AN A319 CREW MISSED AN ALT XING RESTR BECAUSE THE AIRBUS FMGC SYS DROPPED A FIX XING ALT RESTR.

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WE WERE GIVEN A XING RESTR BY CTR TO CROSS 25 MI N OF DRK VOR AT FL330. AS THERE WAS NO ESTABLISHED WAYPOINT AT THIS LOCATION; THE CAPT MANUALLY BUILT A WAYPOINT 25 MI N OF DRK; THEN ENTERED THE RESTR INTO THE FMS (FL330). THE AIRBUS HAS A UNIQUE FEATURE THAT WHEN THE CAPT MANAGED (STARTED) THE DSCNT; THE FMS DEFAULTED TO FL330 AS A NEW 'CRUISE' ALT AND DROPPED OUT THE PREVIOUSLY PROGRAMMED XING RESTR. WHEN CHANGING 'CRUISE' ALTS THE FMS AUTOMATICALLY DSNDS AT A CONSERVATIVE 1000 FPM. WE NOTICED AT 15 MI N DRK THAT WE WERE DSNDING THROUGH FL338 AND OBVIOUSLY HAD BLOWN OUR 25 DME N DRK RESTR. IT APPEARED TO BE A CTLR HDOF RESTR; AS ATC DIDN'T INFORM US OF ANY TFC CONFLICTS. MOST PLTS DON'T USE THIS FMS PROGRAMMING TECHNIQUE DUE TO THE OBVIOUS SOFTWARE PROGRAMMING CONFLICT. I WAS CAUGHT OFF GUARD BY THIS UNIQUE AIRBUS HABIT. WE EXPEDITED OUR DSCNT UPON REALIZING WE HAD MISSED OUR RESTR AND WERE NOT INFORMED BY ATC OF ANY PROBS OR CONFLICTS CREATED BY OUR HIGH XING. FATIGUE MAY HAVE BEEN A FACTOR AS DEP TIME WAS AFTER XA00 WITH ARR AT XY00. THIS WAS PURE BACK SIDE OF THE CLOCK FLYING AND CONTRIBUTED TO NOT CATCHING THE DEV SOONER. WHILE FAMILIAR WITH THIS SOFTWARE SCENARIO FROM INITIAL TRAINING; I AM NOW MORE GREATLY AWARE OF THIS 'GOTCHA' FROM PERSONAL EXPERIENCE. AN FMS SOFTWARE CHANGE WOULD ALSO HELP ALLEVIATE THIS FROM HAPPENING IN THE FUTURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE CONDITION CAUSING THE TRANSITION TO A CRUISE DSCNT MODE OF 1000 FPM IS DESCRIBED IN HIS CARRIER'S PLT HANDBOOK. THE CREW DID NOT REALIZE UNTIL AFTER THE FACT THAT THIS TRANSITION WAS POSSIBLE AND; THEREFORE; DID NOT ANTICIPATE ITS OCCURRENCE. AS A CONSEQUENCE; THEY MISSED THE ASSIGNED ALT CONSTRAINT.