B737 FIRST OFFICER REPORTS AIRCRAFT FAILS TO INITIATE DESCENT TO FL200 AS PROGRAMMED AND CITES DIFFERING FMS SOFTWARE VERSIONS AS LIKELY CULPRIT.

Narrative

WHILE LEVEL AT FL280; ATC CLRED US TO CROSS 50 MI S OF SEY AT FL200. I SET AND CONFIRMED FL200 IN MCP ALT WINDOW; AND ENTERED AND CONFIRMED A POINT 50 MI S OF SEY AT FL200 IN THE FMS. I THEN CHKED THE FMS DSCNT PAGE AND VERIFIED THAT IT HAD COMPUTED A REASONABLE TOP OF DSCNT POINT. HAVING TAKEN THESE ACTIONS; THE ACFT SHOULD HAVE CROSSED 50 MI S OF SEY AT FL200; BUT IT NEVER INITIATED A DSCNT. ABOUT 40 MI S OF SEY; ATC ISSUED A REVISED CLRNC OF 'DSND AND MAINTAIN FL200.' I INITIATED A RAPID DSCNT AND STILL MANAGED TO CROSS ABOUT 52 NM S OF SEY AT FL200. FACTORS: BOTH PLTS DID NOT MONITOR WELL ENOUGH TO MAKE SURE THE ACFT STARTED DOWN AT THE TOP OF DSCNT POINT THAT IT SHOWED THAT IT WAS GOING TO START DOWN AT; AND THIS AIRLINE OPERATES THIS ACFT WITH 3 DIFFERENT FMS SOFTWARE FMS INPUTS THAT ARE IDENTICAL; HAVE DIFFERENT EFFECTS UPON WHAT THE ACFT PHYSICALLY DOES; DEPENDING UPON WHICH VERSION OF FMS PROCESSOR AND SOFTWARE IS BEING USED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER HAS EXPERIENCE IN SEVERAL MODERN JET AIRCRAFT WITH ELECTRONIC DISPLAYS BUT HAS RECENTLY BEEN FLYING B737'S WITH FMC'S AND NO GLASS. THE PROBLEM IS THAT ALL FMC INPUTS MUST BE ENTERED AND MONITORED USING VARIOUS FMC PAGES AND IS DIFFICULT WITHOUT THE MAP DISPLAY OF THE EARLIER AIRCRAFT. ALL OF THE SOFT WARE VERSIONS ARE CAPABLE OF MAKING PROGRAMMED CROSSING RESTRICTIONS SO IT IS NOT CLEAR WHY THIS ONE DID NOT.