AN A320 FLT CREW ON DESCENT INTO JFK DESCENDED BELOW CLEARED ALT WHEN THEY WERE GIVEN A LATE RWY CHANGE.

Narrative

FLIGHT WAS TO JFK. CREW WAS EXPECTING ROUTING THROUGH CCC OVER ROBER AND THEN RADAR VECTORS FOR THE VOR RWY 13L AT JFK. AFTER CROSSING CCC AT 250 KTS AND 12000 FT; THE CREW WAS GIVEN A FAIRLY LATE CLEARANCE TO CROSS ROBER AT 9000 FT; THEN DESCEND TO 5000 FT. ROBER AT 9000 FT WAS INSERTED INTO THE MCDU AS MANAGED DESCENT POINT AND 5000 FT WAS ENTERED INTO THE FCU. A MANAGED DESCENT PROFILE WAS ENTERED BY PUSHING THE ALTITUDE BUTTON ON THE FCU. THE AIRCRAFT WAS WELL ABOVE THE MANAGED DESCENT PROFILE SO THE AIRCRAFT PITCHED DOWN AND ACCELERATED; AND THE VSI INCREASED TO SOMEWHERE ABOVE 2500 FPM. DESCENDING THROUGH 11000 FT; JFK APPROACH CHANGED THE ARRIVAL RWY FROM VOR RWY 13L TO VOR DME RWY 22L. THE PF ENTERED THE NEW RWY INTO THE MCDU AND BEGAN CHANGING THE DATA FOR THE ARRIVAL ON THE PERFORMANCE PAGE. JUST BEFORE 9000 FT THE PNF NOTICED THE AIRCRAFT WAS NOT LEVELING OFF AND IMMEDIATELY PUSHED THE 'PUSH TO LEVEL OFF' BUTTON ON THE FCU. THE AIRCRAFT LEVELED OFF AT 8460 FT APPROXIMATELY 2 MILES BEFORE REACHING ROBER. THE PF IMMEDIATELY STARTED A CLIMB BACK TO 9000 FT. AFTER ROBER THE DESCENT TO 5000 FT COMMENCED. THE UNTIMELY CHANGE OF THE ARRIVAL RWY DUMPED ALL VERTICAL NAV DATA ON THE ARRIVAL AND THE ANTICIPATED MANAGED LEVEL OFF DID NOT OCCUR AS EXPECTED. THERE WAS NO MODE CHANGE 'CLICK-CLICK-CLICK' NOTED WHEN THIS HAPPENED. THE CREW DID NOT CONFESS TO THE MISSED CROSSING ALTITUDE AT THE TIME. THE PURGING OF VERTICAL NAVIGATION DATA WHEN CHANGING RWYS IS A WELL KNOWN ISSUE WITH THIS AIRCRAFT AND SHOULD HAVE BEEN TAKEN INTO ACCOUNT BY BOTH PILOTS; HOWEVER; WITH THE RAPID DESCENT RATE; THE TIME TO CATCH THE PROBLEM WAS MINIMIZED. THIS IS A COMMON THING WITH THIS AIRCRAFT BUT THE CREW USUALLY HAS TIME TO CORRECT THE PROBLEM WHEN RE-BRIEFING FOR THE CHANGES IN THE APPROACH. THE PNF SHOULD HAVE NOTICED THE AIRCRAFT NOT LEVELING OFF SOONER; BASED ON THE DESCENT RATE OF THE AIRCRAFT. THE PF DID NOT TURN OFF THE AUTOPILOT TO STOP THE RATE OF DESCENT MORE RAPIDLY. IF HE HAD TAKEN OVER; THE AIRCRAFT WOULD STILL HAVE BEEN 400 FT LOW. THE CREW AGREED AFTERWARDS THAT THE 9000 FOOT RESTRICTION SHOULD HAVE BEEN PUT INTO THE FCU AS A HARD ALTITUDE; AND THEN THE DESCENT TO 5000 FT COULD BE COMMENCED AFTER REACHING ROBER.