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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 and as such need to be examined and reexamined. While area navigation (RNAV) and required navigation performance (RNP) procedures and routes are designed according to criteria contained in Federal Aviation Administration (FAA) orders, FMC manufacturers build their systems in accordance with minimum aviation system performance standards (MASPS) and minimum operational performance standards (MOPS) for area navigation systems, technical service orders and advisory circulars. It is anticipated that the resulting performance of the aircraft FMC will meet the procedure design requirements identified in the FAA criteria. Airlines and air traffic controllers have as their goal flight procedures where aircraft operations meet expectations for repeatability and predictability to levels of performance sufficient to support performance based operations in the National Airspace System (NAS). Sometimes, due to the nearly independent development of procedure design criteria and aircraft performance standards, the paths of various aircraft on the same procedure do not overlap and do not match the expectancy of the procedure designer. These differences may result from any or all of the following: variations in FMC equipment installed on the aircraft; variations and errors in procedure coding in the FMC navigation database; variations in aircraft-to-FMC interface and associated aircraft performance capabilities; and variations in flight crew training and procedures. The hypothesis of this paper is that the basic FMCs built by avionics manufacturers and installed as the core of the FMC/FMS combinations in various airframe platforms perform differently and we will attempt to quantify those differences. This paper focuses on aspects of lateral and vertical flight FMC - performance when processing mandatory block altitudes, aircraft bank angle on turns above flight level nineteen thousand five hundred feet (FL195), determining the vertical transition point at fly-by waypoints, and execution of optimized profile descents (OPDs). Public instrument procedures flown using RNAV are used as the baseline for measuring performance variations. Controlled field observations trials were made using thirteen test benches and four simulators at seven major FMC manufacturers and three airlines. The intent of this report is to contribute technical data as a foundation for the acceptance of mandatory block altitude usage in RNAV and basic RNP procedures; allow standard instrument departure (SID) and standard arrival (STAR) procedure design criteria to utilize bank angles in excess of five degrees above FL195; satisfy an open FAA/Industry Aeronautical Charting Forum issue concerning the vertical transition point at fly-by waypoints; and assess FMC processing of an optimized profile descent.