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.

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Original NASA ASRS Text

Title: 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.

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.

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.