Consequently, when we look forward through time as the natural process of any event or accident, we will see something like a tree instead of a chain, i.e., ramification instead of a sequence. Consequently, any event caused by any type of error will generally produce not only one outcome, but several despite the likeliness that only one of the outcomes will be found to be located on the final accident chain. Common-mode represents dual or even several functions carried by a single device. The main reason is from the so-called common-mode function. With these two characteristics, the potential for unexpected nonlinear interactions between subsystems to occur will be likely very high. Aviation is no doubt a complex system, and for any complex system, there are two characteristics highly complex and tightly coupled. But, when we look forward following the advancement of time, it is a different picture. When we look back through time, it is natural to conclude that the accident is really a chain of events. Although the sequential type of thinking is generally accepted by the aviation community, what really happens in any real case of aircraft accident may or may not be simply sequential. After the first cause is identified, the final sequence of causes is completed and the investigation process is basically finished. The general procedure of accident investigation is to collect all the evidence and look backward, according to the causal relation, to identify the chain of events. Moreover, the sequential type of thinking is further reinforced by accident investigation. Western languages have an alphabet system, every word has to be read and written in an exact certain sequence without any exception. It is thought by the authors that the basic reason for the generation of this type of thinking is from the language. Sequence is the key of all these models obviously. There is one basic feature in all of these models which is the sequential-type thinking. There are certainly other theories which will not be discussed here. When the hole of each piece of cheese lines up, the light will pass through all the pieces of cheese representing that the process of an accident is complete. The other well-known and similar theory is the so-called cheese model. From this, by removing any link in the chain, it is possible that the accident can be prevented. Based on Heinrich’s domino theory, Boeing established the concept of “accident chain” to de-scribe the process of an accident induced by a chain of real events. According to Heinrich’s theory, any accident can be described as the result of a chain of sequential events – like a line of dominos falling over. Domino theory, proposed by Heinrich, described the basic characteristics of industrial accidents. There are many models or theories used in the aviation community regarding flight safety, such as the domino theory, the accident chain, and the cheese model among others. Meaningful results are obtained although there is still much room for improvement. The present methodology has been tested with the FOQA data from final approach in real cases including the Nagoya and Da-Yuang accidents. The expert sys-tem using neural network can thus provide the quantitative flight safety margin given situation parameters from real flight condition. With the chosen situation parameters as the inputs, the surveyed results are then converted to the flight safety margin, representing the outputs of the training examples. A questionnaire is designed to measure the perceived needed performance. The flight safety margin describing how far the present flight situation is from the accident boundary is de-fined as the inverse of the needed performance of the crew to recover the present situation back to the standard condition and scaled from zero to one. Any normal real flight will thus be delineated by a time-varying continuous curve around the centerline defined by the standard flight condition. The flight operation is viewed moving on a virtual terrain in the abstract situation space. Flight Safety Margin, based on the situation of flight and from an operation point of view, provides a new tool whereby flight safety can be analyzed numerically.
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