Narrative:

While level at FL280; ATC cleared us to cross 50 mi south of sey at FL200. I set and confirmed FL200 in MCP altitude window; and entered and confirmed a point 50 mi south of sey at FL200 in the FMS. I then checked the FMS descent page and verified that it had computed a reasonable top of descent point. Having taken these actions; the aircraft should have crossed 50 mi south of sey at FL200; but it never initiated a descent. About 40 mi south of sey; ATC issued a revised clearance of 'descend and maintain FL200.' I initiated a rapid descent and still managed to cross about 52 NM south of sey at FL200. Factors: both pilots did not monitor well enough to make sure the aircraft started down at the top of descent point that it showed that it was going to start down at; and this airline operates this aircraft with 3 different FMS software FMS inputs that are identical; have different effects upon what the aircraft physically does; depending upon which version of FMS processor and software is being used. Callback conversation with reporter revealed the following information: 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.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.