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 job may be grouped into two classes or types - installation diagrams and schematic diagrams.

Installation Diagrams

Figure 2-19 is an example of an installation diagram. This is a diagram of the gust lock systems of an aircraft. It identifies each of the components in the systems and shows their location in the aircraft. Each letter (A, B, C, etc.) on the principal view refers to a detail view located elsewhere on the diagram. Each detail view is an enlarged drawing of a portion of a system. The numbers on the various views are referred to as call outs, and serve to identify each component.

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 Diagram

Schematic diagrams do not indicate the location of individual components in the aircraft, but do locate components with respect to each other within the system. Figure 2-20 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; however, it is 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.

In tracing the flow of fluid through the system, it can be seen that the engine driven pumps receive a supply of fluid from the reservoir. One way check valves are installed in both left and right pump pressure lines so that failure of one pump will not render the pressure from the other pump ineffective. Fluid flows to the relief side of the unloading and relief valve, and through the check valve, which will hold pressure built up beyond this point. Pressure is then directed through all lines leading to each selector valve, where it is checked if no units are being operated.

Pressure builds up in the line routed to the control port of the unloading valve and begins to charge the system accumulator. Pressure to charge the brake accumulator is routed through a check valve incorporated in the thermal relief valve; this prevents the pressure from returning to the general system.

Although the general system accumulator starts charging at the same time, it will not charge as fast, because the fluid passes through a restrictor valve. The general system pressure will bleed into the brake system whenever the brake pressure drops below system pressure.

As soon as the pressure reaches the relief valve setting, the valve will open slightly. General system pressure increases until it reaches the value established as the system operating pressure. At this point, through the line leading to the control part of the unloading valve, the pressure will force the unloading and relief valve completely open. The pressure trapped in the system by the one way check valve holds the valve open to create an idling circuit, which prevails until some unit of the hydraulic system is operated.

Schematic diagrams, like installation diagrams, are used extensively in aircraft manuals.