Very little maintenance is required on most pressurization and ACSs other than making the required periodic inspections and operational checks. Many of the components are repairable at the depot level of maintenance rather than at lower levels of maintenance because of the high cost of special equipment required for making adjustments necessary to proper operation. In most instances, a maladjusted or mal-functioning component must simply be removed and replaced. There are, however, certain components that require periodic servicing, cleaning, and inspection so the component will function properly and efficiently and may be considered reliable for flight. Specific require-ments for servicing, cleaning and inspection are listed in the daily, postflight, and special/ conditional, Maintenance Requirements Card (MRC) decks as well as the MIM for each aircraft.

Electrical Failures

Since all pressurization and ACSs have electrically controlled components, maintenance of these systems must include the related electrical circuits. Although an AE is generally called upon to locate and correct electrical troubles, the AME should be able to check circuits for loose connections, and even perform continuity checks when necessary. A knowledge of electrical symbols and the ability to read circuit diagrams is therefore necessary. Figure 3-22 illustrates the

electrical symbols commonly found in schematic diagrams. Loose connections are located by checking all connectors in the circuit. A connector that can be turned by hand is loose and should be tightened handtight. A continuity check is simply a matter of determining whether or not the circuit to the valve, or other electrically controlled unit, is complete. The check for continuity may be made with a test lamp, which can be drawn from supply. To perform a continuity check, the connector at the electrically controlled unit is first dis-connected. Then, with all necessary switches and circuit breakers closed, the test lamp is connected into the circuit at the electrical connector. The lamp thus indicates whether or not the circuit is complete. Continuity checks may also be made with the use of a multimeter. A multimeter is an instrument used for measuring resistance, voltage, or amperage.

Troubleshooting

Troubleshooting is the process of locating a malfunctioning component or other unit in a

Figure 3-22.- Electrical symbols.

system or mechanism. For the AME, troubleshooting is an important responsibility and one to which he/ she will devote a lot of squadron time. When a malfunction is reported concerning any of the components or systems that are main-tained by the AME, he/ she must be able to locate the trouble and correct the difficulty. To troubleshoot intelligently, the AME must be familiar with the system( s) at hand. He/ she must know the function of each component in the system and have a mental picture of the location of each component in the system in relation to other components, as well as the location of the component in the aircraft. This can be best achieved by studying the installation and schematic diagrams of the systems found in the applicable MIM. Troubleshooting procedures are similar in practically all applications. The procedures covered in this section are adaptable to almost all aircraft systems. Auto mechanics use these steps to find and repair automobile malfunctions. The AME can use these procedures to find and repair malfunctions within all the aircraft system for which he/ she is responsible. Basically, there are seven distinct steps to follow during troubleshooting.

These steps are as follows:

1. Conduct a visual inspection. This inspec-tion should be thorough and searching- checking all lines, linkages, and components for obvious damage, evidence of leakage, looseness, security, material condition, and proper installation; and servicing when applicable.

2. Conduct an operational check. The malfunctioning system or subsystem is checked for proper operation. This may be accomplished by using special support equipment such as the environmental control test set or by using aircraft power and equipment with the engines running. Each aircraft maintenance manual provides the steps to be taken in performing the operational checkout of all the aircraft's systems. The operational checks and troubleshooting charts for each system are numbered so that when a malfunction occurs during a step in the operational checkout, the malfunction can be located under the same step number in the troubleshooting chart. The troubleshooting chart will provide a list of the most probable causes of the malfunction in the order of probability, along with a recommended remedy. In any case, the AME must check the system out thoroughly, observing proper operation, sequence of events, etc.

3. Classify the trouble. Malfunctions usually fall into three basic categories- electrical, mechanical, and/ or improper installation. Using the information acquired in steps 1 and 2, the AME determines under which category the malfunction occurs. Proper use of the test set or a multimeter will identify whether the trouble is electrical or mechanical. Use of the MIM when performing all maintenance tasks should prevent improper installation. Something affecting the flow of gas or liquid (as could be the case in the vapor cycle ACS) could be categorized as a combination electrical/ mechanical failure. Most mechanical failures should be found on the visual inspection; however, drive shaft failure on some of the air-conditioning valves is not readily apparent until the valve is operated. In some cases it may even be necessary to disconnect the valve from the ducting so that the butterfly valve can be observed through the end opening. The position indicator on some valves can indicate that the valve is changing positions, which would be a false indication if the shaft was broken after the indicating mechanism, or if the butterfly valve was damaged in such a manner that the shaft would rotate without actually repositioning the valve.

4. Isolate the trouble. This step calls for sound reasoning and a full and complete knowledge of how each component and the system should operate. During this step, the AME can make full use of his knowledge and the system schematics to trace system operation and systematically eliminate components. He can then arrive at a reasonable conclusion concerning the cause of the malfunction based on facts and deductive reason-ing. Usually the trouble can be pinned down to one or two areas. By checking each individual area or component, the trouble can be isolated.

5. Locate the trouble. This step is used to eliminate unnecessary parts removal, thus saving time, money, and man-hours. Once the AME has isolated the trouble to a certain area or component, a closer observation of the valve or component in operation should provide some obvious indication that it is not operating as specified in the MIM. If all evidence indicates that the problem is electrical, the assistance of an AE should be requested.

6. Correct the trouble. This step is performed only after the trouble has been definitely pin-pointed and there is no doubt that the AME's diagnosis is correct. Removal, replacement, or repair of the unit or system is done using the instructions provided in the applicable aircraft MIM. NOTE: While performing maintenance on any system, ensure that the step-by-step procedures outlined in the MIM including CAUTIONS, WARNINGS, and SAFETY notes concerning the specific procedures are strictly complied with.

7. Conduct a final operational check. The affected component or system must be given an operational check following installation or repair to verify proper system or component operation. The MIM will provide the procedures for conducting the operational check. It will usually require operation of the system in various modes (manual and automatic for air-conditioning and pressurization systems) or through several cycles, as applicable. Specified steps throughout the repair procedure and operational check must be observed and certified by a quality assurance representative or a collateral duty quality assurance representative from the work center performing the work. These steps are usually identified in the MIM by underlining, italics, or some other obvious method.