Safety as a management concept in the air transport sector: A systematic literature review

Purpose: The main purpose of the present study is to conduct a literature review of the contribution made by safety in air transport, based on the existing international academic literature in the field of the social sciences. It primarily attempts to examine and verify the relationship between safety-related concepts (co-occurrence), the link between the different authors (co-authorship) and the corresponding citations (co-citations). Design/methodology: To achieve the established objectives, a systematic literature review (SLR) has been conducted using the Scopus database between the years 1990 and 2016, identifying international academic papers related to the research topic of the present study. Findings: It has been verified, on the one hand, that safety in the air transport sector is a field of growing interest, as the number of papers has increased considerably in recent years, thus demonstrating the importance that this topic has acquired over time. On the other hand, however, it must be mentioned that the total quantity of papers related to the topic is low in terms of absolute numbers. The results of the co-occurrence analysis show that the most important aspect of safety is safety management, while the strongest link is between safety management and aircraft accidents, a fact that is foreseeable a priori.


Introduction
The air transport sector is an essential provider of a wide range of services, and is a fast-growing industry faced with a large number of challenges when it comes to generating benefits, due to the numerous events and trends that influence it, both social and economic (IATA, 2014;ATAG, 2014).
Within the large variety of circumstances that have an influence on the air transport sector, a key element is offering operations with a high level of safety and efficiency to passengers on a global level (ICAO, 2015), as well as ensuring the safety and security of the people who work around the aircraft and the safety of the aircraft themselves (AERTEC, 2013).
Air transport stands out from other alternative means due to factors such as speed, cost, efficiency, comfort and flexibility (Kelemen, 2003;ATAG, 2014), in addition to its safety conditions, which are much higher than those of other types of transport, such as road, rail, etc. (Pacheco, Ferna ndes & Domingos, 2014).
Improved safety standards have always been one of the main priorities in the sector, if not the top priority (Liou, Yen & Tzeng, 2008).There is little doubt that it is a concept that plays an essential role and its improvement has become a topic of growing interest in the field of operation management (McFadden & Hosmane, 2001).The air transport industry is also known for its efforts and the unending challenge of always wanting to be known for its high level of safety and security, therefore reducing the number of accidents and incidents (Shappell, Detwiler, Holcomb, Hackworth, Boquet & Wiegmann, 2007;Liao, 2015).
While the number of passengers in this sector is continuously on the rise, the number of passengers who are victims of accidents is decreasing in absolute values, and therefore, to a greater extent, in relative values (Aviation Safety Network, 2016;ICAO, 2015).
However, due to the difficulty of defining the aspects that are included within the framework of operational safety, the main objective of the study is to obtain a broad view of the contributions made in the literature about safety in air transport and its evolution over time, as well as to determine the concepts that are most closely related to it (co-occurrence).
To reach said objective, the necessary information has been examined and collected through a systematic revision of the literature, using the Scopus database.
Other more specific objectives, taking into account the present research topic at all times, are the following: • To see the evolution of the academic publications related to safety in air transport during the period 1990-2016 (understanding 2016 as the review completed prior to February 29 of this year).
• To obtain an overview of the main journals considering the research topic.
• To identify which are the most frequently cited academic articles and their corresponding researchers.
• To determine the relationship among the different researchers (co-authorship).
• To see the co-citation among the documents included in this study.
• To show and analyze the origin of the authors of the related articles.
The main contribution of this article is therefore the description and dissemination of what has been said about safety in the air transport sector in recent decades, the authors that stand out and the relationship between the established variables and the concept of safety.
The present document has been structured as follows: first, the field of interest and research topic are introduced, that is to say, safety in air transport; secondly, the methodology used to conduct the research is described; the third section presents the results obtained, while the last part discusses the main conclusions drawn.

The conceptual framework and evolution of operational safety
The intent of this section is to analyze the concept of operational safety, as well as to show the evolution of said concept over time.

