Date: 201302
Local time of day: 1201-1800
Location: IAD.Airport, VA
Flight conditions: VMC
Light: Daylight
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E PCT
ATC / advisory: TRACON PCT
Aircraft component: FMS/FMC
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Training / Qualification; Human-Machine Interface; Distraction; Communication Breakdown
Second reporter function: First Officer; Pilot Flying
Anomaly: ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Contributing factors: Chart Or Publication; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: First Officer flying GIBBZ arrival. Runway assignment changed and First Officer reprogrammed FMC for new arrival. In doing so; some previous altitude restrictions were dumped and fixes already flown past were added. As aircraft crossed fix with 10;000 feet restriction; First Officer realized that VNAV had failed to engage and MCP was in undesired mode that would not capture altitude restriction and took over manually to make a smooth level off at 10;000 feet. In doing so; we descended not more than 300 feet below the altitude restriction before returning back to assigned altitude. No query was received from ATC. Two threats should be mentioned. First; it was known to us that the GIBBZ arrival is complicated by the fact that fixes after GIBBZ dependent on runway direction are not always loaded with the runway. Special vigilance is required to assure that the proper fixes are included. First Officer performed this properly; but it would constitute a distraction at that phase of flight; compromising his normal vigilance of the autopilot. Also; in loading the new runway with the arrival; the FMC goes back to previous fixes which must be deleted; another distraction. Second threat; my attention was divided at this time between flight deck and communication with cabin crew because of an ill flight attendant. I had assigned communication duties with ATC to the First Officer as well as flying. My ability to verify all the changes in FMC programming at that moment was compromised by the interphone communications in the back. I joined the First Officer when he remarked that the aircraft was not going to make the level off and monitored his progress in correcting the situation. He began his action just before passing 10;000 feet and dished out the level off at about 9;700 feet before returning to assigned altitude.


Second narrative: On arrival into Dulles on the GIBBZ; we had a runway change. We enter the arrival of the GIBBZ1 again with the new runway; hit execute and the FMC went back to the approach transition; of which we did not select on the execute. We quickly put the correct waypoint at the top of the FMC pg 1 and executed. The automation had gone into speed hold with the loss of the arrival. We were approaching 11;000 feet when we executed the new runway. When the Aircraft left VNAV and entered speed hold we were at a 1;500 FPM descent. Realizing the change of mode that occurred the autopilot was disconnected to alleviate descent and return aircraft to 11;000 feet. From the point of automation change and autopilot disconnect the aircraft descended to an altitude of 10;700 feet. We were cleared to descend via the GIBBZ with the lowest altitude being 6;000 feet. That was why there was no protection on the 11;000 feet restriction when the aircraft entered speed hold.


Synopsis: PCT changed a B757's arrival to the IAD GIBBZ1 and while the flying First Officer reprogrammed the FMS; the active waypoints with altitude constraints dropped out and the SPEED mode engaged as VNAV disengaged. So he disconnected the autopilot when he realizing SPEED gave no altitude protections and over shot the descent altitude constraint.