**4. Human factors**

Case studies in the preceding section have shown that human errors can cause catastrophic aircraft accidents. We have heard of the popular saying, "to err is human," meaning that it is normal for people to make mistakes. Analogous to manufacturing tolerances that are used to control the inherent variations in aircraft parts to ensure greater consistency, interchangeability, and intended performance, we can think of human error as a deviation from an intended action that does not lead to undesirable consequence. An intentional deviation would amount to violation. In this section, we will introduce the concept of human factors and the various models that have been in use to understand their role in aviation accidents.

In the FAA, Human Factors are defined as a "multi-disciplinary effort to generate and compile information about human capabilities and limitations and apply that information to equipment, systems, facilities, procedures, jobs, environments, training, staffing, and personnel management for safe, comfortable, and effective human performance" [22].

The concept of human factors can be understood by referring to the SHEL model, developed by Edwards and modified by Hawkins as shown in **Figure 4**. The SHEL model is represented by five blocks whose edges are not simple and straight. The most critical and flexible component is in the center of this model and corresponds to Liveware (human). This is flanked by four blocks corresponding to Software, Hardware, Environment, and Liveware. Achieving proper matching of each of these peripheral blocks with that in the center is vital, as any mismatch can be a source of human error [23]. **Table 2** lists the relevance of blocks in the SHEL model.

The PEAR Model is one of the popular concepts related to the science and practice of Human Factors, especially for developing aviation maintenance activities. This mnemonic comprises four key elements, namely People who do the job; Environment in which they work; Actions that they perform; Resources that are necessary to carry out the job [24]. **Table 3** lists the parameters addressed in the PEAR model.

While there are over 300 conditions that can result in human error [25], the aerospace industry very frequently uses a set of 12 (called the "dirty dozen") when discussing human factors. The elements of this set are "lack of communication," "complacency," "lack of knowledge," "distraction," "lack of teamwork," "fatigue," "lack of resources," "pressure,' "lack of assertiveness," "stress," "lack of awareness," and "norms" [26]. Although primarily used in aircraft maintenance programs, this

**Figure 4.** *SHEL model as modified by Hawkins.*


#### **Table 2.**

*Relevance of blocks in the SHEL model.*


#### **Table 3.**

*Parameters addressed in the PEAR model.*

concept has been extended to all areas of the aviation industry and has enhanced aviation safety. Factors associated with pilot errors have been discussed in [27, 28].

The Swiss-Cheese model of accident causation, originally proposed by James Reason, likens human system defenses to a series of slices of randomly holed Swisscheese arranged vertically and parallel to each other with gaps in-between each slice [29, 30]. The Swiss-cheese model, adapted from [29], is shown schematically in **Figure 5**. Each slice of cheese represents the defense of an organization against failure. Holes in each slice represent individual weaknesses in individual parts of the system and have a dynamic nature. It means that the holes vary continually in both size and position on the slice. A limited window of accident opportunity is created when the holes in all the slices align momentarily. This in turn facilitates a hazard to pass through all the holes leading to an accident.

Boeing has used procedural event analysis tool (PEAT) to help the airline industry effectively manage the risks associated with flight crew procedural deviations. Similarly, Boeing has used maintenance error decision aid (MEDA) to help airlines shift from blaming maintenance personnel for making errors to systematically investigating and understanding contributing causes [31]. A variety of operators have witnessed substantial safety improvements, and some have also experienced significant economic benefits because of reduced maintenance errors.

*Role of Human Factors in Preventing Aviation Accidents: An Insight DOI: http://dx.doi.org/10.5772/intechopen.106899*

**Figure 5.** *Concept of Swiss-cheese model developed by reason.*
