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The differences in performance of various manufacturers' flight management systems (FMSs) and their associated flight management computers (FMCs) have the potential for significant impact on the air traffic control system. While area navigation (RNAV) and required navigation performance (RNP) procedures and routes are designed according to criteria contained in Federal Aviation Administration (FAA) orders, FMCs are built to meet minimum aviation system performance standards (MASPS) (RTCA, 2003) and the minimum operational performance standards (MOPS) (RTCA, 2003) for area navigation systems, technical service orders and advisory circulars. The expectation is the resulting performance of the aircraft FMC will meet the procedure design requirements identified in the FAA criteria. The airspace design goal is procedures where aircraft operations result in repeatable and predictable paths. However, actual aircraft performance frequently does not match the expectations of the procedure designer. Studies referenced in this paper such as assessment of operational differences among flight management systems (Steinbach, 2004), analysis of advanced flight management systems (FMSs) (Herndon et al., 2005) and analysis of advanced flight management systems (FMSs), FMC field observations trials (Herndon et al., 2006) have shown that these differences result from variations in FMS equipment; variations and errors in data collection and processing; variations in pilot training and airline operating procedures; and variations in aircraft performance. This paper presents the hypothesis that given a standardized performance-based (RNAV/RNP) procedure with coded altitudes, variations in vertical path performance will exist among the various FMC/FMS combinations that are tested. Controlled observations were made using twelve test benches at five major FMC manufacturers and three full-motion simulators at the FAA and two airlines. This focus on vertical navigation (VNAV) path conformance follows the MITRE Corporation's analysis of lateral navigation (LNAV) path conformance described in analysis of advanced flight management systems (FMSs), FMC field observations trials (Herndon et al., 2006).