A giv crew experienced dual flight guidance computer (FGC) failures during an RNAV arrival; but rese...

Narrative:

Cleared to descend via gibbz 2 STAR into iad. At about FL240 FMS and VNAV were confirmed armed for descent with altitude preselect dialed down to 7;000 ft. Watched profile and waited for VNAV bug in pfd; but did not appear. Rechecked FMS programming and confirmed correct. Still no vertical VNAV bug on pfd. Started descent via vs to stay on profile. Then selected direct to next crossing altitude; then FGC fail - FGC 1 failed then auto reverted to FGC 2. About 15 seconds later; FGC 2 failed. About 17;000 ft; requested ATC radar vectors off of STAR to [run] checklist and address issue while continuing descent. During level off; descended about 300 ft low to about 6;700 ft very briefly while assessing current status before getting established back at 7;000 ft. Pm pfd displayed different crossing altitudes and began to display vertical descent bug.when FGC's failed; we lost autopilot; auto throttles; FGC panel functions. Per checklist; reset by pulling FGC breakers for about 15 seconds each and reset. Normal operations afterward. During assessment of initial situation; we were continuing on STAR profile until we decided we would not be able to properly meet the profile speed and altitude restrictions while assessing and running the checklist. It is possible that we may not have been exactly on the crossings up to the point of asking for vectors off the STAR to run the checklist; however I believe we met them.we now believe there may have been a possible disagreement between the FMS's that the FGC's could not resolve which may have triggered the FGC's failing. No further issues have been seen during the following flights. In the future; if a similar event were to occur; I will suggest immediately removing ourselves from the STAR and away from the traffic flow instead of continuing for a while on the STAR. There were no traffic conflicts or communication from ATC regarding any missed altitudes or airspeeds. Vectored to ILS visual 19C for VFR landing in VMC conditions.

Original NASA ASRS Text

Title: A GIV crew experienced dual Flight Guidance Computer (FGC) failures during an RNAV arrival; but reset them while completing with reset checklist. A suspected unresolved disagreement between FMS's caused the FGC shutdown.

Narrative: Cleared to descend via GIBBZ 2 STAR into IAD. At about FL240 FMS and VNAV were confirmed armed for descent with altitude preselect dialed down to 7;000 FT. Watched profile and waited for VNAV bug in PFD; but did not appear. Rechecked FMS programming and confirmed correct. Still no vertical VNAV bug on PFD. Started descent via VS to stay on profile. Then selected direct to next crossing altitude; then FGC fail - FGC 1 failed then auto reverted to FGC 2. About 15 seconds later; FGC 2 failed. About 17;000 FT; requested ATC radar vectors off of STAR to [run] checklist and address issue while continuing descent. During level off; descended about 300 FT low to about 6;700 FT very briefly while assessing current status before getting established back at 7;000 FT. PM PFD displayed different crossing altitudes and began to display vertical descent bug.When FGC's failed; we lost autopilot; auto throttles; FGC panel functions. Per checklist; reset by pulling FGC breakers for about 15 seconds each and reset. Normal operations afterward. During assessment of initial situation; we were continuing on STAR profile until we decided we would not be able to properly meet the profile speed and altitude restrictions while assessing and running the checklist. It is possible that we may not have been exactly on the crossings up to the point of asking for vectors off the STAR to run the checklist; however I believe we met them.We now believe there may have been a possible disagreement between the FMS's that the FGC's could not resolve which may have triggered the FGC's failing. No further issues have been seen during the following flights. In the future; if a similar event were to occur; I will suggest immediately removing ourselves from the STAR and away from the traffic flow instead of continuing for a while on the STAR. There were no traffic conflicts or communication from ATC regarding any missed altitudes or airspeeds. Vectored to ILS visual 19C for VFR landing in VMC conditions.

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	1188960
Time
Date	201407
Local Time Of Day	1201-1800
Place
Locale Reference	IAD.Airport
State Reference	DC
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Gulfstream IV / G350 / G450
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	STAR GIBBZ2
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain
Qualification	Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 85 Flight Crew Total 13000 Flight Crew Type 1000
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance

A giv crew experienced dual flight guidance computer (FGC) failures during an RNAV arrival; but reset them while completing with reset checklist. A suspected unresolved disagreement between FMS's caused the FGC shutdown.