B737-700 FLT CREW DESCENDS BELOW CROSSING RESTRICTION ON LDA-A IN VMC TO LGA. CITE WORKLOAD; CRM ISSUES AND LACK OF PROCEDURE IN FMS DATABASE AS CONTRIBUTORS.

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DURING DSCNT INTO LGA AREA; RECEIVED ATIS INDICATING LDA-A RWY 22 APCH IN USE. SINCE LDA-A APCH NOT IN FMS AS AN APCH AVAILABLE TO BE SELECTED; FIXES ON APCH HAD TO BE MANUALLY BUILT IN FMS TO USE BEST AVAILABLE ACFT AUTOMATION. THIS TOOK TIME FOR INSTRUCTION FROM CAPT TO FO (CAPT FLYING); AND CAUSED SOME RUSHING TO ACCOMPLISH IN-RANGE CHKLISTS AND ARR PROCS IN TIMELY MANNER. NEW YORK APCH RADIO TFC WAS ALSO VERY BUSY. FLT WAS CLRED FROM HAARP ON RKA2 ARR DIRECT TO CASLE ON LDA-A APCH; AND CLRED FOR LDA-A APCH. WX WAS VMC; AND ARPT WAS IN SIGHT BEFORE CASLE FIX. SINCE MANUALLY BUILT APCHS CAN'T USE LNAV/VNAV PROCS; CAPT SELECTED VOR/LOC AND VERT SPD FOR DSCNT AT CASLE. CAPT INADVERTENTLY SELECTED 1000 FT (CIRCLING MINIMA) IN ALT SELECTOR VERSUS 1600 FT FOR COHOP STEPDOWN FIX. LOC WAS CAPTURED AT CASLE; AND ACFT WAS S-ING DOWN FINAL DUE TO LOC COURSE DEVIATING BACK-AND-FORTH (PROBABLY DUE TO XING ACFT ON GND; LGA TKOFS WERE BEING CONDUCTED ON RWY 13). CAPT SELECTED LNAV FOR STEADY COURSE CTL. APPROX 1 MI BEFORE COHOP; CAPT RECOGNIZED ALTDEV AT APPROX 1450 FT; DISCONNECTED AUTOPLT; CORRECTED BACK TO 1600 FT AT COHOP; AND CONTINUED APCH TO LAND AT LGA RWY 22. DIGITAL ATIS CAPABILITY WOULD HAVE HELPED. WITH DIGITAL ATIS; DEST STATION WX AND ACTIVE APCHS CAN BE STUDIED AND ANALYZED ENRTE RATHER THAN IN DSCNT PHASE; ALLOWING FOR MORE TIME TO DISCUSS AND PLAN ALL ASPECTS OF THE APCH TO BE FLOWN. WHEN VOICE ATIS MUST BE RECEIVED BY ONE OF THE PLTS DURING DSCNT INTO BUSY ARPT AREAS; THERE IS MUCH MORE POTENTIAL FOR ERROR IN MANY ASPECTS OF THE DSCNT AND APCH.