B767 Undifferentiated or Other Model
Flight Crew Last 90 Days 82; Flight Crew Total 11271; Flight Crew Type 4273
Situational Awareness; Troubleshooting
Aircraft Equipment Problem Less Severe
Aircraft Equipment Problem Dissipated
Aircraft
I was the IRO (Relief pilot) on this leg; but in the right seat when the event occurred.  We were over the ocean when we experienced an UNABLE RNP FMC message.  On the FMC POS REF pages we noticed that we no longer had GPS reception; but there was no associated GPS1 or GPS2 EICAS message.  The RNP default value showed 2.0 and ANP was between 2.0 and 3.0; thus the UNABLE RNP message.  The Captain manually changed the RNP value to 12.0 and the UNABLE RNP message went away.  Shortly thereafter I deleted the manually selected RNP value and noticed that the default RNP value had returned to 12.0.  ANP slowly kept increasing; however it never exceeded 5.0.  The IRS (3) ACTUAL POS REF 2/4 at that time was around 11.0.  At landfall I watched ANP drop to 0.36.  Sometime later over land; I don't know when since I was on break sleeping; GPS reception resumed.    In the systems manual there is a note that states:  The FMC stops GPS updating if GPS data accuracy degrades due to satellite availability or unfavorable geometry.  Subsequently; the FMC receives updates from another system.  I have no way of knowing why GPS reception dropped off; but after the drop off the system functioned as designed.  I am guessing the RNP 2.0 value was there for a short time maybe because we crossed an ATS RNAV route. I was the IRO; but both the First Officer (FO) and I verified that both the post flight check IRS tolerances checks we do were within limits.      Suggestions:  1.  Determine whether GPS reception drop-offs is a known fleet problem.  2.  Educate crewmembers in an ASAP newsletter if this happens to be more than an isolated incident.
Operating as the First Officer during oceanic crossing I entered my scheduled rest period with all aircraft systems operating normally.  After returning to my duty station the Captain and Relief Officer informed me we had received the EICAS message 'UNABLE RNP' during my rest period.  The aircraft was now over land; under radar contact with ATC.  The aircraft FMC positions was being calculated from the IRS and corrections to the positions being made by DME/DME data from ground stations as the aircraft was over land.  The aircraft Actual Navigational Performance was approximately  0.36 and no GPS positions for GPS 1 or GPS 2  where being displayed on either FMC.  I compared the HSI map with data from navigation receivers and adequate radio updating was available the rest of the flight with no other abnormalities noted.  Sometime before our arrival at destination the GPS 1 and GPS 2 positions again displayed on both FMCs. The Captain also informed me that an ACARS message was sent to the company to alert maintenance of potential operational impact.  I conducted the Post Flight IRS Accuracy checks and with no discrepancies noted.  After reflecting on the events a logbook entry should have been made by the crew to document the Navigation System degraded capability encountered.  Simply stated- GPS where not updating the FMCs during flight.
A B767 crew reported that during an oceanic crossing; the aircraft's EICAS alerted UNABLE RNP FMC with loss of the GPS signal. No loss of navigation accuracy was discovered; and later normal GPS functions returned.