## 3.3.3 Defining the System / NAS Change 

## 3.3.3.1 Describe the System and the NAS Change 

## 3.3.3.1.1 Overview 

System descriptions need to exhibit two essential characteristics: correctness and completeness. Correctness means that the description accurately reflects the system without ambiguity or error. Completeness means that nothing has been omitted and that everything stated is essential and appropriate to the level of detail. 

The system description provides information that serves as the basis for identifying all hazards and associated safety risks. The system/operation must be described and modeled in sufficient detail to allow the safety analysis to proceed to the hazard identification stage. For example, modeling might entail creating a functional flow diagram to help depict the system and its interface with the users, other systems, or sub-systems. 

As discussed, the system is always a component of some larger system. For example, even if the analysis encompasses all services provided within an entire Air Route Traffic Control Center, that Center can be considered a subset of a larger body of airspace, which in turn is a subset of the NAS. 

## 3.3.3.1.2 Considerations when Defining the System 

Complex NAS changes may require a detailed system description that includes numerous charts, drawings, design descriptions, and/or narratives. Simple NAS changes may only require one or two paragraphs describing the system and NAS change. The description must be clear and complete before continuing the safety analysis. Questions to consider include: 

• What is the purpose of the NAS change? 

• What issue is necessitating the NAS change? 

• How will the change be used/function in the NAS? 

• What are the boundaries and external interfaces of the NAS change or system? 

• In what environment will the system or NAS change operate? 

• How is the system or NAS change interconnected/interdependent with other systems? 

• How will the NAS change affect system users/maintainers? 

• If the NAS change is a waiver/renewal, how could other waivers in effect interact with it? 

The following are examples of information to consider when describing the system: 

• Average annual approaches to each runway 

• Fleet mix 

• Number and type of airport operations 

• Number of aircraft controlled (ground, pattern, and transitions) 

• Number of hours the airport operates and number of aircraft controlled under Visual Flight Rules versus Instrument Flight Rules 

• Availability and reliability of both hardware and software 

