Narrative:

We were assigned the GIBBZ1 RNAV STAR transitioning from mgw to gibbz. The captain was utilizing the VNAV function after the crew confirmed all waypoints and altitudes were properly set in the FMS for descent and approach into iad. ATC delayed our descent from FL450 and that delay required at least a 4.0 degree descent via the GIBBZ1. Initial descent appeared to be correct based on manually confirming required descent rate. Later; a disparity between FMS 1 and FMS 2 presented a different descent profile. The FMS 1 clock froze and shallowed out our descent multiple times despite the PF's attempts to correct the descent rate. FMS 2 presented a level-off pitch which we knew was incorrect. Since there are multiple altitude restrictions with this STAR and an aggressive descent [was required]; we missed an altitude restriction; at/below FL290; at brndn by approximately 1;000 feet. In addition; the FMS approach mode did not provide proper course and vertical guidance during the ILS approach requiring us to utilize LOC2 guidance for the approach. We wrote up FMS 1 with maintenance after landing. A common technique prior to flying a complex arrival with multiple descent limits is to begin the descent from cruise altitude early (fuel permitting). Also; don't trust VNAV; stick with using vs in the citation X and mentally calculating required descent rates (old school method).

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Original NASA ASRS Text

Title: When they experienced malfunctions of both FMS systems while flying the GIBBZ RNAV STAR to IAD; a Citation 750 flight crew failed to comply with an altitude crossing restriction at BRNDN.

Narrative: We were assigned the GIBBZ1 RNAV STAR transitioning from MGW to GIBBZ. The Captain was utilizing the VNAV function after the crew confirmed all waypoints and altitudes were properly set in the FMS for descent and approach into IAD. ATC delayed our descent from FL450 and that delay required at least a 4.0 degree descent via the GIBBZ1. Initial descent appeared to be correct based on manually confirming required descent rate. Later; a disparity between FMS 1 and FMS 2 presented a different descent profile. The FMS 1 clock froze and shallowed out our descent multiple times despite the PF's attempts to correct the descent rate. FMS 2 presented a level-off pitch which we knew was incorrect. Since there are multiple altitude restrictions with this STAR and an aggressive descent [was required]; we missed an altitude restriction; at/below FL290; at BRNDN by approximately 1;000 feet. In addition; the FMS Approach mode did not provide proper course and vertical guidance during the ILS approach requiring us to utilize LOC2 guidance for the approach. We wrote up FMS 1 with Maintenance after landing. A common technique prior to flying a complex arrival with multiple descent limits is to begin the descent from cruise altitude early (fuel permitting). Also; don't trust VNAV; stick with using VS in the Citation X and mentally calculating required descent rates (old school method).

Data retrieved from NASA's ASRS site as of July 2013 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.