Functional flow block diagram

Functional flow block diagram

The Functional Flow Block Diagram (FFBD) is a multi-tier, time-sequenced, step-by-step flow diagram of the system’s functional flow. [The first version of this article is completely based on the [ NAS SYSTEM ENGINEERING MANUAL SECTION] 4.4 VERSION 3.1 06/06/06.]

The diagram is developed in the 1950s and widely used in classical systems engineering. The Functional Flow Block Diagram is also referred to as "Functional Flow Diagram", "functional block diagram", and "functional flow".


Functional Flow Block Diagrams (FFBD) usually define the detailed, step-by-step operational and support sequences for systems, but they are also used effectively to define processes in developing and producing systems. The software development processes also use FFBDs extensively. In the system context, the functional flow steps may include combinations of hardware, software, personnel, facilities, and/or procedures. In the FFBD method, the functions are organized and depicted by their logical order of execution. Each function is shown with respect to its logical relationship to the execution and completion of other functions. A node labeled with the function name depicts each function. Arrows from left to right show the order of execution of the functions. Logic symbols represent sequential or parallel execution of functions. FAA (2006). [ NAS SYSTEM ENGINEERING MANUAL SECTION] 4.4 VERSION 3.1 06/06/06.]

The purpose of the FFBD is to indicate the sequential relationship of all functions that must be accomplished by a system. FFBDs depict the time sequence of functional events. That is, each function (represented by a block) occurs following the preceding function. Some functions may be performed in parallel, or alternate paths may be taken. The duration of the function and the time between functions is not shown, and may vary from a fraction of a second to many weeks. The FFBDs are function oriented, not equipment oriented. In other words, they identify "what" must happen and do not assume a particular answer to "how" a function will be performed. NASA (1995). "Techniques of Functional Analysis". In: " [ NASA Systems Engineering Handbook] " June 1995. p.139-140.]

A key concept in modeling functional flow is that for a function to begin, the preceding function or functions within the “control” flow must have finished. For example, an “eat food” function logically would not begin until a “cook food” function was completed. The logical sequence of functions (i.e., the functional flow) describes the “control” environment of the functional model. In addition to a function being enabled, it may also need to be triggered with an input. So, in the example, the “eat food” function is enabled once the “cook food” function is completed, and once it receives the “prepared food” as input. This second aspect—triggering a function speaks to the “data” environment, which the N2 diagram captures. Most system functionality can be modeled using the standard symbols discussed below. If an extended set of symbols is required, then it should be defined in the resulting Functional Analysis Document (FAD) to ensure that all stakeholders are able to accurately interpret the diagrams.


The first structured method for documenting process flow, the flow process chart, was introduced by Frank Gilbreth to members of American Society of Mechanical Engineers (ASMR) in 1921 as the presentation “Process Charts—First Steps in Finding the One Best Way”. Ben B. Graham (2002). "Detail Process Charting". p.2.] Gilbreth's tools quickly found their way into industrial engineering curricula. In the early 1930s, an industrial engineer, Allan H. Mogensen began training business people in the use of some of the tools of industrial engineering at his Work Simplification Conferences in Lake Placid, New York. A 1944 graduate of Mogensen's class, Art Spinanger, took the tools back to Procter and Gamble where he developed their Deliberate Methods Change Program. Another 1944 graduate, Ben S. Graham, Director of Formcraft Engineering at Standard Register Corporation, adapted the flow process chart to information processing with his development of the multi-flow process chart to displays multiple documents and their relationships. In 1947, ASME adopted a symbol set as the ASME Standard for Operation and Flow Process Charts, derived from Gilbreth's original work.

