# Classification of Systems and Maintenance Models  
[PERSON]  
1 Department of Electronics, Robotics, Monitoring and IoT Technologies, National Aviation University, 03058 Kyiv, Ukraine 1,1 Department of Electronics, Robotics, Monitoring and IoT Technologies, National Aviation University,  
necessary to maintain and restore the object of maintenance (OM). The difference between maintenance and repair is that maintenance is conducted to prevent unexpected equipment downtime, whereas repair is conducted following downtime to restore the OM operable state and reduce losses. Under MRS effectiveness, we refer to a set of properties that characterize the ratio between the costs of resources (material, time, or labor) to maintain and restore the health of the OM and the effect achieved. The effectiveness of MRS depends on the reliability, maintainability, and durability of the OM as well as the trustworthiness of inspections or condition monitoring. The quantitative characteristics of various MRS properties are indicators of the reliability, maintainability, durability, and trustworthiness of the inspections. For example, indicators of inspection trustworthiness quantitatively characterize the properties of the inspection tool, which is a subsystem in the MRS, to objectively display the actual condition of the OM based on the inspection results. Currently, technical and technical-economic indicators are used to quantify maintenance effectiveness. Technical indicators include the achieved availability, inherent availability, mission availability, and operational reliability. Technical-economic indicators include the long-run average profit per unit time and long-run average cost per unit time. Choosing maintenance effectiveness indicators of a system requires identifying the key features that distinguish it from others. Therefore, for the proper selection of MRS effectiveness indicators, the development of a technical system classification is required according to signs that consider the unique features of their design, intended use, and operating conditions. After selecting the MRS effectiveness indicators, a maintenance model should be developed or chosen for the calculation. This model should reflect the features of the structural construction of the system in terms of reliability, the presence of external manifestations of failure, and the main signs of maintenance operations. Since any MRS necessarily includes an inspection subsystem and a recovery subsystem, the maintenance model should consider the characteristics of operations, including inspection and repair. The efficiency of using a system for its intended purpose depends on the type of inspection. Therefore, the maintenance model should consider the type of inspection and the degree of system repair. Based on the literature review in Section 2, the following findings can be drawn:  
1. Performance indicators are considered at three levels of maintenance control: strategic, tactical, and operational. 2. An analysis of the published studies revealed that diverse types of maintenance performance indicators exist in the literature for each level of maintenance control. 3. At the operational stage, the most frequently used maintenance indicators are instantaneous availability, steady-state availability, average availability, inherent availability, mission availability, operational reliability, long-run average profit per unit time, long-run average cost per unit time, and average lifetime maintenance costs. 4. To date, several classifications of maintenance performance indicators have been developed, including, for example, such categories of indicators as equipment-related, maintenance task-related, cost-related, and so on. However, the signs by which indicators should be selected in each group for systems of various purposes have not been indicated. Since a formalized approach to selecting the indicators has not been developed, users are forced to subjectively choose suitable indicators for their circumstances from a set of known indicators. 5.