A Boeing 737 MAX 9 aircraft of American Airlines recently experienced a sudden drop in the emergency hatch on the left center rear shortly after takeoff, which has raised renewed concerns about aviation industry safety. What is the reason that caused the cabin door to fall off in the air? Industry experts believe that there are three possible reasons for this situation, including problems with cabin door fasteners. Academician of the Overseas Engineering Academy and senior expert who has worked at Boeing for many years, Li Xiaoguang, said in an interview with Global Times reporters on the 8th that a rigorous investigation is needed to determine how the aircraft cabin door ultimately fell off. However, after carefully observing the news pictures attached to the accident, it can be found that the door frame of the aircraft cabin door is basically intact, with only a slight damage below, and it did not tear under severe external drag, This indicates that the quality of the cabin door itself is not a problem, so the most likely issue is with the door fasteners. A Boeing 737 MAX 9 aircraft experienced an abnormal situation in the air on the 5th, and images circulating on social media showed that one of the cabin doors of the aircraft in question had disappeared. Li Xiaoguang has worked at Boeing for many years, participating in the research and development, and production of various Boeing aircraft models. He analyzed that during cruise altitude flight, in order to ensure passenger comfort, the cabin interior needs to be pressurized. At this moment, the pressure inside the cabin is greater than the external environmental pressure. During this process, if some components of the cabin door have problems, the door will fall off. "But there are two possibilities that can be ruled out. Firstly, metal fatigue of components is impossible. The aircraft involved in this accident was delivered only two months ago, and the phenomenon of metal fatigue usually occurs on aircraft with a flight history of 30 years. Secondly, Boeing is an" old shop "with over a hundred years of history, and its aircraft design, manufacturing standards, and airworthiness certification have a complete set of mature processes, so the probability of aircraft standards problems is also high." It's not big. " After careful observation and analysis of the photos of the accident plane, Li Xiaoguang said that it appeared that the plane door had fallen as a whole, and there was no significant deformation in the body structure and door frame. The door frame was basically undamaged, with only slight damage on the lower side. "This indicates that the door was not torn down by strong external forces.". Therefore, the most likely issue is with the fastener. Li Xiaoguang explained that, just like ordinary doors, airplane cabin doors are actually connected to the door frame by screws and bolts. If the quality of the fasteners is not up to standard, it will cause fractures. Li Xiaoguang stated that another possible cause of this accident is that the assembly workers did not follow the process, resulting in insufficient number of bolts or insufficient tightening strength during the installation process. "This door is a closed door that is not frequently used, and workers may not have followed the construction process; and it was not discovered during the subsequent review process." Zhao Xin from the Civil Aviation Flight Academy of China listed a third possibility in a media interview: there is a side window in the middle of the accident cabin door, and the middle side window on the cabin door is different from the side window installed on the aircraft structure, which cannot be ruled out as having insufficient structural strength, During the aircraft pressurization process, the window first ruptured, causing the entire cabin door to break out. The expert said that whether the side window on the cabin door broke first or the edge of the cabin door