The Functional Flow Block Diagram (FFBD) is developed by the TRW Incorporated, a defense-related business, in the 1950s. [Tim Weilkiens (2008). "Systems Engineering with SysML/UML: Modeling, Analysis, Design". Page 287.] In the 1960s it was exploited by the NASA to visualize the time sequence of events in a space systems and flight missions. [ Harold Chestnut (1967). "Systems Engineering Methods". Page 254.] It is is further widely used in classical systems engineering to show the order of execution of system functions. Thomas Dufresne & James Martin (2003). [ "Process Modeling for E-Business"] . INFS 770 Methods for Information Systems Engineering: Knowledge Management and E-Business. Spring 2003] The FFBD is one of the classic process modeling methodologies, with Flow charts, Data Flow Diagrams, Control Flow Diagrams, Gantt charts, PERT diagrams, and IDEF, which is probably the most common technique of traditional business process modeling. These represent just a fraction of the methodologies used over the last thirty years to document business processes. Modern methods here are Unified Modeling Language and the Business Process Modeling Notation.

Development of functional flow block diagrams

FFBDs can be developed in a series of levels. FFBDs show the same tasks identified through functional decomposition and display them in their logical, sequential relationship. For example, the entire flight mission of a spacecraft can be defined in a top level FFBD, as shown in Figure 2. Each block in the first level diagram can then be expanded to a series of functions, as shown in the second level diagram for "perform mission operations." Note that the diagram shows both input (transfer to operational orbit) and output (transfer to space transportation system orbit), thus initiating the interface identification and control process. Each block in the second level diagram can be progressively developed into a series of functions, as shown in the third level diagram on Figure 2.

These diagrams are used both to develop requirements and to identify profitable trade studies. For example, does thespacecraft antenna acquire the tracking and data relay satellite (TDRS) only when the payload data are to be transmitted, or does it track TDRS continually to allow for the reception of emergency commands or transmission of emergency data? The FFBD also incorporates alternate and contingency operations, which improve the probability of mission success. The flow diagram provides an understanding of total operation of the system, serves as a basis for development of operational and contingency procedures, and pinpoints areas where changes in operational procedures could simplify the overall system operation. In certain cases, alternate FFBDs may be used to represent various means of satisfying a particular function until data are acquired, which permits selection among the alternatives.

Building blocks

Key attributes

An overview of the key FFBD attributes:
* "Function block": Each function on an FFBD should be separate and be represented by single box (solid line). Each function needs to stand for definite, finite, discrete action to be accomplished by system elements.
* "Function numbering": Each level should have a consistent number scheme and provide information concerning function origin. These numbers establish identification and relationships that will carry through all Functional Analysis and Allocation activities and facilitate traceability from lower to top levels.
* "Functional reference": Each diagram should contain a reference to other functional diagrams by using a functional reference (box in brackets).
* "Flow connection": Lines connecting functions should only indicate function flow and not a lapse in time or intermediate activity.
* "Flow direction": Diagrams should be laid out so that the flow direction is generally from left to right. Arrows are often used to indicate functional flows.
* "Summing gates": A circle is used to denote a summing gate and is used when AND/OR is present. AND is used to indicate parallel functions and all conditions must be satisfied to proceed. OR is used to indicate that alternative paths can be satisfied to proceed.
* "GO and NO-GO paths": “G” and “bar G” are used to denote “go” and “no-go” conditions. These symbols are placed adjacent to lines leaving a particular function to indicate alternative paths.

Function symbology

A function shall be represented by a rectangle containing the title of the function (an action verb followed by a noun phrase) and its unique decimal delimited number. A horizontal line shall separate this number and the title, as shown in see Figure 3 above. The figure also depicts how to represent a reference function, which provides context within a specific FFBD. See Figure 9 for an example regarding use of a reference function.

See also

* Block diagram
* Business Process Mapping
* Flow chart
* Flow diagram
* Function model
* Function block diagram
* N2 Chart


Further reading

* DAU (2001) [ "Systems Engineering Fundamentals."] Defense Acquisition University Press.
* FAA (2007) [ System Engineering Manual] . Federal Aviation Administration Washington.

External links

* [ Functional Flow Block Diagram (FFBD)] , NASA PBMA.

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