Schematic Diagrams

Troubleshooting Flight Hydraulic Power System

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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Figure 3-9.—Installation diagram of a motor-driven hydraulic pump. There are two classes of diagrams you will be concerned with in gaining a complete knowledge of a specific system. These are the schematic and installation diagrams. A diagram, whether it is a schematic diagram or an installation diagram, maybe defined as a graphic representation of an assembly or system. Schematic Diagrams Figure 3-8 is another example of a schematic diagram. Diagrams of this type do not indicate the actual physical location of the individual components in the aircraft. They do locate components with respect to each other within the system. Various components are indicated by symbols in schematic diagrams, while drawings of the actual components are used in the installation (pictorial) diagrams. The symbols used in the schematic diagrams conform to the military standard mechanical symbols provided in MIL-STD-17B-1 and MIL-STD-17B-2. Most manufacturers improve upon these basic symbols by showing a cutaway portion on each component. These cutaways aid in clarifying the operation of that component. You should be able to trace the flow of fluid from component to component. On most diagrams of this type, an uncolored legend or different colors are used to represent the various lines. The legend identifies the lines in relation to their purpose and the mode of operation being represented. Each component is further identified by name, and its location within the system can be determined by noting which lines lead into and out of the component. Since many systems are electrically controlled, you should be capable of reading the electrical portion of a schematic diagram. Knowledge of the electrical symbols and the use of a multimeter in making voltage and continuity checks will contribute significantly to efficient troubleshooting. If a malfunction is caused by electrical problems, the assistance of AE personnel may be required. All electrical wiring in the aircraft is marked at specified intervals with a wire identification code. These identification codes are defined in the electrical volume(s) of the MIM, and they are useful in tracing wires throughout the aircraft. If an elusive malfunction is reasonably traced to or considered to be of an electrical nature, the electrical circuit should be checked by a qualified AE. Many wires can give a good continuity reading under a no-load or low-current condition and still be malfunctioning when under a load condition. NOTE: Electrical schematics are especially useful in determining annunciator panel malfunctions. Installation Diagrams Figure 3-9 is an example of an installation diagram. This is a diagram of the motor-driven hydraulic pump 3-13

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