Systems Engineering

Functional Diagrams are prepared during analysis to illustrate the component operations a system performs and their inputs and outputs. They are a model of how a system operates in visual form. They start at the top level with external inputs and outputs which cross the system boundary, then are broken down into multiple levels of detail. They are often in a step by step time sequence, but can be more complex networks of operations with decision points and loops (Figure 1.5-4). For example, for an airplane, the main functions would be Load Passengers and Cargo, Taxi to Runway, Takeoff, Fly, Land, Taxi to Gate, and Unload Passengers and Cargo. Each of these main functions are further divided into smaller steps, then assigned to system elements to carry out. For example, the landing gear might be assigned multiple functions such as "absorb landing loads" and "provide steering for taxiing". Those then become requirements for detailed design and testing of that element.

Individual functions in a diagram transform inputs into outputs (Figure 1.5-5). The diagram typically shows functions as boxes and input/output flows as arrows connecting the boxes running from left to right. Flows can contain any sort of item, including information, matter, energy, labor, etc, or a combination thereof. They may divide and combine on a diagram, but the divided flow must sum to the contents of the undivided one. This derives from the physics concept of conservation laws, where matter and energy do not arise out of nothing. Similarly the flows within a system do not arise from or vanish into nothing, they must enter from outside or be converted by a functional task. By following the Conservation of Flows logic, then all the inputs and outputs of a system will be accounted for.

Control inputs regulate the operation of a function. By convention they are shown entering the top of a function box. Mechanisms perform the function, but are not transformed themselves, and are shown entering from the bottom. A mechanism example is a stamping press, which converts flat steel blanks to shaped stampings. Tne blanks and stampings are the inputs and outputs, respectively. For a complex system, the diagrams form a hierarchy, with one box on a given level expanded to a full diagram with multiple boxes at the next level down. Developing the levels of diagrams is a continuing task done incrementally, rather than all at once. The diagrams are a way to record and communicate the structure and operation of a system. They allow numerical calculations, for example noting the time required for each step to find the total operation time, or summing staff required for each function to get total staff needed to operate the system. Functional diagrams can also be converted to mathematical simulations of system operations, typically with computer software made for that purpose. Any amount of description or other information may be attached to items in a diagram, by means of a unique function or flow reference number. By convention, expanded lower level diagrams use the same number as the parent box (i.e. 9.2), with another period followed by another number (9.2.1, 9.2.2, etc.). This is not required, but it makes tracing the connections between diagrams easier.