# 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  
Comprising subsystems like approach control, airspace environment, and cockpit human-machine interaction, the terminal area control system is a dynamic system. Using technical tools, it monitors and controls aircrafts to prevent collisions both in the air and on the ground, and to ensure timely and safe aircraft takeoffs and landings. To assess the operational safety situation of the terminal area control system, it is crucial to first identify the elements that influence the system's safety. We describe the operational safety condition of the terminal area control system as the system's current operational state, which comprises numerous aspects impacting safety and predicts changes over a specific time period based on the features of terminal area control activities. Aspects of the safety situation represent the variables that could influence the safety of operations in the terminal area. Scholars often use the following two techniques to identify system risk sources: active identification, which is based on elements such as humans, machines, the environment, and management, or passive identification, which involves analyzing aspects that lead to dangerous situations. This study combines both active and passive risk identification approaches to ensure thorough factor identification and to capture all of the safety situation facets of the terminal area control system. The combination of active and passive risk source identification methods has significant and comprehensive advantages in identifying elements of the operational safety situation of the terminal area control system, and the two can play a powerful complementary role in broadening the breadth and increasing the depth of element identification. This combination not only helps to identify potential risk elements in a more comprehensive way, and to improve the safety and reliability of the system, but also provides a richer and more in-depth basis for the formulation of more effective preventive and control measures. The Process Failure Mode and Effects Analysis (PFMEA) method first divides the aircraft handover, aircraft monitoring, conflict detection and resolution, and approach and departure control subprocesses of the terminal area control process. It is believed that every subprocess is a \"component\" within a \"product\". We then break down these elements into smaller bits, analogous to the \"parts\" of a product. We further break down these components to identify the elements influencing operational safety. Based on the PFMEA method, the operational process of terminal area control is divided into the following four subprocesses: aircraft handover, aircraft monitoring, aircraft conflict detection and release, and aircraft control, and each subprocess is regarded as a \"part\" in the FMEA; then, these are subdivided step by step from the top to the bottom to determine the component elements of the \"parts\", which correspond to the \"components\" in the FMEA; these are further subdivided to analyze the impact elements of the \"parts\". To further split the \"parts\" and analyze their influencing elements, we extracted the posture elements that may affect the operational safety of the terminal area. The flowchart of this process is shown in Figure 1.  
We then used the \"2-4 model\" to conduct a reverse study of unsafe occurrences, identifying passive risk sources from unsafe operation and dangerous states. We then gathered data from the last five years' worth of unsafe incident investigations. This is in addition to the proactive identification of variables that may affect the operational safety of the terminal area.