation unable required navigation performance.' after conferring with the carrier's maintenance department GPS updating of the FMS was disabled. No apparent deviations from track were noted by the crew or ATC.