Dual of interpretation
Before defining this concept, it is necessary to comment on the need for a correct interpretation of two concepts highly related in the air transport field: safety and security.
One concept is safety, which refers to concepts related to incidents and accidents involving air transport (AERTEC, 2013), while the other aspect is the security, related to, for example, "criminal acts and illicit interference" (AERTEC, 2013), which can affect the protection of both passengers and ground and air crews, among others.Security can thus affect, to a greater or lesser extent, safety.
Both safety and security have always been two key objectives and a great challenge in the air transport sector (Fox, 2014), and more so since the events of September 2001 in the United States.According to Barnett (2009), the two factors that passengers most commonly relate to safe travel is one, avoiding airplane accidents, and two, the prevention of criminal and terrorist incidents.

Operational safety
According to AERTEC (2013), safety works when the possibility of accidents and incidents and injuries or damage to passengers, property and staff members is reduced, thanks to the ongoing process of analysis and management of both dangers and risks.
The International Civil Aviation Organization (ICAO) defines safety in a similar manner, as a state in which the possibility of personal injury or property damage is reduced, and an attempt is made to keep it low and controlled through continuous risk analysis, among other measures.
In other words, safety refers to the act of reducing both accidents and incidents as much as possible.
Per AERTEC (2013), three criteria need to be considered in order to define safety: first of all, how to evaluate and detect risks in order to keep them at an acceptable low level; secondly, how to investigate incidents in order to make the right preventive decisions and finally, how to define safety variables and indicators that make it possible to measure and monitor the safety level.

Evolution of safety in recent years
As mentioned earlier, the number of passengers has continually increased, while the number of fatal accidents has significantly decreased in recent decades, as shown in Figure 1. Figure 1 shows the constant growth in the number of passengers, which reached 3.5 billion in 2015, representing an increase of 8.9% over 2014.
According to the European Aviation Safety Agency (EASA, 2015), the number of fatal accidents in 2014 was below the average total for the last 10 years (2004-2013), in spite of the fact that 2014 (see Figure 1) was a year that stands out for the number of fatalities (n=904).
The year 2014 includes two unique accidents, Malaysia Airlines Flight 370 (which disappeared in March 2014; there is still no evidence as to its cause), and Flight 17 from the same company, which was attacked as it flew over Ukraine in July 2014.

Methodology
The methodology followed in the present study is as follows: first, a systematic review was conducted of the literature (hereinafter referred to as the systematic literature review or SLR), using the bibliographic database Scopus, which provides a broad overview of international academic articles, including a large database of summaries and citations (Elsevier, 2015).
The systematic literature review methodology is an essential tool for any project in academia, as it helps add to the knowledge researchers need about a specific topic (Webster & Watson, 2002).It also has the advantage of providing rigorous, exhaustive, reliable and unbiased information (Kitchenham, 2004).Kitchenham (2004) also mentions two fundamental aspects that make SLR necessary, namely, the identification of gaps in order to propose future lines of research and also to present background information as a framework.
In most cases, SLR has been used in the field of health (CRD, 2009) and other disciplines, such as public policy, but it has also been applied in relation to the air transport sector (Ginieis, Sánchez-Rebull & Campa-Planas, 2012a-b).
All this shows that SLR is a very useful tool for the evaluation and analysis of the available information and its later interpretation in response to the research question to be studied (Kitchenham, 2004).
The SLR conducted for the present study covers the period between 1990 and February 2016, thus encompassing an extended period, which enables us to obtain an overview of the evolution of the concept of safety in air transport.
Secondly, for the co-authorship and co-citation analysis among the different articles, as well as to compare the variables (co-occurrence) related to safety, the program NodeXL Template for Microsoft Excel 2007 was used to facilitate the identification of the concepts and to create a network analysis that is visualized in a way that makes it easy to see the existing relationships among the different variables (NodeXL, 2013).

Identification of key words and article sampling
To identify the publications related to the present study, different combinations of a total of nine key words (three primary and six secondary key words) were established, as shown below: As can be seen in Table 1, considering the different combinations of primary and secondary key words, the total number of articles found in the Scopus database within the field of the social sciences after the first search was 1,021.This figure includes conference presentations and book chapters which were later eliminated for the final review.The search criterion and subsequent filtering of the articles after eliminating duplicates was based on the requirement that at least one of the key words must be used in the title, the article abstract or the keywords specified by each of the authors.
According to this procedure, the final bibliographic database consisted of 59 international academic articles, which were used as the basis for the present study, as shown in Table 2.

