Guidelines for FAST Diagrams
Figure 10.1 depicts the diagramming conventions to be used in preparing a FAST diagram. The relative positions of functions as displayed on the diagram are also levels of activity. The FAST diagram is a horizontal graphical display based on system functions rather than system flowcharting or components. Level 1 functions, the higher-level functions, appear on the left side of the FAST diagram, with lower-level activity successively graphed to the right as shown. In most cases, when conducting a VE study, various levels of activity of verb-noun functions will be automatically suggested as the basic function of an item or a system.
The FAST diagram is just a tool. It is the process used in creating the diagram that is important, not the final diagram itself or its appearance. There is no such thing as a
“right” or perfect schoolbook solution that each diagrammer should be able to create, if he or she had perfect knowledge of the technique and theory. Yet if the diagram logic is logical to the diagrammer, it will normally be logical to a reviewer. And if it is not, then the FAST diagram will have served another purpose—communication of a misunderstanding in statement of the problem. That is also valuable to know. With these things in mind, consider the following guidelines in preparing a diagram:
1. Show the scope of the problem under study by two vertical dashed lines, one to the extreme left and one to the extreme right of the diagram. Everything that lies between the two scope lines is defined as the problem under study.
2. Every FAST diagram will have a “critical path of functions” going from left to right across the scope lines.
3. On that critical path should be found only required secondary functions, the basic function(s), and the higher-order function.
4. The higher-order function will lie to the immediate left of the left scope line.
5. The basic function(s) will always lie to the immediate right of the left scope line.
6. All other functions on the critical path will lie to the right of the basic function and will be the required secondary functions (not normally aesthetic or unwanted secondary functions).
7. Any “assumed” functions lie to the right of the right-hand scope line.
8. All other secondary functions the item performs will lie either above or below the critical path of functions. These functions can be required secondary functions, aesthetic functions, or unwanted functions.
9. If the function “happens at the same time as” and/or “is caused by” some function on the critical path, place the function below that critical path function.
10. If the function happens “all the time” the system is doing its work, place it above the critical path function to the extreme right of the diagram.
11. If there are specific design objectives or general specifications to keep in mind as the diagram is constructed, place them above the basic function and show them as dotted boxes.
12. All “one-time” actions are placed above the critical path and in the center area of the diagram.
13. All functions that lie on the critical path must take place to accomplish the basic function. All other functions on the FAST diagram are subordinate to the critical path function and may or may not have to take place to accomplish the basic functions.