B757 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Flight Engineer; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 160; Flight Crew Total 11400; Flight Crew Type 4500
Situational Awareness; Human-Machine Interface; Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Procedure; Human Factors; Chart Or Publication; Aircraft
We were cleared to descend via the GIBBZ RNAV STAR to IAD. We started the arrival in VNAV PATH allowing the FMC to calculate our vertical navigation. The airplane started down on VNAV PATH; however after looking at our distance and altitude we were going to cross JIMVE Intersection very high; probably 2;000 FT high. I disconnected the autopilot and immediately started a steeper descent with speed brakes deployed. Despite our effort we crossed JIMVE 600 FT high.The same occurred at the next waypoint on the arrival; VNAV path was calculating a high crossing again. We discontinued VNAV and reverted to manual vertical navigation. The FMC had been programmed correctly and wind information input. We had flown this same aircraft on a previous leg that same day and no problems occurred on a previous RNAV arrival. The remainder of the flight was normal.
When their autoflight system; operating in VNAV PATH mode; failed to comply with the first two altitude constraints on the GIBBZ RNAV STAR to IAD the flight crew of a B757-200 reverted to manual control on their descent path. The STAR had been properly programmed including descent wind forecasts and the autoflight system had conducted a previous RNAV STAR descent without incident.