Encoding operational safety variables
After reviewing and reading the aforementioned 59 articles about operational safety and with the collaboration of a panel of experts, 11 key variables were determined that were derived from their reading.The variables detected were the following: 1. Aircraft accidents: Related to airplane accidents, collision, etc.; events that generate the death or serious injury of people, and in which the aircraft is substantially damaged.

Aircraft incidents:
An event different from an operational accident, it is related to the operation and use of an aircraft that can affect safety.
3. Aircraft related issues: Aspects related to the aircraft, i.e., the type of aircraft, age of the fleet, etc.
4. Customer related issues: Includes aspects such as customer satisfaction, perception, behavior, etc.

Human factors:
This variable covers both human errors and human resource management, among other aspects.
6. Maintenance: Aircraft maintenance is related to aspects such as the repair, inspection, general inspection, failure mechanisms, etc.
7. Safety culture: Includes everything related to the culture surrounding safety, such as the commitment and dedication to safety, the definition of obligations and the responsibilities of employees, etc.

Safety management:
A management variable of safety that includes aspects such as safety management programs and systems, as well as risk management and analysis.9. Safety performance: Aspects associated with financial stability, economic and financial profitability, the financial health of the airline, etc.All variables were necessary in order to establish and then demonstrate the relationship that exists among the different concepts included (co-occurrence) during the SLR, and thus determine which topics were dealt with most often during the analyzed period.

Descriptive results
This section shows the descriptive results obtained from the 59 articles from the Scopus database, along with the 134 authors included in the study and resulting of said articles, for the period 1990-February 2016 covered by the present study.

Evolution of the academic articles on operational safety
Figure 2 shows the evolution over time of the final selection of international academic articles (n=59) that have been selected following an exhaustive SLR.
In particular, during the period 2012-2015, the number of publications has increased significantly, representing 42.37% of the total analyzed.This total reflects the growing importance of the topic studied in recent years.Kalemba & Campa-Planas, 2016).

Most productive and most frequently cited journals
This section analyzes which journals are the most productive and most often cited in this study, with reference to the topic addressed in this SLR, taking into account the 59 articles obtained from the systematic literature review and their corresponding ranking.Table 3 shows the most productive journals and those that have at least 2 or more articles included in the present study (n=8), as well as their impact factor at 5 years and the quartile to which each journal corresponds, according to its category.
The first two journals, the Journal of Air Transport Management (United Kingdom) and Safety Science (Netherlands) represent more than 50% of the publications out of the total number of articles included in the study (30 of 59 articles), thus demonstrating that the articles are concentrated in a small number of journals.
The Journal of Air Transport Management (n=16) is in first position, with 463 citations, and it is also the most specific journal in terms of air transport.
It should also be mentioned that at the same time it is the journal with the lowest impact factor at 5 years, as a journal in the first/second quartile in the subcategories of law, management and transport (SCImago Journal & Country Rank, 2014).
On the other hand, the journal Safety Science is found in the first quartile in the subcategories of health and safety (SCImago Journal & Country Rank, 2014).

Most frequently cited articles
Table 4 provides information on the articles most frequently cited, considering the authors, the title and the year of publication, as well as the total number of citations since publication.The most cited article was that by Liou, Tzeng and Chang (2007), entitled Airline safety measurement using a hybrid model, which received a total of 164 citations.It was followed by an article by the same author, Liou et al. (2008), entitled Building an effective safety management system for airlines, which was cited a total of 83 times.Both articles were published in the aforementioned Journal of Air Transport Management.

Relationship among authors (co-authorship)
In order to demonstrate the collaboration among the different authors established by the articles selected as part of the SLR, a co-authorship map has been created.
During the evolution of the time considered in the study, the scientific works were increasingly coauthored, and following this trend, the phenomenon of co-authorship has attracted interest in recent years (Acedo, Barroso, Casanueva & Galán, 2006).
The 59 academic articles selected were written by a total of 134 authors, of whom 13 have contributed to two and even three articles.The two most productive authors have been Chen, C.F. (2012,2014,2014) and Chen, S.C. (2012,2014,2014), both of whom have collaborated on the three occasions with one another.
Figure 3 shows this information on co-authorship in graphic form.It should be mentioned that 32.20% of the 59 articles have been written by two authors (n=19), 30.51% by a single author and 23.73% by three authors.The remaining 13.56% were written by 4 to 6 authors.

