The AV-3M system uses codes to process information. Items of information, such as a number of hours on aircraft or equipment and aircraft bureau numbers, are normally expressed in numerical terms and do not need to be converted into code. Also, most of the codes used in the aircraft inventory and reporting system (AIRS) are used in the AV-3M system without change.    In most other cases, data must be converted into code to be computer processed. The basic codes used on the various source documents are prescribed for Navy-wide use; therefore, they cannot be changed locally. Some codes are structured so that flexibility is provided, allowing expansion to meet local requirements. Some of the principal codes used by aircraft maintenance activities are described in the following paragraphs. NOTE: A list of MDS codes is contained in the appendix section of the latest edition of the Naval Aviation Maintenance Program, OPNAVINST 4790.2. Organization code. A structured three- character alphanumeric code that identifies an activity within a major command. Permanent unit code (PUC). A six-character numeric code assigned to each aircraft reporting custodian for identification. Work center code. A three-character alpha- numeric code used to identify work centers within an organization. Type equipment code (TEC). A four-character alphanumeric code that identifies the end item of equipment on which work is performed. There are 16 categories of equipment, identified by the first character of the TEC. For example, the first code character for aircraft is A; the first code character for jet engines is J; and the first code character for reciprocating engines is R. Julian date. A four-character numeric code used to express the date. The first character of the code is the last digit of the year, and the last three characters show the day of the year. For example, 1101 is the 10lst day of 1991, or 10 April 1991. When used on the VIDS/MAF as part of the job control number, the first position identifying the year is omitted. All dates used on source documents are expressed as Julian dates. Job control number (JCN). A 9- through 11-character alphanumeric code used to separately identify each maintenance action. This code is composed of four parts—an organization code, a Julian day, a serial number, and a suffix letter or number. The first two parts have been discussed. The serial number is either a three- character number that runs sequentially from 001 to 999 or a three-character alphanumeric number with an alphabetic first character, and the last two numbers running sequentially from 00 to 99. Three-character sequence numbers are used to identify routine day-to-day maintenance actions. An example of this type of JCN is AC4-030- 216. Three-character alphanumeric serial numbers are used only to document major inspections other than preflight, postflight, turnaround, daily, special, conditional, corrosion, and acceptance/ transfer inspections (discussed later in this chapter). An example of this type of JCN is AC4-030-A00. The JCN suffix is an alphanumeric code that is used by intermediate maintenance activities. It identifies a subassembly or sub-subassembly repair action completed separately from the major component repair action. This suffix is added to the basic JCN; for example, AC4-030-216-A or AC4-030-216-1A. — Work unit code (WUC). A one-, three-, five-, or seven-character numeric or alphanumeric code that normally identifies the system, subsystem, and component or part of the end item being worked on. The first two characters are standardized and identify the system or the end item. This block is also used to identify major inspections that are performed; for example, 030000A is used for a 7- and 14-day special inspection. NOTE: An end item is a final combination of products,   components, parts, etc., with a designated use. An example of an end item is an aircraft, a mobile electric power plant, or an uninstalled aircraft engine. Action taken code. A one-character alphabetic or numeric code that describes what maintenance was performed on an item identified by a WUC. Manufacturer’s code. A five-character numeric or alphanumeric code that identifies the manufacturer of a component, part, or end item. 4-5