Narrative:

[We were] cleared to mco via the cwrld two RNAV arrival. A briefing on the descent plan was conducted prior to starting the descent from cruise. During that time; the captain verbally noted the expected crossing restriction at lamma of 12;000 ft. This was input into the aircraft FMS as well as an arrival runway of 35R. The captain distinctly remembers doing so; as a descent angle of 3.6 degrees was also input for the planned descent based on expected winds aloft. The flight was given progressive descent clearances from cruise altitude; and prior to the ormond beach VOR; was cleared direct lamma. A clearance to cross lamma at 12;000 ft was also issued by jax center. The aircraft was established in a descent from cruise; with the autopilot on; when it intercepted the VNAV path; as displayed on the primary flight display. The captain selected VNAV mode on the control panel; the aircraft joined; and began descending on the selected FMS vertical navigation path. For unknown reasons the vertical path that the aircraft followed did not result in an altitude of 12;000 ft at lamma intersection. The entire time the magenta VNAV 'diamond' remained centered and a crossing indication at lamma was visible; however; the aircraft ended up being approximately 10;000 ft high at lamma; crossing that point at about 22;000 ft MSL. The crew did not notice the altitude discrepancy until approximately five miles from lamma. ATC queried the flight if they were descending to 12;000 ft. The crew responded that they were descending to 12;000 ft; but that they would be high at lamma. A greater vigilance on the part of the crew is essential. As has been emphasized in training; there should not be a heavy reliance on automation and the programming of the FMS. The crew needs to also verify that what is displayed is correct and that the aircraft is being operated accordingly. In this case; the crew should have better monitored the altitude and rate of descent after the crossing restriction was issued. A moderately long duty day and some fatigue after a deadhead flight and break may have been contributing factors. In addition the FMS system on this aircraft requires constant supervision. On many occasions; often related to vertical navigation planning; the system produces behavior that is unexpected or undesired. In this case a crossing altitude was entered into the FMS at the desired fix. In any system that should have been sufficient for the FMS to perform adequately. Instead there are certain programming issues; 'glitches;' or bugs that could cause the FMS to drop or remove data that has been input. The fact that different software loads seem to perform differently with these functions can serve to aggravate these problems. For example; one FMS might behave one way with certain inputs but another FMS might behave slightly differently because it has a different software version loaded. A crew expecting one outcome might find something different due to that. This is not meant to excuse a crew's need to observe and fly the aircraft properly but is intended to highlight one possible contributing factor.

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Original NASA ASRS Text

Title: An ERJ-170 flight crew reported failing to make a crossing restriction even though their FMS was engaged in VNAV Path and showed on profile.

Narrative: [We were] cleared to MCO via the CWRLD TWO RNAV arrival. A briefing on the descent plan was conducted prior to starting the descent from cruise. During that time; the Captain verbally noted the expected crossing restriction at LAMMA of 12;000 FT. This was input into the aircraft FMS as well as an arrival runway of 35R. The Captain distinctly remembers doing so; as a descent angle of 3.6 degrees was also input for the planned descent based on expected winds aloft. The flight was given progressive descent clearances from cruise altitude; and prior to the Ormond Beach VOR; was cleared direct LAMMA. A clearance to cross LAMMA at 12;000 FT was also issued by JAX Center. The aircraft was established in a descent from cruise; with the autopilot on; when it intercepted the VNAV path; as displayed on the primary flight display. The Captain selected VNAV mode on the control panel; the aircraft joined; and began descending on the selected FMS vertical navigation path. For unknown reasons the vertical path that the aircraft followed did not result in an altitude of 12;000 FT at LAMMA Intersection. The entire time the magenta VNAV 'diamond' remained centered and a crossing indication at LAMMA was visible; however; the aircraft ended up being approximately 10;000 FT high at LAMMA; crossing that point at about 22;000 FT MSL. The crew did not notice the altitude discrepancy until approximately five miles from LAMMA. ATC queried the flight if they were descending to 12;000 FT. The crew responded that they were descending to 12;000 FT; but that they would be high at LAMMA. A greater vigilance on the part of the crew is essential. As has been emphasized in training; there should not be a heavy reliance on automation and the programming of the FMS. The crew needs to also verify that what is displayed is correct and that the aircraft is being operated accordingly. In this case; the crew should have better monitored the altitude and rate of descent after the crossing restriction was issued. A moderately long duty day and some fatigue after a deadhead flight and break may have been contributing factors. In addition the FMS system on this aircraft requires constant supervision. On many occasions; often related to vertical navigation planning; the system produces behavior that is unexpected or undesired. In this case a crossing altitude was entered into the FMS at the desired fix. In any system that should have been sufficient for the FMS to perform adequately. Instead there are certain programming issues; 'glitches;' or bugs that could cause the FMS to drop or remove data that has been input. The fact that different software loads seem to perform differently with these functions can serve to aggravate these problems. For example; one FMS might behave one way with certain inputs but another FMS might behave slightly differently because it has a different software version loaded. A crew expecting one outcome might find something different due to that. This is not meant to excuse a crew's need to observe and fly the aircraft properly but is intended to highlight one possible contributing factor.

Data retrieved from NASA's ASRS site as of April 2012 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.