Relationship among cited articles (co-citation)
Co-citation analysis has become a necessary and predominate method for scientific works in recent years, and it has already been applied in a number of studies (Seguí-Mas, Sarrión-Viñes, Tormo-Carbó & Oltra, 2016; Alves, Fernandes & Raposo, 2016).It is a tool that helps analyze different groups of researchers, revealing their publications and fields of application (Alves et al., 2016), as well as observing similarities in the content by the cited authors (Gmür, 2003).
Figure 4 shows a co-citation map of the authors of the articles with the largest number of citations in relation to safety, taking into account the criterion that the number of citations was greater than or equal to 50 (n=4).For all cases, only the first author of each article was included and then all authors with less than two citations were eliminated in order to obtain a better view of the co-citations among the authors.The most frequently cited authors were Liou (2007Liou ( , 2008) ) and Janic (2000), with 116 and 24 citations, respectively.The greatest co-occurrence of citations exists between Liou & Huang (n=13) and Liou & Tsai (n=13).

Origin of the authors
In order to observe the spatial distribution and to study where else the present research topic has been given attention, the origin of each of the authors was analyzed.Figures 5 and 6 show the 17 countries in which the topic of operational safety in air transport has been studied.The countries belonging to the European Union with the largest proportion of authors are the United Kingdom (10.4%) and the Netherlands (6.0%).

Analysis of the network of variables related to safety
In order to demonstrate the relationship among the different concepts that are linked to safety, as discussed in the basic literature, a co-occurrence analysis was conducted on the keys aspects.
A map of the co-occurrences was created (see Figure 7) using the NodeXL tool (NodeXL, 2013), considering the eleven variables mentioned earlier, which were determined based on the 59 articles included in the SLR.
This figure is shown according to the design by Fruchterman and Reingold (1991), based on the one hand on edges, which show the importance and the frequency of the co-occurrence between two variables, and on the other hand, the volume of each of the nodes, which represent the relevance of each of the variables.As can be seen in Figure 7, the node with the greatest weight, i.e., the concept most used in relation to safety in the international academic literature is Safety management (n=30), followed by Aircraft accidents (n=22), while the strongest link is shown between the variables of Safety management and Aircraft accidents, and between Safety management and Aircraft incidents.

