Module 6 (chapter 5 pg 192-207)

Simply put, system availability is

is the portion of time in which a system is in its operational or functional state under the specified environmental conditions.

The goal of maintenance is to

perform the tasks in the least amount of time and with the least amount of cost.

Maintainability is a design-derived decision, a result of design. As defined in MIL-HDBK-470A, system maintainability is

"the relative ease and economy of time and resources with which an item can be retained in, or restored to, a specified condition when maintenance is performed by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair."

Inventory management answers two fundamental questions:

(1) When should an order be placed? (2) How many units should each order have? The factors that regulate these two questions are the various costs that are involved in inventory management.

To prolong system operational time, the system needs to be reliable, and, more importantly,

, a well-defined maintenance policy to complement the reliability design is necessary to make sure the system and its components are well maintained while preventing failures from occurring in the most effective and efficient manner. Maintenance requires logistical support, including facilities, personnel, tools, equipment, and spare parts. These support functions are performed on a continuous basis, throughout the system life cycle and beyond. To ensure an effective maintenance performance, the necessary support infrastructure needs to be in place and operate efficiently. In the current social and economic environment, it is essential to consider the support functions within the context of the global supply chain, as this has become an integral component for the operations of all businesses and organizations.

Within the supply chain organizations, management and operations have different responsibilities for ensuring an effective and efficient supply chain. From the management perspective at higher levels, the strategic responsibilities include

1. Aligning the supply chain strategies with the system mission and overall organizational strategic planning. Any decisions on outsourcing and procurement should be based on the overall system mission requirements. 2. Supply chain structure configuration: determining the number and locations of the suppliers, warehouses, distribution centers, and support facilities 3. Information technology determination: selecting the application of information technology to manage supply chain information, including data collection, information processing and sharing, inventory status checking, and event tracking. Technology and methods such as ERP may be applied to integrate internal and external operations across the entire organization, facilitating the flow of information between different support functions. 4. Capacity planning of the supply chain: based on the system requirements, determining the long-term and short-term capacity needs, and the level of flexibility required to accommodate the risks involved. 5. Risk reduction and management: identifying the potential sources of risks and making appropriate decisions so that these risks can be measured and controlled to a minimum level.

The commonly used measures for supply chain factors within supportability are

1. Capacity. To what extent can the support functions can be accomplished, and what is the capability and ability of handling large volumes and uncertain transactions? Together with reliability and flexibility, long-term capacity is planned to cope with the large degree of uncertainty involved in the supply chain, as many unexpected events may occur within the process. With a well-defined supply chain capacity, an organization can fulfill a need or service with high probability. The capacity of the supply chain measures its capability of dealing with uncertain demands and fluctuation of transactions; it refers to the combined effects of the demand pattern and cost functions that are involved in the supply chain. 2. Reliability and availability. The availability of the supply chain measures the inherent capability of being readily available for smooth transactions whenever required. A more realistic measure for this capability is to use operational availability (Ao) to measure overall organizational efficiency. 3. Quality of the supply chain. The quality measure of the supply chain consists of the following subcategories: a. Response time: since all the activities and transactions take time within the supply chain, with many administrative/transportation delays and the handling of lead time, it is desired that the response time should be as short as possible. The response time is a composite function of many other related factors, including purchasing and order fulfillment, inventory management, and supplier flexibility/cooperation. The response time is one of the primary measures of supply chain management (SCM) effectiveness. b. Total processing time/cycle time: The total time to fulfill a need, from the identification of the need for a product until it is delivered in the required quantities and quality to the customer and service is completed, should be as short as possible. This has a much wider scope than the response time for a certain event. Modern technologies, such as e-business, electronic bar codes,and radio frequency identification (RFID) are often applied to fast tracking order statuses and sharing information quickly among partners c. Total cost. This includes all the cost factors of processing transactions from the sources of the materials to the end users, including product/service cost, transportation cost, inventory cost, and costs involved in risks, such as returns and defects. Total cost should be minimized, and is often traded off with other supply chain measures, such as response time and quality.

Basic assumptions for the deterministic EOQ model include

1. Constant demand rate: to simplify the problem, it is assumed that demand is deterministic and occurs at a constant rate, denoted as D units/time period. 2. Shortage cost is ignored. 3. Although a lead time may be involved, as seen in Figure 5.16, the lead time does not really have an impact on the overall total cost for the simple EOQ model. For simplicity of calculation, we assume that the order arrives immediately. So, we have identical cycles (the large triangle) for the EOQ model. We just need to look at each cycle to derive the total cost per period.

