Narrative:

We were cleared to descend on the pdz 4 arrival to lax, the clearance limit was 8000 ft. Aircraft was in a 'managed' descent, so that the autoplt would automatically level off at the FMC programmed crossing restrs on the arrival. About 3 mi east of trtle waypoint, we received a runway change from runway 25L to runway 24R. When the captain (PF) inserted the new runway into the FMC, the waypoints were missequenced and the aircraft reverted to open descent which resulted in us crossing trtle waypoint at 15400 ft as opposed to the charted minimum crossing altitude of 16000 ft. Although I was monitoring our flight path using raw data, in a brief moment of miscoms I could not impress on the PF the altitude trend/situation. In addition, my workload increased at the time he inserted the new runway and unexpected waypoints came up, as he asked me for assistance. We did eventually stop the descent. We would have probably missed the crossing altitude by a greater margin had I not used raw data to back up the FMC. ATC did not seem to notice. Some recommendations: 1) alter the A320 FMC programming to allow an easier method of switching runways when on a STAR (this is not the first time waypoint dropped flight plan problem has occurred for me on the pdz 4 arrival as well as others). 2) alter company SOP's requiring the use of open descent and hard altitudes in the FCU instead of a 'managed' automatic descent which would eliminated the open descent reversion problem. Supplemental information from acn 526644: ATC did not call out any altitude deviation, but the first officer who was the PNF thought we may have crossed trtle intersection too low.

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Original NASA ASRS Text

Title: AN A320 CREW, PREOCCUPIED WITH REPROGRAMMING THE FMS, MISSED THEIR ASSIGNED XING RESTR.

Narrative: WE WERE CLRED TO DSND ON THE PDZ 4 ARR TO LAX, THE CLRNC LIMIT WAS 8000 FT. ACFT WAS IN A 'MANAGED' DSCNT, SO THAT THE AUTOPLT WOULD AUTOMATICALLY LEVEL OFF AT THE FMC PROGRAMMED XING RESTRS ON THE ARR. ABOUT 3 MI E OF TRTLE WAYPOINT, WE RECEIVED A RWY CHANGE FROM RWY 25L TO RWY 24R. WHEN THE CAPT (PF) INSERTED THE NEW RWY INTO THE FMC, THE WAYPOINTS WERE MISSEQUENCED AND THE ACFT REVERTED TO OPEN DSCNT WHICH RESULTED IN US XING TRTLE WAYPOINT AT 15400 FT AS OPPOSED TO THE CHARTED MINIMUM XING ALT OF 16000 FT. ALTHOUGH I WAS MONITORING OUR FLT PATH USING RAW DATA, IN A BRIEF MOMENT OF MISCOMS I COULD NOT IMPRESS ON THE PF THE ALT TREND/SIT. IN ADDITION, MY WORKLOAD INCREASED AT THE TIME HE INSERTED THE NEW RWY AND UNEXPECTED WAYPOINTS CAME UP, AS HE ASKED ME FOR ASSISTANCE. WE DID EVENTUALLY STOP THE DSCNT. WE WOULD HAVE PROBABLY MISSED THE XING ALT BY A GREATER MARGIN HAD I NOT USED RAW DATA TO BACK UP THE FMC. ATC DID NOT SEEM TO NOTICE. SOME RECOMMENDATIONS: 1) ALTER THE A320 FMC PROGRAMMING TO ALLOW AN EASIER METHOD OF SWITCHING RWYS WHEN ON A STAR (THIS IS NOT THE FIRST TIME WAYPOINT DROPPED FLT PLAN PROB HAS OCCURRED FOR ME ON THE PDZ 4 ARR AS WELL AS OTHERS). 2) ALTER COMPANY SOP'S REQUIRING THE USE OF OPEN DSCNT AND HARD ALTS IN THE FCU INSTEAD OF A 'MANAGED' AUTOMATIC DSCNT WHICH WOULD ELIMINATED THE OPEN DSCNT REVERSION PROB. SUPPLEMENTAL INFO FROM ACN 526644: ATC DID NOT CALL OUT ANY ALTDEV, BUT THE FO WHO WAS THE PNF THOUGHT WE MAY HAVE CROSSED TRTLE INTXN TOO LOW.

Data retrieved from NASA's ASRS site as of July 2007 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.