Conclusions
In spite of the fact that safety is a key element in the air transport sector, it must be mentioned that no specific literature review article in the sector has considered the contribution made by the concept studied in this article.
Therefore, the study has attempted to bridge this gap and conduct a systematic literature review (SLR), using the Scopus database.This review enabled us to identify international academic articles corresponding to the proposed research topic.
The most frequently cited journals and articles were examined, in addition to determining the degree of co-occurrence among the variables established by the authors in relation to safety, with the help of a panel of experts; an overview of the co-authorship and co-citations among the different authors was also provided.Starting with a total of 1,021 articles, once the selected articles were refined, a sample of 59 articles published between 1990 and February 2016 were analyzed.
The first conclusion that we can comment on is that the research topic is one of growing interest, even though the total number of articles in terms of absolute value continues to be very low.
Most of the articles obtained after the preliminary search have been primarily related to the field of psychology and engineering, and only a few articles are based on management topics and relevant concepts in relation to safety.
The most productive journal in terms of both the total number of articles included (n=16) and the total number of citations (n=463) was the Journal of Air Transport Management.In second place was the journal Safety Science, in terms of the total number of articles included in the study (n=14), while the journal with the second largest number of citations was the International Journal of Industrial Ergonomics, with 110 citations.
The two most frequently cited articles were both published by Liou et al., in 2007 and2008, andwere entitled Airline safety measurement using a hybrid model and Building an effective safety management system for airlines, respectively.
Through a network analysis, it was possible to obtain a co-occurrence map of the different variables established as the result of the SLR and their subsequent definition.Thanks to this, it was possible to acquire an overview of the structure of the basic literature dedicated to this research topic.
The most relevant concepts with the greatest relative weight with regard to operational safety were Safety management and Aircraft accidents, while the edges with the highest degree of interaction were found between the same variables, and between Safety management and Aircraft incidents.
Furthermore, it was revealed that collaboration among the authors in recent years has increased, with 69.5% of the aforementioned 59 articles published by two or more authors.
There are a total of 13 researchers with a minimum of two and even three published articles, among whom Chen, C.F. (2012,2014,2014) and Chen, S.C. (2012,2014,2014) stand out for having collaborated three times with one another.However, the remaining 11 authors have collaborated with a wide variety of different authors.
Finally, it is believed that the study conducted may promote the view that operational safety contributes to business management in the air transport sector.It should be stressed that the SLR carried out offers an opportunity for future lines of research with a more management-oriented focus, as the study has shown that operational safety, as an essential topic in the field of air transport, provides little academic literature related to management.
Other possible lines of future research may, therefore, include the following: • Analyzing whether safety is profitable for the airlines.In other words, the impact of safety on the profitability of the airlines could be analyzed.
• Studying the extent to which the airlines invest in safety.In other words, what percentage of expenses and/or investment is dedicated to improving safety?
• Analyzing how safety can be measured in air transport.Some sector indicators are already available, such as the JACDEC Safety Index, for example, developed by Jan-Arwed Richter and Christian Wolf (Richter, 2014), which provides information on aircraft accidents and incidents in recent years.Webster, J., & Watson, R. (2002).Analyzing the past to prepare for the future: Writing a literature review.MIS Quarterly, 26(2), 13-23.
Los resultados del análisis de co-ocurrencia muestran que la variable que mayor relevancia obtiene referente a la seguridad operacional es la gestión de la seguridad operacional, mientras que el vínculo más fuerte se observa entre la gestión de la seguridad operacional y los accidentes de aviación, hecho a priori previsible.
No obstante, y debido a la dificultad de definir los aspectos que se incluyen en el marco de la seguridad operativa, el objetivo principal del estudio es obtener una amplia visión sobre las aportaciones realizadas en la literatura sobre la seguridad operacional en el transporte aéreo, su evolución en el tiempo y ver cuáles son los conceptos que están más relacionados a ella (co-ocurrencia).
Para conseguir dicho objetivo, se ha examinado y recopilado la información necesaria a través de una revisión sistemática de literatura haciendo uso de la base de datos Scopus.
Por lo tanto, la principal contribución de este artículo es describir y dar a conocer que se ha dicho sobre la seguridad operacional en el sector de transporte aéreo en las últimas décadas, cuales son los autores que más destacan y ver cuál es la relación que existe entre las variables establecidas y el concepto de seguridad operacional.
El presente documento se ha estructurado de la siguiente manera.Primero de todo se presenta el campo de interés y el tema de investigación, es decir, la seguridad operacional en el transporte aéreo.En segundo lugar, se describe la metodología utilizada para llevar a cabo la investigación.La tercera sección presenta los resultados obtenidos, mientras que en la última parte se muestran las principales conclusiones obtenidas.

Marco conceptual y evolución de la seguridad operacional
En este apartado se pretende analizar el concepto de seguridad operacional, así como también mostrar la evolución de dicho concepto en el tiempo.

Dualidad de interpretación
Antes de definir la seguridad, es necesario comentar, que un problema que se puede encontrar en el momento de utilizar el concepto de seguridad en el ámbito de transporte aéreo, es su dualidad de interpretación, ya que la palabra tiene un significado ambiguo.
La Organización de Aviación Civil Internacional (ICAO) define a la seguridad operacional de manera similar, como un estado en el que la posibilidad de daño a personas o propiedades es reducida y se intenta mantener baja y controlada a través de un continuo análisis de riesgos, entre otros.
El criterio de búsqueda y el posterior filtro de artículos a partir de la eliminación de duplicados se ha basado en que mínimo una de las palabras clave debía mostrarse o en el título, en el resumen del artículo, o en las keywords establecidos por cada uno de los autores.
De acuerdo a este procedimiento, la base bibliográfica final ha estado compuesta por 59 artículos académicos internacionales, en base a los cuales se ha llevado a cabo el estudio presente, tal como se muestra en la Tabla 2.