One thing to keep in mind is that, as with other design parameters and TPMs, it is difficult to design a hard and fast maintainability plan, due to the dynamic nature of system design process. With changing requirements, design for maintainability should also be flexible and evolve continuously. Such design is an iterative process, evolving with the test and evaluation processes. It primarily includes five major activities:

1. Derive requirements from the systems requirements. Maintainability is a DDP; it is design derived, based on the mission requirements for the systems availability and reliability profile. Analytical modeling and experiments are necessary to aid in the translation process. 2. Define resources and constraints for system maintainability. These resources and constraints cover the whole spectrum of system availability, including support facilities, tools and equipment, personnel skills levels and training requirements, and management style/policies. These factors will all play a role in determining the maintainability policy for the system. 3. Define the maintenance level. Maintenance levels need to be specified for the system after the system functional structure is designed and physical models are configured. These levels include the nature of maintenance tasks and detailed information for each of them, both corrective and preventive (i.e., who, where, when, and how these tasks are performed). Sometimes, analysis models such as task analysis can be applied to aid in deriving this information. 4. Maintenance function identification and allocation. As stated earlier, the functional structure of maintenance needs to be identified. This structure is based on the system functional architecture, with consideration of no-go functions, further expanding them into system maintenance functions. These maintenance functions are allocated in a top-down process to lower levels; trade-off studies and decisionmaking models are sometimes necessary to balance the requirements concerned with different aspects, such as system life cycle cost, reliability, usability, and supportability. 5. Establish the maintenance program management plan. As part of the system engineering management plan (SEMP), the factors described above are organized into one management document to guide system maintenance activities throughout the system life cycle. The major sections of the maintenance program plan should include a. Maintenance requirements and TPM objectives b. System maintenance functional structure and relationships with other system functions c. Maintenance organization and personnel structure and their requirements d. Logistics, supply, facility, and tools and equipment support for maintainability e. Job training and documentation requirements for maintenance personnel f. Test, evaluation, and demonstration methods, and models/data related to system maintainability

In terms of maintenance personnel and key human factors issues, the general design should consider the following factors:

1. Human performance consideration 2. Human machine interface/usability 3. Maintainer skills and training programs 4. Environmental conditions (noise, vibration, humidity, temperature, etc.) 5. Design simplicity 6. Safety These may be found in any human factors text, and need to be tailored to each individual system design.

So based on different perspectives , , there are three different measures for availability which include

1. Inherent availability (Ai ). This is "the probability that a system, when used under stated conditions or design specified ideal environment, will operate satisfactorily at any point in time, as required." Ai excludes preventive maintenance, logistics, and administrative delays; it is only concerned with random failure-induced maintenance actions. It primarily reflects the quality of the system; the higher the reliability (larger MTBF), the shorter time required to fix failures (smaller Mct), the higher inherent availability. Ai can be expressed as in Equation 5.34: 2. Achieved availability (Aa). This is the probability that a system will operate or function in a satisfactory manner in the ideal supporting environment. Compared to Ai , achieved availability considers both corrective and preventive maintenance activities; it is a more practical measure than Ai , since preventive maintenance activities will help to avoid failures from occurring. Aa can be expressed as in Equation 5.35: 3. Operational availability (Ao). This is the probability that the system will operate in a satisfactory manner in the actual operational environment. The actual delays within the system consist of both technical aspects (corrective and preventive maintenance) and nontechnical factors (logistical and administrative delays). Operational availability gives the most realistic and practical measure for system availability, as it considers all the aspects of the system delay factors and reflects the efficiency of the maintenance at the organizational level. Ao can be expressed as in Equation 5.37:

Supportability is an inherent system characteristic; it is a derived DDP, developed for a specific system configuration. The basic elements for system supportability include:

1. Maintenance support requirements. One of the important elements of system supportability is to define a clear set of requirements based on the requirements for reliability, maintainability, and availability. Requirements should define the goals for support functions, infrastructure, activities, organizations, and hardware/software that are involved in system support activities at the system level, iteratively refined and decomposed to lower levels. These requirements are defined starting from the conceptual design stage. 2. Support personnel. This category addresses the personnel required to perform support functions, including system users, maintainers, and logistical/supply chain management personnel. Support functions cover a wide range of activities throughout the system life cycle, from initial system installation and system-sustaining support all the way to system retirement. 3. Training and training support. System support should address training for system operators and maintainers throughout the system life cycle, and support for implementing the training, including documentation, training materials, and the necessary training resources. This training covers the initial training program for new personnel, daily on-the-job training, and training program assessment (i.e., feedback, data analysis, and improvement). 4. Inventory and supply support. As an integral part of system logistics and supply chain management, inventory control and management plays an important role; the quantities and qualities of the inventory items have a significant impact on the overall supply chain operation. Common inventory items include spare parts, repair parts, consumables, special supplies, and supporting supplies. 5. Support tools and equipment. This category of support elements includes the tools and equipment that are necessary for carrying out support functions. Tools include those that are required for performing maintenance activities; testing, measuring, and diagnosing the system; and calibration equipment. These tools should be maintained and kept in an operational state whenever they are needed. They also include any designated computer hardware and software that will perform the support functions. 6. Packaging, handling, storage, and transportation. A large part of system support functions involves the flow of material from one location to another. From the procurement of the parts to their final destination, support items need to be properly packaged, handled, stored, and transported. This category of system support addresses supplier relationships, the global supply chain infrastructure, and resources for an effective and efficient material flow. 7. Facilities. This category includes all the facilities that are necessary to support all the scheduled and unscheduled maintenance activities at different levels: user site level, depot level, and central headquarters level. These facilities may include the buildings, laboratories, vehicles, and any other fixed or mobile units that will house the support functions. 8. Data, documentation, and analysis. Data may include all the technical information concerning the system configuration and procedures that involve system installation, operation and maintenance instruction, procurement, and modifications. This data may be in quantitative TPM format, or in graphic format, as in blueprints or schematic drawings, or in electronic format in information systems, such as CAD/CATIA data, databases (e.g., the enterprise resource planning [ERP] database in supply chain management). This data is collected on an ongoing basis throughout the system life cycle and should be well maintained for documentation and analysis to improve the support functions.

Generally speaking, system maintainability can be broken down into two categories, preventive maintenance and corrective maintenance. What are they

1. Preventive maintenance: also called proactive maintenance or scheduled maintenance, this refers to systematic methods of maintenance activities to prolong system life and to retain the system at a better level of performance. These activities include tests, detection, measurements, and periodic component replacements. Preventive maintenance is usually scheduled to be performed in a fixed time interval; its purpose is to avoid or prevent faults from occurring. Preventive maintenance is usually measured in preventive time, or Mpt. 2. Corrective maintenance: also called reactive maintenance or unscheduled maintenance. Corrective maintenance is performed when system failure occurs. Corrective maintenance tasks generally include detecting, testing, isolating, and rectifying system failures to restore the system to its operational conditions. Typical actions of corrective maintenance include initial detection, localization, fault isolation, disassembly of system components, replacing faulty parts, reassembly, adjustment, and verification that system performance has been restored. Corrective maintenance is usually measured in corrective time, or Mct

It has been found that most of the repair times fall into one of the three following distributions (Blanchard and Fabrycky 2006)

1. The normal (or Gaussian) distribution: The normal distribution is most commonly used in systems with relatively straightforward and simple maintenance actions; for example, where system repairs only involve simple removal and replacement actions, and these actions are usually standard and with little variation. Repair times following the normal distribution can be found for most maintenance tasks. Another reason for its popularity is perhaps due to the famous central limit theorem, which states that the mean of a sufficiently large number of independent random variables (or asymptotic independent samples), each with a finite mean and variance, will be approximately normally distributed. This is the reason that the normal distribution is used for such conditions if the true distribution is unknown to us. 2. The exponential distribution: This type of distribution most likely applies to those maintenance activities involving faults with a constant failure rate. As the failure occurs independently, the constant failure rate will result in a Poisson process (see Appendix I for details) so that the general principles of queuing theory may apply The lognormal distribution: This is a continuous probability distribution of a random variable whose logarithm is normally distributed. Lognormal distribution has been used commonly in maintenance tasks for large, complex system structures, whose maintenance usually involves performing tasks and activities at different levels, and usually involves a nonstationary failure rate and time duration.

