ALT XING RESTR NOT MADE WHEN THE FO OF AN A320 MIS-PROGRAMS THE FMS; DELETING ALT CONSTRAINTS PRIOR TO XING DAWNA INTXN; CA.

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ALTDEV: LEVEL AT (I THINK) FL240. ZLA CLRED US TO DSND TO 13000 FT BY DAWNA ON KAYOH 4 TO SNA. INSTEAD; WE CROSSED AT APPROX FL190. WHY: I HAD PROGRAMMED FMS WITH EXPECTED 13000 FT DAWNA CONSTRAINT. THEN NOTICED AN UNWANTED 'VIA' THE FMS HAD INSERTED BTWN THE ARR AND THE SNA RWY 19R ILS. I DELETED THE 'VIA' BUT DID NOT KNOW THAT WOULD DELETE CONSTRAINTS I MANUALLY INPUT TO THE ARR. WHEN GIVEN THE 13000 FT DAWNA CONSTRAINT BY ZLA; I SELECTED DSCNT MODE AND I STARTED 1000 FPM DSCNT. APPROX 10 MI FROM DAWNA; ZLA ASKED ABOUT OUR DSCNT. I IMMEDIATELY SELECTED OPEN DSCNT; TURNED OFF AUTOPLT; AND USED SPD BRAKES. WE CROSSED DAWNA AT APPROX FL190 AND QUICKLY REACHED 13000 FT. THERE WERE NO APPARENT TFC CONFLICTS. CONTRIBUTING FACTORS: I WAS NEVER TOLD THAT DELETING FMA 'VIA' WOULD DELETE MANUALLY INPUT ALT CONSTRAINTS -- SIMILAR TO WHAT FMS RWY CHANGES DO AND WHICH I WAS WELL TRAINED FOR. I DID NOT TELL THE CAPT I DELETED THE 'VIA.' HE WOULD HAVE KNOWN TO RECHK THE CONSTRAINTS. RECOMMENDATIONS: INSTRUCTOR AND IOE TEACH THAT DELETING 'VIA' WILL DELETE MANUALLY INPUT FMS CONSTRAINT. MY PLAN: ALWAYS RECHK FMS CONSTRAINTS STARTING DSCNT AND MONITOR LEVELOFF ESTIMATE SYMBOL. I WILL ALSO ONLY USE OPEN DSCNT MODE VERSUS DSCNT MODE TILL I HAVE MORE EXPERIENCE. SUPPLEMENTAL INFO FROM ACN 541705: THE AUTOPLT REDUCED THE DSCNT RATE; WE WERE UPDATING OTHER APCH INFO INTO THE MCDU.