Codificación de variables acerca de la seguridad operacional
Después de una revisión y lectura de los referidos 59 artículos sobre la seguridad operacional y con la colaboración de un panel de expertos, se determinaron 11 variables clave que se derivaban de la lectura de los mismos.Las variables detectadas han sido las siguientes: Todas las variables han sido necesarias para poder establecer y demostrar posteriormente la relación existente entre los diferentes conceptos incluidos (co-ocurrencia) durante la SLR y por lo tanto ver también cuales son los temas que fueron tratados con mayor frecuencia durante el período analizado.
La revista más productiva, tanto en referencia al número total de artículos incluidos (n=16) y el total de citas (n=463) ha sido el Journal of Air Transport Management.En segundo lugar, está la revista Safety Science, en cuanto al número total de artículos incluidos en el estudio (n=14), mientras que la segunda revista con mayor número de citas es el International Journal of Industrial Ergonomics, con 110 citas.
Los dos artículos más citados, ambos han sido publicados por Liou et al. en los años 2007 y 2008 con los títulos "Airline safety measurement using a hybrid model" y "Building an effective safety management system for Airlines", respectivamente.
A través de un análisis de redes ha sido posible la obtención de un mapa de co-ocurrencia entre las diferentes variables establecidas, resultado de la SLR y su posterior definición.Gracias a ello se ha podido adquirir una visión general sobre la estructura de la literatura básica de este tema de investigación.
Los conceptos con mayor relevancia y peso relativo en referencia a la seguridad operacional han sido la gestión de la seguridad operacional (safety management) y los accidentes de avión (aircraft accidents), mientras que las aristas con mayor grado de interacción se han podido encontrar entre las mismas variables, como también entre la gestión de la seguridad operacional (safety management) y los incidentes de avión (aircraft incidents).

Figure 1 .
Figure 1.Number of passengers and number of fatal accidents 2010-2015 (author's own work based on data of the World Bank (2015) and ICAO (2015)) Safety AND Airlines OR Air Transportation AND Profitability • Safety AND Airlines OR Air Transportation AND Quality • Safety AND Airlines OR Air Transportation AND Cost Maintenance • Safety AND Airlines OR Air Transportation AND Management • Safety AND Airlines OR Air Transportation AND Performance • Safety AND Airlines OR Air Transportation AND Success

10. Security : 11 .
Includes everything related to physical security, such as the protection of passengers, ground and flight crews, the aircraft itself, etc. Technical flight crew: The technical flight crew variable includes aspects such as the communications skills of the employees, their training and the pilot's experience (i.e. total flight time), among other aspects.

Figure 2 .
Figure 2. Evolution in the number of international academic articles during the years 1990-2015

Figure 5 .
Figure 5. Origin of authors by country

Figure 6 .
Figure 6.Origin of authors by country

Figure 7 .
Figure 7.The relationship among the established variables in reference to safety (author's own work, using NodeXL) Figure 7 also shows that the other variables most commonly used in relation to the concept of safety are Human factors, Safety performance and the Technical flight crew.

Figura 1 .
Figura 1. Número de accidentes fatales y número de pasajeros 2010-2015 (Elaboración propia a partir de datos del Banco Mundial (2015) y ICAO (2015)) Safety AND Airlines OR Air Transportation AND Profitability • Safety AND Airlines OR Air Transportation AND Quality • Safety AND Airlines OR Air Transportation AND Cost Maintenance • Safety AND Airlines OR Air Transportation AND Management • Safety AND Airlines OR Air Transportation AND Performance • Safety AND Airlines OR Air Transportation AND Success

Table 1 .
Results of the search in the Scopus database

Table 2 .
Final sample of articles included in the literature review
9 Lee, W.K. Risk assessment modeling in aviation safety management 2006 30 10 Taylor, J.C.The evolution and effectiveness of Maintenance Resource Management (MRM) 2000 22 Table 4.Most frequently cited academic articles base on the systematic literature review Intangible Capital -http://dx.doi.org/10.3926/ic.918