scope:

Most of the systems comprising the National Airspace System (NAS) fall into one of three general categories:

• Automated information systems that continuously integrate and update data from remote services to provide timely decision-support services to Air Traffic Control (ATC) specialists

• Remote and distributed subsystems that provide services such as navigation, surveillance, and communications to support NAS ATC systems

• Infrastructure systems that provide services such as power, heating, ventilating, and air conditioning (HVAC) systems, and telecommunications to support NAS facilities

This document primarily allocates NAS-Level requirements to the information systems that provide consolidated ATC services. These systems involve software-intensive air traffic control automation and communications capabilities. They have stringent availability requirements and, as a consequence of the large amounts of custom software that must be developed for them, entail significant cost and schedule risks. These programs provide the most critical operational services and have the most visibility. For these reasons, it is appropriate that they be given the most attention in this handbook

Remote and distributed subsystems achieve the necessary overall availability through their reliance upon diversity tailored to meet specific regional considerations. The availability of the individual elements comprising these subsystems is furthermore determined by life-cycle considerations, not by top-down allocations from NAS-level requirements.

Because infrastructure systems such as power systems, heating ventilation and air conditioning (HVAC) systems typically violate the independence assumption underlying RMA calculations, they can directly cause failures in the systems they support. Therefore, top-down allocations of availability requirements are not appropriate for these systems. Instead, the aviation community needs to prepare and standardize a new, well defined set of configurations to use with infrastructure systems.

This handbook is for guidance only and cannot be cited as a requirement.