A diagram may be defined as a graphic representation of an assembly or system, indicating the various parts and expressing the methods or principles of operation.
There are many types of diagrams; however, those with which the aviation mechanic will be concerned during the performance of his or her job may be grouped into four classes or types: (1) installation, (2) schematic, (3) block, and (4) wiring diagrams.
Figure 2-18 is an example of an installation diagram. This is a diagram of the installation of the flight guidance control components of an aircraft. It identifies each of the components in the systems and shows their location in the aircraft. Each number (1, 2, 3, and 4) on the detail shows the location of the individual flight guidance system components within the cockpit of the aircraft. Installation diagrams are used extensively in aircraft maintenance and repair manuals, and are invaluable in identifying and locating components and understanding the operation of various systems.
Schematic diagrams do not indicate the location of individual components in the aircraft, but locate components with respect to each other within the system. Figure 2-19 illustrates a schematic diagram of an aircraft hydraulic system. The hydraulic pressure gauge is not necessarily located above the landing gear selector valve in the aircraft. It is, however, connected to the pressure line that leads to the selector valve.
Schematic diagrams of this type are used mainly in troubleshooting. Note that each line is coded for ease of reading and tracing the flow. Each component is identified by name, and its location within the system can be ascertained by noting the lines that lead into and out of the unit.
Schematic diagrams and installation diagrams are used extensively in aircraft manuals.
Block diagrams [Figure 2-20] are used to show a simplified relationship of a more complex system of components. Individual components are drawn as a rectangle (block) with lines connecting it to other components (blocks) that it interfaces with during operation.
Wiring diagrams [Figure 2-21] show the electrical wiring and circuitry, coded for identification, of all the electrical appliances and devices used on aircraft. These diagrams, even for relatively simple circuits, can be quite complicated. For technicians involved with electrical repairs and installations, a thorough knowledge of wiring diagrams and electrical schematics is essential.