Many standards and published guidelines provide some general recommendations for the selection of components and personnel for system design.General guidelines for components selection include:

1. Use standardized components and materials. These are easier to find, quicker to replace, and, most importantly, due to standardized production and the existence of large suppliers for these components, are most likely less expensive to procure. 2. Limit the need for special tools and equipment. This is based on similar reasons to (1), to minimize the time and cost involved for maintenance tasks. 3. Design for the consideration of ease of maintenance. This includes modular parts to minimize the impact to other components, separate control adjustability, the use of self-diagnosis and self-detection to rapidly identify failures, provisions to preclude errors in the installation phase, the provision of easy accessibility to avoid obstruction of the items to be serviced, ensuring access to spaces for test equipment and tools, making the most frequently serviced components the most accessible, avoidance of short-life components, especially for critical system items, and using proper labeling and identification for effective failure identification

Nowadays, one cannot talk about supportability without talking about the supply chain, as every business organization is part of at least one supply chain, and it is not uncommon to see that many organizations are part of multiple supply chains. What is a supply chain

A supply chain is a sequence of organizations, people, information, resources, and activities that are involved in producing and/or delivering a product or service. A product in the supply chain starts with the raw materials; through a sequence of processes in various facilities (e.g., warehouses, factories, distribution centers, retail stores, and offices), it evolves to its final form and is delivered to its users. A typical supply chain is illustrated in Figure 5.15. make sure to get figure 5.15.

System availability is highly related to system reliability. however

However, availability is not just reliability; as seen in the previous sections, it includes factors that are not covered by system reliability. Reliability only addresses system failures caused by breakdowns; failures occur randomly and maintenance activities are primarily corrective. Availability may also be increased by making strategic plans of preventive maintenance activities, by regularly testing and replacing parts before they fail to prolong the time between failures occurring.

Who provides good sources for design guidelines

Many standards, such as MIL-STD-1472D (Human Engineering Design Criteria for Military Systems and Facilities), MIL-STD-470B (Maintainability Program Requirements for Systems and Equipment), MIL-STD-471A (Maintainability Verification/Demonstration/Evaluation), MIL-HDBK-472 (Maintainability Prediction), and DOD-HDBK-791 (Maintainability Design Techniques) (U.S. Department of Defense 1966, 1973, 1988, 1989a, 1989b), provide good sources for design guidelines for maintenance issues. Although primarily focused on military systems, most of the standards are very general and universally valid for most other types of systems.

To make clear assumptions for the EOQ model, the following costs are considered.

Ordering cost and setup: For most orders, there is a fixed cost factor involved, regardless of the size of the order; for example, the cost of labor to set up the order (cost of communication, paper, billing process, etc.) and, sometimes, a flat-rate transportation cost. The order and setup cost is assumed to be fixed for each order placed and denoted as K. Unit purchasing cost :This is simply the variable cost (or price) for each unit of the product purchased. This cost sometimes includes the shipping cost if that cost depends on the quantity ordered. The unit purchasing cost is denoted as p. Unit holding cost:This is the cost of holding one unit of inventory for one time period. The holding cost usually includes the storage cost, insurance cost, taxes on the inventory, and costs due to unexpected losses such as theft, spoilage, and damage. The holding cost is denoted as h/unit/time period.

How is system reliability and maintainability different

Reliability is an inherent system characteristic; it deals with the internal quality of the system itself, the better design of the system, and better system reliability. Maintainability, on the other hand, is derived based on the system reliability characteristics; it is a design-dependent parameter that is developed to achieve the highest level of system availability. Although maintainability is inherent to a specific system, one usually cannot specify maintainability until the system requirements on reliability and availability are determined.

what is logistic delay and administrative delay time

Sometimes, delays in fixing the system are caused by nontechnical factors. For example, when a system breaks down, we find out that the replacement part is not in stock; we need to order it and it takes some lead time to arrive. This type of delay is called logistic delay, and time taken due to logistic delay is logistic delay time (LDT). Besides LDT, there are also periods of administrative delay time (ADT). ADT is referred to as the time delay for administrative reasons, such as supervisor approval, board review, organizational structure flow, and so forth. Neither LDT nor ADT are technical factors for maintenance but they both produce similar effects on maintenance efficiency, preventing the system from being restored on time, and they inevitably happen, as logistics and administration are two key components of system operations. Considering LDT and ADT gives us a more realistic picture of system maintenance requirements; thus, a more realistic measure of the maintenance time is mean down time (MDT), given by Equation 5.33:

Here is the summary of all the symbols used in the deterministic EOQ model:

TC: total cost per time period Q: quantities ordered each time (this is the variable we are trying to determine) D: demand rate (number of units consumed per time period) T: number of periods in each ordering cycle K: ordering and setup cost per order p: unit cost (price) h: unit holding cost (per period)

