Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 85; Flight Crew Total 13000; Flight Crew Type 1000
Confusion; Distraction; Human-Machine Interface; Workload
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Aircraft Equipment Problem Dissipated; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Procedure; Aircraft
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.
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.