A B767 crew reported that during an oceanic crossing; the aircraft's EICAS alerted unable rnp FMC wi...

Narrative:

I was the international relief officer (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 position reference 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 position reference 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 international relief officer; but both the first officer (first officer) 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 as soon as possible newsletter if this happens to be more than an isolated incident.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site 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.

Attributes
ACN	1245571
Time
Date	201503
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B767 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Oceanic
Flight Plan	IFR
Component
Aircraft Component	GPS & Other Satellite Navigation
Person 1
Function	Relief Pilot Pilot Not Flying
Experience	Flight Crew Last 90 Days 82 Flight Crew Total 11271 Flight Crew Type 4273
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 100 Flight Crew Type 3000
Events
Anomaly	Aircraft Equipment Problem Less Severe

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.