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As fleet avionics continue to evolve, the use of the solutions provided by performance-based navigation (PBN) will continue to grow and provide benefits to the aircraft operators. Due to these potential benefits, several air carriers in the United States have announced plans to retro-fit their fleets with PBN capabilities. However, it is important to understand the current snapshot of avionics, as well as the historic growth before analyzing the future forward-fit scenarios. Since 2004, the MITRE Corporation's Center for Advanced Aviation System Development (CAASD) has monitored the navigational avionics for Part 121 United States airlines. In this time span, the number of Part 121 airframes has dropped by more than 400 aircraft while the number of Part 121 aircraft with a flight management computer (FMC) has grown at approximately the same rate. This has resulted in a fleet wide growth in the percent of aircraft operating with an FMC from 79% to 90%. Also, the number of aircraft operating with a Global Positioning System (GPS) (also known as Global Navigation Satellite System (GNSS) navigational sensor) has grown with 1,100 additional aircraft operating with a GNSS sensor today. This has resulted in a growth from 47% of the aircraft in 2004 to 66% of the aircraft. There have been three primary reasons for the growth of PBN operational capability: equipped aircraft delivered to the system, current fleet retro-fits, and new knowledge of the airline operators. This paper will discuss the current equipage trends, capability, and identify the evolutionary change in Part 121 PBN avionics from 2004 through 2008. Additionally, this paper will discuss the next generation of aircraft and potential future scenarios for fleet avionics.