AN A320 PLT RPTS THAT WHEN A RTE FIX ALT RESTRICTION IS ENTERED INTO THE FMGC; IT AUTOMATICALLY DROPPED THE CONSTRAINT AND THE ACFT CROSSED HIGH.

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ATC GAVE US CLRNC TO CROSS 40 NW OF MSN AT FL290. CROSSING WAS PLACED IN THE FMGC AND WE BEGAN OUR DSCNT. THE FMGC AUTOMATICALLY DELETED THE CROSSING RESTRICTION AND WHEN WE NOTICED IT; WE WERE UNABLE TO MAKE THE RESTRICTION. WE CROSSED THE RESTRICTION ABOUT 1500 FT HIGH. THIS IS TO ME A CRITICAL PROGRAMMING PROB IN THE AIRBUS. WE TRY TO WATCH WHAT THE COMPUTER DOES; BUT YOU HAVE TO LIE TO THE COMPUTER TO TAKE THIS KIND OF INPUT. THE AIRBUS PROGRAMMING NEEDS TO BE CHANGED SO THAT A PLT INPUT IS NOT DELETED.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATES THAT A DESIGN FEATURE OF THE AIRBUS FMGC IS THAT IF THE ACFT IS GREATER THAN 200 NM FROM ITS DEST AND AN ALT CONSTRAINT IS ENTERED WITH A FIX ON THE RTE; THAT NEW ALT BECOMES A CRUISE ALT AND THE ACFT IMMEDIATELY BEGINS A DSCNT TO THE ALT AT 1000 FT PER MINUTE. IF THE DISTANCE IS LESS THAN 200 NM FROM DEST; THE ALT IS ATTACHED TO THE RTE FIX AS A MANDATORY CONSTRAINT AND THE DSCNT WILL OCCUR SO AS TO CROSS THE FIX AT THE CONSTRAINT ALT. IN THE CASE OF FIX CONSTRAINT ASSIGNED GREATER THAN 200 NM FROM DEST; THE FIX REMAINS BUT THE CROSSING ALT IS DELETED BECAUSE THE ALT IS TREATED AS A CRUISE ALT. IN THIS CASE; THE ACFT MAY NOT DSND FAST ENOUGH AND CROSS THE FIX HIGH IF THE AUTOMATIC 1000 FT RATE OF DSCNT DOES NOT MEET THE CROSSING RESTRICTION. THE CREW MUST SELECT AN OPEN DSCNT AND MANUALLY SELECT THE RATE OF DSCNT NEEDED TO MEET THE ALT CROSSING RESTRICTION.