surface that pose additional challenges to capacity expansion. Finally, this is not only a commercial aviation issue, as military readiness is also being challenged by restrictions on training and operations. These effects will be exacerbated by aviation growth.

Compounding the environmental issues is the fact that aviation has features that distinguish it from other transportation modes and industries. The high premium placed on safety demands the incorporation of only proven and technically sound environmental technologies in aircraft, as well as on the ground (e.g., deicing for aircraft and airport runways). Aircraft are expensive and have a long life span, requiring long lead times for new technologies to be widely incorporated in the fleet and close attention to financial feasibility. Airborne systems must be lightweight and fuel-efficient. Noise, local and regional air quality, and potential climate effects are engendered by an interdependent set of technologies and operations, so that action to reduce impacts in one area (e.g., aircraft engine noise) can increase the impacts in another (e.g., nitrogen oxides emissions). All these factors combine to make it challenging to quickly incorporate new technologies, rapidly change fleets, or manage multiple environmental impacts without trade-offs.

Key Uncertainties in NGATS Architecture

In developing the environmental roadmap for implementing the draft NGATS architecture, a number of uncertainties remain that will have a large influence on the success of tackling the environmental dimension in delivering the NGATS plan. Some critical ones include:

  • Capacity results. It is uncertain whether the draft architecture will produce the targeted three times growth in capacity of the NGATS endeavor. Further, it is not clear what the definition is for capacity—whether it’s measured in passengers or operations. Both these factors will have a large influence on potential environmental impacts and whether the current plans and initiatives have any prospect of success.

  • Number and location of “air portals.” It is not clear at this juncture whether NGATS will be delivering aircraft in the same airport patterns of today or something vastly different. Will the majority of traffic involve the top 50 airports, or will it spread to hundreds or thousands of air portals? The potential environmental footprint of aviation—and hence the investments required to shrink that footprint—will be vastly different depending on this number.

  • Required environmental performance. It remains to be developed how to work in the environmental performance requirements for air traffic services, aircraft, and airports in a system that operates on multiple, differentiated service levels aligned with each user’s ability to meet different levels of Required Total System Performance. Further, given the capabilities that new technologies may offer, a balance between applying these abilities in expanding capacity versus minimizing environmental impacts will need to occur.

  • Cost and timing of delivery. There are (understandably) significant gaps in information on the costs of implementation and timing of delivery of capabilities in the transition from the existing mode of NAS operations to the planned architecture. Both the scale of costs, especially in terms of enabling technology—e.g., GPS overlay procedures and fleet retrofitting—and timing issues will have large potential impacts on the ability to manage the resulting environmental impacts.

  • Identification of choke points in the system. The initial architecture has focused on managing traffic through the sky. Given the early stage of development, it is not surprising that the key choke points have not yet been identified. However, just as important will be identification of the choke points in NGATS, to ensure correct targeting of investment in different aspects of the plan to provide the necessary capacity growth.

Other Key Uncertainties
  • Environmental targets for 2025. The required scope of reduction in both noise and local air quality emissions for the system of 2025 has yet to be determined. While we have committed to absolute reductions in both areas, the investments actually required will again depend on how aggressive we are in changing these metrics. It is also unclear what additional impacts may arise from improved scientific understanding of aviation’s influence on climate change. Finally, it is uncertain what new requirements may arise from potential introduction of new aircraft types—for example, supersonic business jets—or new environmental concerns—for example, high-altitude noise over national parks.

  • Composition and environmental performance of the aircraft fleet. The large, subsonic commercial aircraft fleet we have today will—without intervention—in large part be the fleet we have in 2025. This poses a significant obstacle to improved environmental performance, especially for a system that expands threefold. For example, while navigation capabilities can be upgraded relatively quickly and cheaply through plug and play, changing the environmental performance of an aircraft is a more costly and difficult task given the safety and operational issues. Further, the role and size of other aircraft—very light jets, very large jets, UAVs, supersonic business jets, etc.—in the system of 2025 are unclear.

  • Technological research funding gap. The next 5 to 7 years of research and development are critical for the fleet of 2025 given the long lead times involved in

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