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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 operational impact on the air traffic control system and as such need to be examined on a recurring basis. Performance-based navigation (PBN) is a fundamental principle for aircraft operations that will facilitate the transition to future airspace systems. A critical element of PBN is the FMS's capability to fly a consistently repeatable and predictable flight path trajectory that will meet the expectations of air traffic control. FMS manufacturers build their systems in accordance with and for area navigation systems, Technical Standard Orders and Advisory Circulars. Area Navigation (RNAV) and Required Navigation Performance (RNP) procedures and routes are published by the Federal Aviation Administration (FAA) according to criteria contained in FAA orders. It is anticipated that the resulting performance of the aircraft FMC will meet the procedure design requirements identified in the FAA criteria. 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 coincide and therefore do not match the expectations 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 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 FMC performance when flying Standard Instrument Departu- - res (SIDs) and their associated waypoints and leg types (path terminators) and combinations as described in. Public instrument procedures flown using RNAV equipment are used as the baseline for measuring the performance variations. Controlled field observations trials were made using twelve FMS test benches and three simulators at seven major FMC manufacturers and two airlines. Analysis of data from the trials confirms differences and the details are presented. The intent of this report is to contribute technical data as a foundation for the acceptance of required navigation performance (RNP) departures.