FAA Handbook Reliability, Maintainability, and Availability (RMA) HANDBOOK

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4.1.1 Purpose of NAS-Level RMA Requirements

The primary purpose of defining NAS-Level RMA requirements is to relate NAS Architecture Capability functional requirements to verifiable specifications for the hardware and software systems that support these capabilities. An intermediate step in this process is the introduction of the concept of generic Service Threads that define specific services, provided to controllers and/or pilots, which support the various NAS Architecture Capabilities. The Service Threads serve to bridge the gap between un-allocated functional requirements and the specifications for the systems that support them. They also provide the vehicle for allocating NAS-Level RMA-related requirements to specifications for the systems that comprise the Service Threads.

NAS-Level RMA requirements are provided to satisfy the following objectives:
• Provide a bridge between NAS-Level user needs and System-Level Specifications.
• Establish a common framework upon which to justify future additions and deletions of requirements.
• Provide uniformity and consistency of requirements across procured systems, promoting common understanding among the specifying engineers and the development contractors.
• Establish and maintain a baseline for validation and improvement of the RMA characteristics of fielded systems.

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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.

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