As an inherent DDP, the effectiveness and efficiency of maintainability is primarily measured using

Time and cost factors

For most system designs, availability is a more realistic measure for the overall efficiency, considering system reliability and maintainability together. As mentioned earlier, reliability is a measure of dealing with random failures; it depends on the quality of the design, and once the design is finalized, reliability cannot be directly controlled. System maintainability, on the other hand, offers full control for the system designers to improve the degree of availability by providing well-planned maintenance strategies. These strategies are determined with the system reliability characteristics in mind, as there is a trade-off relationship between reliability and maintainability. What does this look like

To achieve a higher availability, a system with better reliability may require less frequent maintenance actions—both preventive and corrective—and vice versa. Understanding the trade-off relationships between reliability and maintainability will help us to create a more efficient system maintainability plan, both in terms of cost and time.

With both scheduled (or corrective) maintenance time and unscheduled (preventive) time being defined, we can obtain the mean time required for a piece of maintenance, either scheduled or unscheduled, as both activities cause system unavailability; this is called

With both scheduled (or corrective) maintenance time and unscheduled (preventive) time being defined, we can obtain the mean time required for a piece of maintenance, either scheduled or unscheduled, as both activities cause system unavailability; this is called assuming that all required tools and parts are available when a maintenance action is required.

The ultimate goal of system operation is to make the system operational as far as possible;

a more realistic measure for this operational capability is system availability. This is because if a system is not available, whether due to failure or routine maintenance, the consequence is similar in the sense that if the system is not operational, it is not generating profits or providing the functions that it is supposed to. So, system reliability and maintainability are two separate but highly related factors concerning the same objective, which is to increase the degree to which the system is available.

why is supply chain sometimes referred to as value chain

a supply chain is also referred as a value chain; as the material progresses through the chain, value is added to the materials. Increasing the value-added activity efficiencies and minimizing the non-value-added activities are the key concepts of supply chain management (SCM). This is the process of planning, implementing, and controlling the operations for more efficient supply chain operations.

MBTM is the mean time between maintenance; what is it

it is the measure of maintenance time considering both corrective and preventive maintenance activities. MBTM is given by Equation 5.36:

Compared to corrective maintenance, preventive time has relatively less variability, as it is usually scheduled at fixed time intervals and the activities involved are very specific and standard. In other words,

preventive maintenance activities occur in certain frequencies, or fpt, that is to say, the number of preventive maintenance actions per time period. So, the mean preventive time Mpt is a function of the frequency, as shown in Equation 5.31.where Mpti is the individual preventive maintenance time for the ith element of the system.

As one of the key design considerations and design-dependent measures, maintainability should be considered in the early planning of the design phase, starting where

starting from the conceptual design stage.

What does system maintainability measure

the ability with which a system is maintained to prevent a failure from occurring in the future and restore the system when a failure does occur.

For corrective maintenance, the primary time measurement is the mean corrective time Mct. Nevertheless, due to the random nature of system failures, the time taken to fix them, Mct, is also a random variable. As a random variable

the distribution function to interpret Mct varies from system to system. Just like any other random variable, the common measures for Mct include the probability distribution function (p.d.f.), cumulative distribution function (c.d.f.), mean, variance, and percentile value. Practically, one can approximate these parameters by observing the Mct sample and analyzing the sample data, assuming each of the observations is individually independently distributed (IID).

System supportability refers to the

the ease and economy of design, installation, and implementation of the support infrastructure that enable effective and efficient system maintenance and support of the system throughout its life cycle. The goal of system supportability is to develop a cohesive support infrastructure that is highly responsive to demand from system maintenance activities, and that is efficient in terms of time and cost with minimum impact to other system functions.

The key to SCM is

to support system maintenance functions by having the highest quality of parts in the shortest period time with minimum cost involved. It is a trade-off between the various cost factors (i.e., holding cost, shortage cost, procurement cost, etc.) and the demand rate. Here, we use a simple economic order quantity (EOQ) model to illustrate how to determine the proper quantity to minimize the total cost of the transaction.

An effective supply chain design to support system supportability relies on the integration of all factors within the supply chain; these factors include

trust among partners, effective communication, fast information flow, visibility and transparency of the supply chain, management capability of handling uncertain events, and appropriate performance measure metrics

Murphy's Law states

""anything that can go wrong will go wrong." Having a high level of reliability and fixing failures quickly when they occur are really the "two blades of the sword"; we need both to improve the level of system availability.