Maintenance strategy is defined as a framework outlining procedure on when, how and where maintenance tasks take place (Blanchard, et al., 1991). A brief overview of the framework with respect to its application to repair practice was presented by ISO/IEC (2004). The framework depicts the relation between the repair echelons, the system indentures and the levels of repair which is considered as the most relevant to any system components. Hence, this framework is viewed as a general methodology of the maintenance and its support resource provision based on the policies of manufacturers and system users (ISO/IEC, 2004).
A repair echelon is a location where a predefined group of maintenance tasks are carried out on specific components. Blanchard (1998) claimed that the structure of the configuration of the repair locations is subject to the system-of-interest, its mission characteristics, its operation zone, the interrelationship with other repair echelon, and cost effectiveness of maintenance activities. However, the indenture levels are the system decomposition from the maintenance action point of view. For instance, indenture levels are: system, subsystem, equipment and component (Blanchard, 1998).
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Fig. (3.3): Preventive and corrective maintenance strategy (ISO/IEC, 2004)
Figure 3.3 describes the optimal development of maintenance concepts for preventive and corrective tasks by considering both repair structure and system breakdown structure. Spare parts are in general provided through a multi-echelon structure, which is a top-down structure consisting of many layers of repair shops as shown in Figures 3.1 & 3.2 (Rustenburg, et al., 2001; de Smidt-Destombes, et al., 2005 and Sleptchenko, et al., 2002). The main objectives of these facilities are twofold: (1) stoking and supplying spare parts and (2) repairing failed components. Shops at the down layers directly provide spares to installed systems, while those at a top layer provide spares to its subsequent layers. Besides, repair shops at a top layer are typically equipped with the most sophisticated repairing capacities. That is, a part that cannot be repaired by a particular facility would be sent upward to the central maintenance facility.
The dynamic and reactivity of repair tasks linked to maintenance of complex systems stresses on the importance of appropriate LORA to maintenance policies (Barros, 2001; Basten, 2009 and Gutin, 2005). As stated above, the main objective of LORA is to ensure a prompt spare and repair provision to failed systems by optimising both system performances and maintenance costs, i.e. making the right spare and repair available at the right time and at the right place (United Kingdom Ministry of Defence, 1996). Analysing
Maintenance strategy
System breakdown structure
System
Subsystem Subsystem Subsystem
equipment equipment equipment
component component component
Repair breakdown structure
Manufacturer’s maintenance shop Intermediate repair shop Intermediate repair shop Intermediate repair shop System location System location System location Repair resources: manpower, equipment, databases, documensts … Maintenance concept Indenture levels
Central repair shop
Repair echelons
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level of repair is: (1) a maintenance content management, which addresses “what and why to provide”; (2) a maintenance period management, which deals with “when to provide”; (3) a maintenance approach management, which refers to “how to provide” and (4) a maintenance location management, which addresses “where to deliver”. In addition, maintenance engineers classify all elements of the system structure into two categories: replaceable and discard items (Wu et al., 2008). The replaceable units or components stand for items that can be repaired; which in turn entail two types of units: line replacement units (LRU) and shop replacement unit (SRU). An LRU is a failed item that can be removed from the installed system individually, replaced by a new item, and shipped out to repair echelons for repairing. Inversely, an SRU is a failed item that cannot be removed from the installed system individually alike an LRU. In this case, its LRU parent is removed from the system, replaced by a new LRU and sent to repair shops where it can be disbanded, and the failed SRU can be removed for repairing, and replaced by a new one (Jones, 2006 and Blanchard, 1991).
Table (3.2): Repair configuration in a Multi-Echelon Logistics Support (Petch, 2009)
System location or Organisational
Level
Failures originate at the organisational level and are isolated to a line replaceable unit (LRU).
The faulty item is removed from the system and replaced with a spare one.
The system is checked for proper operation. The faulty item is sent to the nearest repair shop.
Intermediate Level
The LRU is repaired by isolating the faulty shop replaceable unit (SRU).
The faulty SRU is removed and replaced with a spare one. The repaired LRU is checked for proper operation.
Once the LRU is repaired, it is sent to the organisational level or to an inventory control or storage point.
If no fault is found, the LRU is also sent to the inventory control or storage point.
Occasionally, the LRU cannot be repaired by the intermediate level and it is sent to the depot for repair.
Depot
The SRUs (and sometimes LRUs) are repaired. The faulty component is removed and replaced. The SRU (or LRU) is checked for proper operation.
Once the repair is complete, the repair unit is sent back to the intermediate or depot level inventory control or storage point.
A discard units or components stand for items that are non-repairable, which also entail two types: a discard unit DU and a discard part DP. In a similar way, DU is an item that
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can be removed individually from the installed systems, whereas DP cannot but its parent LRU can be removed individually. A failed DU is directly removed from the system, replaced, and then discarded. However a failed DP involves taking its LRU out of the system and sending it out to repair shop where the defeated DP is removed and discarded (Rustenburg, et al., 2001; de Smidt-Destombes, et al., 2005 and Sleptchenko, et al., 2002). The table 3.2 enumerates the mean repair tasks through the repair network.
In summary, the preventive and corrective maintenance strategy is intended to reduce the time of system malfunctioning by identifying the most effective decisions, which are related to repair, replacement and discard tasks. Since repair facilities and spare parts are quite expensive for complex systems, much of maintenance effectiveness requirement is the trade-off between the stocking policies and repair investment (Kleynera, et al., 2008). Consequently, the LORA plays an important role in addressing these maintenance questions. Its main objective is to emphasize on the optimal provision of spare and repair services with respect to maintenance needs. Another objective is to emphasize the need to include the aspects of the whole supply chain in the analysis and to increase the collaboration between the parties at planning stages.