# Security Situation Assessment for Terminal Area Control System Operation Based on BN-ISSA-ELM
## 2 Materials and Methods
### Construction of the Operational Safety Situation Indicator System
In the passive situation element identification, unsafe behavior is defined as an obvious and direct behavioral action to the occurrence of the terminal area control operation incident, and the emitters of the action include, but are not limited to, all of the personnel involved in the terminal area flights, the management personnel who formulate the rules and regulations, the related training personnel, etc., so as to obtain the indicators under the personnel factor. The unsafe object state in the model refers to the state of the object that has a direct influence on the unsafe event in the terminal area control operation. According to the analysis of the results of the accident investigation of past unsafe events, the subcategories under unsafe object state are divided into two categories, namely, theairspace environment object state and the technical environment object state, after statistics, generalization, and summarization. Airspace environment objects mainly refer to the airspace environment, meteorological conditions, control room operating environment, etc., while technical environment objects include the degree of completeness of the control equipment, navigation equipment, and facilities. In the process of mining the indirect causes of unsafe behaviors and unsafe objects, we identify the elements of management and supervision. The flow chart of this process is shown in Figure 2.  
In the process of establishing the safety situation indicator system for the terminal area control system, experts, system safety leaders, and front-line air traffic controllers provided their opinions through a questionnaire. Based on past data on operations in the terminal area, the three most dangerous events are height deviation, air danger proximity, and unstable approach. These are the main indicators of the operational safety situation, as shown in Figure 3. We used the combined active-passive risk identification approach to identify the 19 safety situation aspects that form the secondary indicators, categorized under the liveware, the hardware, the environment, and the management framework. We obtained the expert views of front-line controllers and safety leaders to verify the identification process. Figure 1: PFMEA active identification flowchart. Figure 2: Theoretical framework of the “2–4 model”. The questionnaire was conducted in two parts. The first part was a survey concerning the agreement of the factors influencing the operational safety posture of the terminal area control system. The agreement of the safety posture elements identified by the PFMEA and the 2-4 model was used to assess the operational safety posture of the terminal area control system, which was categorized into the following five grades: strongly disagree, disagree, average, agree, and strongly agree. A total of 60 questionnaires were collected so to retain the proportion of the average, as well as those above the 90 th percentile, of the posture elements for the collection of the second part of the questionnaire. In the second part, the degree of influence of the operational safety posture elements retained in the first part on the operational safety posture of the control system in the terminal area was divided into the following seven grades: very small, small, small, average, large, big, and very big, and 120 questionnaires were collected. It was found that the proportion of the general influence of each of the retained posture elements on the operational safety posture of the control system in the terminal area was higher than 90%, which confirmed the questionnaire method used in this article. The 19 operational safety posture elements of terminal area control system are based on man-machine-environment-management relationships.