# Semi-Markov Model of the System of Repairs and Preventive Replacements by Age of City Buses
## 2 Description of the States of the Model of the Repair and Preventive Replacement System  
In the study the object of research is the bus exploitation system of public transport, in which technical objects (city buses) can stay in one of the five states of the considered model of the renewal system (repairs and preventive replacements):  
State 1--the state of operational availability of the technical object--it is the state, when the technical object (a city bus) is fully fit and supplied and can perform the assigned transport task in accordance with the accepted assumptions, i.e., in accordance with the accepted plan and schedule of realization of the courses (in the paper this state is considered as the state of failure-free work of the technical object);  
State 2--the state of repair by the Technical Rescue Service without the loss of the course--this is the state, when a damaged technical object (a city bus) is repaired during the realization of the transport task (on the route), which is carried out by the Technical Rescue Service; this type of repair is carried out in a \"short\" time interval or in the gaps between the successive realizations of the transport task (between the next courses), i.e., it does not cause loss of the course in accordance with the accepted plan and schedule of transport realization--it is assumed that this condition does not cause any disturbance (break) in realization of assigned transport tasks (in the paper this condition is considered as a condition of minimal repair of a technical object);  
State 3--the state of the repair by the Technical Rescue Unit with the loss of the course--it is the state, when a damaged technical object (a city bus) is repaired during the realization of the transport task (on the route), which is realized by the Technical Rescue Unit; this type of repair is realized in a \"longer\" time interval than the repair realized in the state 2 of the model, thus the repair causes the loss of the course in accordance with the plan and schedule of transportation realization--it is assumed that this state causes a disruption (break) in the realization of the assigned transportation tasks (in the paper this state is considered as the state of the minimal repair of the technical object);  
State 4--state of repair at the Service Station--it is the state, when a damaged technical object (a city bus) is subject to repair at the specialized service and repair stands of the Service Station assigned for this purpose--it is assumed that this state causes a disturbance (break) in the realization of the assigned transport tasks (in the paper, this state is considered as the state of perfect repair of a technical object);  
State 5--the state of preventive replacement--it is the state, when the technical object (a city bus) is subject to preventive maintenance after a specific hourly mileage and in accordance with the adopted operational strategy (according to the resour) in the technical object the elements and sub-assemblies are replaced--it is assumed, that this state does not cause disruption (break) in the realization of the assigned transport tasks (in the work this state is considered as the state of preventive age-replacement.).  
Figure 1 shows a directed graph of the mapping of the state changes of the renewal system model (preventive repairs and replacements) of the considered technical objects (city buses).