Commercial Fixed Wing
Flight Crew Last 90 Days 250; Flight Crew Total 14000; Flight Crew Type 2600
Confusion; Human-Machine Interface
ATC Issue All Types; Aircraft Equipment Problem Critical; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Procedure; Human Factors; Aircraft
Got late 'descend via' for GIBBZ RNAV arrival; first portion of arrival was deleted by ATC; with instructions to cross MOSLE at 11;000 FT. [We] had to confirm that ATC wanted us to continue to descend via the arrival after MOSLE and wanted published 250 KTS speed at MOSLE. We had been in VNAV PATH with GIBBS altitude of 7;000 FT set when this clearance was received. We then went to FLCH with assigned altitude of 11;000 FT. As we approached 11;000 FT the FMA went to ALT CAP mode; as expected; but the jet descended right through 11;000 FT in full autopilot with ALT CAP annunciated. We deselected the autopilot and bottomed out at 10;750; then returned to 11;000. The remainder of the arrival was normal; but we conducted all further descents in FLCH and selected speed.
ATC initiated alterations to the GIBZZ RNAV STAR may have contributed to an altitude deviation for a Boeing flight crew when the autoflight system failed to arrest their descent at 11;000 per the MCP altitude window and the active FLCH mode.