Introduction
Annual aviation accidents impose a heavy financial burden on airlines and passenger lines directly or indirectly. Military aircraft accidents, in addition to causing heavy financial losses, have political and security consequences. According to the recently announced statistics by the Iranian Ministry of Roads and Urban Development, Iran has experienced an average of more than two plane crashes per year during the last 35 years (74 accidents in total). Meanwhile, more than 2200 Iranians have been victims of plane crashes. Discovering the cause of aviation accidents and preventing them as well as solving plane design and construction defects, are needed to prevent aviation accidents in the future. Aviation accidents have various classifications. In general, the types of aviation accidents include major, minor, moderate, catastrophic, disaster and non-flight. The current study aims to investigate the dynamics of aviation accident management in Iran and propose prevention policies.

Methods
This study focuses on system dynamics. The system dynamics methodology includes five major phases: Problem identification and definition, causal loop modeling, dynamic modeling, model testing and validation and policy definition and implementation.
After reviewing the studies on aviation accidents, the dynamic model of the aviation accident management system was designed with the participation of policymakers and aviation experts. After validation using the boundary adequacy test, the trustworthiness criteria (credibility, transferability, dependability and confirmability), the reproducibility test, and the integration error test, the model was simulated for a 20-year time horizon. According to the results of the Monte Carlo sensitivity analysis, aviation accident prevention policies were then identified based on three strategies: Development of the country’s aircraft fleet, standardization and improvement of flight safety and development of pilots’ flight competencies. After applying the policies of each strategy separately on the tested model, the results were compared.

Results
The subsystem diagram of aviation accidents in Iran included six subsystems: Aviation accidents, flight safety, flight competency, aviation equipment, aviation human resources and aviation financial resources. According to this diagram, the variables of each subsystem were identified and the cause-and-effect relationships between them were plotted by causal loop modeling. Then, the cause-and-effect model turned into a flow diagram.
Considering that none of the strategies or policies alone could reduce the rate of aviation accidents in a sustainable manner, new policies were presented with the combination of selected policies and applied in the model. The combined policies included: 1) Financing and budget allocation for aircraft fleet development and purchasing new aircraft to reduce the obsolete fleet by 50% in 5 years, 2) Supporting the aircraft industries in the country and transferring advanced technologies to the country in contracts, 3) Improving the standards of flight planning and existing aircraft, 4) Increasing the monitoring of flight planning implementation quality according to international standards, 5) Using the aircraft development financial resources for the development of flight safety equipment, 6) Investigating the causes of aviation accidents and implementing risk assessment tests according to international guidelines, 7) Increasing the productivity of aviation higher education courses and improving the competences of pilots and 8) Increasing the monitoring of pilots’ physical and mental fitness for flight. 

Conclusion
This study presented a practical model of the dynamics of Iran’s aviation accident management system to develop air accident prevention policies based on improving flight safety. The combined policies showed favorable results in reducing aviation accidents, and their efficiency was confirmed according to the data of the national civil aviation organization, and the international civil aviation organization (ICAO). Based on the proposed combined prevention policies and the dynamic model of the system, it is recommended:
1) The process of budgeting and providing national and international financial resources should be revised and optimized; 2) The domestic aircraft industries should be supported, advanced technologies should be transferred to the country, and attention should be paid to the country’s aircraft fleet; 3) The productivity of aviation higher education courses and the competence of pilots should be improved; 4) Quality monitoring of flight planning implementations should be increased.

Ethical Considerations

Compliance with ethical guidelines
In this study, all ethical principles were considered. Since no experiments were conducted on animal or human samples, no ethical code was obtained.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for profit sectors. 

Authors' contributions
Conceptualization, validation, data analysis, and supervision:Ali Mohammad Ahmadvand; Investigation, methodology, and writing the initial draft: Mostafa Moradi; Methodology, validation, data analysis, editing & review:Marzieh Samadi-Foroushani.

Conflicts of interest
The authors declare no conflict of interest.

Acknowledgments
The authors would like to thank all participants and the Crisis Prevention and Management Organization in Tehran for their cooperation and support.