Narrative:

The first officer was flying from phx to lax. During the pdz 4 arrival to runway 25L at lax, socal approach cleared 'descend on the pdz 4 arrival.' the descent was 'managed' on autoplt with 8000 ft (the altitude limit) and 'speed' on the FCU. After passing konzl at 17000 ft, approach control reclred 'to maintain maximum forward speed and to change runway to runway 24R. As the first officer made the runway change in the mcdu, the FCU defaulted to 'open descent.' the first officer also changed the FCU to a selected higher speed (about 340 KTS). The aircraft descended to 15000 ft about 2 NM east of trtle. The arrival limits at trtle are 'cross at or below 17000 ft, cross at or above 16000 ft.' the captain and first officer, upon recognizing that actions on the FCU had caused the FCU to default to 'open descent,' and allowed a descent 1000 ft low just prior to trtle, immediately corrected the aircraft back to 16000 ft on autoplt at trtle and the first officer flew the remainder of the arrival and approach to runway 24R in 'select' speeds and altitudes on the FCU. As the first officer was flying the aircraft on autoplt and making clearance changes in the mcdu, the captain was responding to ATC's clearance changes and monitoring changes being entered in the mcdu. The cockpit workload was high trying to accommodate the clearance changes and aircraft positioning monitoring was momentarily neglected, leading to the altitude deviation just short of the next fix. Reclrncs such as this occasionally exceed capabilities of aircraft's automation to comply, leading to such a default. Although these reclrncs are understandably necessary, an attempt to limit them, or give them at a more timely point so that automation can be programmed (ie, prior to initial descent clearance) may prevent this from recurring in the future.

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Original NASA ASRS Text

Title: A320 FO OVERSHOT DSCNT ALT DUE TO MISPROGRAMMING THE AUTO FCU AND CORRECTED BACK IN TIME TO MAKE THE STAR FIX.

Narrative: THE FO WAS FLYING FROM PHX TO LAX. DURING THE PDZ 4 ARR TO RWY 25L AT LAX, SOCAL APCH CLRED 'DSND ON THE PDZ 4 ARR.' THE DSCNT WAS 'MANAGED' ON AUTOPLT WITH 8000 FT (THE ALT LIMIT) AND 'SPD' ON THE FCU. AFTER PASSING KONZL AT 17000 FT, APCH CTL RECLRED 'TO MAINTAIN MAX FORWARD SPD AND TO CHANGE RWY TO RWY 24R. AS THE FO MADE THE RWY CHANGE IN THE MCDU, THE FCU DEFAULTED TO 'OPEN DSCNT.' THE FO ALSO CHANGED THE FCU TO A SELECTED HIGHER SPD (ABOUT 340 KTS). THE ACFT DSNDED TO 15000 FT ABOUT 2 NM E OF TRTLE. THE ARR LIMITS AT TRTLE ARE 'CROSS AT OR BELOW 17000 FT, CROSS AT OR ABOVE 16000 FT.' THE CAPT AND FO, UPON RECOGNIZING THAT ACTIONS ON THE FCU HAD CAUSED THE FCU TO DEFAULT TO 'OPEN DSCNT,' AND ALLOWED A DSCNT 1000 FT LOW JUST PRIOR TO TRTLE, IMMEDIATELY CORRECTED THE ACFT BACK TO 16000 FT ON AUTOPLT AT TRTLE AND THE FO FLEW THE REMAINDER OF THE ARR AND APCH TO RWY 24R IN 'SELECT' SPDS AND ALTS ON THE FCU. AS THE FO WAS FLYING THE ACFT ON AUTOPLT AND MAKING CLRNC CHANGES IN THE MCDU, THE CAPT WAS RESPONDING TO ATC'S CLRNC CHANGES AND MONITORING CHANGES BEING ENTERED IN THE MCDU. THE COCKPIT WORKLOAD WAS HIGH TRYING TO ACCOMMODATE THE CLRNC CHANGES AND ACFT POSITIONING MONITORING WAS MOMENTARILY NEGLECTED, LEADING TO THE ALTDEV JUST SHORT OF THE NEXT FIX. RECLRNCS SUCH AS THIS OCCASIONALLY EXCEED CAPABILITIES OF ACFT'S AUTOMATION TO COMPLY, LEADING TO SUCH A DEFAULT. ALTHOUGH THESE RECLRNCS ARE UNDERSTANDABLY NECESSARY, AN ATTEMPT TO LIMIT THEM, OR GIVE THEM AT A MORE TIMELY POINT SO THAT AUTOMATION CAN BE PROGRAMMED (IE, PRIOR TO INITIAL DSCNT CLRNC) MAY PREVENT THIS FROM RECURRING IN THE FUTURE.

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.