## 2 Theoretical Literature
### Human Factors Analysis and Classification System (HFACS)  
Human error has been implicated in 70 to 80% of all civil and military aviation accidents. The concept of the HFACS model was developed by [PERSON] in 1990 [10] and was generally referred to as the \"Swiss cheese\" model of human error, describing five levels of human failure, each influencing the next [11]. To identify the human causes of an accident, the HFACS model was developed to meet the need to offer analyzing tools as a way to plan preventive training. In addition, it is an important taxonomic nature and can be easily applied in practical application [20]. Therefore, it has been used within the military, commercial, and general aviation sectors for reducing the accidents that happen, including to systematically examine underlying human causal factors and to improve aviation accident investigations [11].  
In addition, the HFACS model of accident investigations is also an important qualitative approach to understanding and managing facility safety that is used by some government departments, such as the United States Coast Guard and the US Department of Defense [18]. To better understand the potential safe implications for avoiding the risks of picking and material-handling accidents in a warehouse facility [21], the HFACS model is used by this article and is described in detail as follows, including the five levels of human negligence [11; 20; 21]:  
* Unsafe behavior (UB): this can be classified into two categories, including errors and violations. Errors represent the mental or physical activities of individuals that fail to achieve their intended outcome, which dominate most accident databases, including three basic error types of skill-based, decision, and perception. Violations refer to the willful disregard for the rules and regulations that govern the safety of flight, including two violating forms of routine and exception.
* Precondition for unsafe behavior (PUB): this can be classified into two major conditions, including substandard conditions of operators (adverse mental states, adverse physiological states, and physical/mental limitations) and the substandard practices they commit (crew resource mismanagement and personal readiness).
* Unsafe supervision (US): the causal chain of events is traced to back up the supervisory chain of command. Four categories of unsafe supervision have been identified, including inadequate supervision, planned inappropriate operations, failure to correct a known problem, and supervisory violations. For example, there is inadequate on-the-job training and inadequate train-the-trainer training.
* Organizational influence (OI): this can be classified into three categories, including the organizational process, resource management, and organizational climate. For example, fallible decisions of upper-level management directly affect supervisory practices, as well as the conditions and actions of operators.
* External environment factors (EEF): it can be the technological environment or the physical environment. The technological environment refers to a broken wire, an improperly installed cable, or a failure of the automated pullback protection system. The physical environment is, for example, working in inadequate lighting of the yard or flying in severe blizzard weather.