The airline industry is a significant global sector, experiencing substantial growth until the Covid-19 pandemic. Technological advancements, deregulation, and increased stock market reliance have reshaped the industry, creating a competitive and riskier environment. Effective financial management and value creation are critical for airline survival and success. This study addresses a gap in the literature by providing a comprehensive review of firm value determinants in the airline industry. The research questions explore the evolution of literature on firm value, main research directions, influential factors, the shift towards sustainability, and potential future research avenues. The study uses the systematic literature review (SLR) method, employing data from the Web of Science Core Collection and Scopus databases.

Existing literature reviews in the air transport context often focus on specific factors like air travel demand, revenue management, service quality, safety, or the impact of the COVID-19 pandemic. However, a comprehensive review of contextual factors affecting firm valuation is lacking. Pereira et al. (2021) identified 114 value-creating innovations, highlighting efficiency, convenience, portfolio differentiation, and sustainability. This study expands upon this by considering a broader range of determinants, including firm- and industry-level factors, health crises, political and economic stability, and customer relationships.

The study employed a systematic literature review (SLR) methodology following the guidelines of CRD (2009) and Denyer and Tranfield (2009). Data were collected from Scopus and Web of Science Core Collection databases using keywords related to firm value and air transport. The search yielded 572 initial results, which were filtered based on inclusion and exclusion criteria (peer-reviewed articles focusing on airline firm value). This resulted in a final sample of 173 articles. Data analysis included descriptive analysis (evolution of publications, citation analysis, geographical scope, co-authorship) and thematic analysis to identify main research strands. R 3.6.0, Vosviewer, and Microsoft Excel were utilized for data analysis.

The analysis revealed a significant increase in publications on airline firm value, particularly between 2008 and 2018 (post-financial crisis) and again in 2020-2021 (due to the COVID-19 pandemic). The Journal of Air Transport Management was the most productive and cited journal. The USA and China were the top contributing countries. Co-authorship analysis indicated a relatively weak collaboration network among researchers. Keyword co-occurrence analysis showed 'airline', 'finance', and 'management' as the most frequent terms. Thematic analysis identified seven major research themes: 1. **Industry-level characteristics:** This theme, comprising 37 papers, focused on mergers and acquisitions (M&As), alliances, competition, oil price volatility, and the impact of unexpected events (e.g., aircraft crashes). Studies showed that oil price volatility significantly impacts airline profitability and stock prices. Alliances were found to influence stock market reactions, though the impact can be complex. 2. **Firm-level value influencers:** This theme (36 papers) explored factors like business model (low-cost vs. full-service), hedging strategies, route announcements, and bankruptcy protection. Findings suggested that full-service airlines outperform low-cost carriers during crises. Hedging strategies were found to influence firm value, though research results are not conclusive. New route announcements could lead to different market reactions depending on the competition. 3. **Sustainability:** This theme (31 papers) examined the link between sustainability initiatives (ESG) and firm value. Studies demonstrated a positive correlation between sustainability activities (environmental, social, and governance) and market value. This is particularly relevant due to the airline industry's environmental impact. 4. **Customer relationship and marketing:** Studies in this theme explored the relationship between customer satisfaction, marketing strategies, and firm value. Customer satisfaction and positive word-of-mouth have been linked to increased profitability and market value. 5. **International political and economic instability:** This theme covered the impact of political uncertainty, disasters, and economic shocks on airline valuations. Terrorist attacks and economic crises were shown to affect airline stock prices, with smaller airlines exhibiting higher sensitivity to these events. 6. **New methods to predict share price:** This theme introduced new methodologies for more accurate stock price prediction, often incorporating financial and operational data, and employing optimization algorithms and various prediction models (PCA, option pricing models, GFS, ANN, etc.). 7. **Health Crisis:** This theme (Covid-19 and SARS) investigated the impact of global health crises on airline stock markets. Event study methodology revealed significant negative effects on share prices, emphasizing the need for effective crisis management and government intervention.

The findings address the research questions by illustrating the evolution of research on airline firm value, identifying key influential factors, and revealing a shift towards sustainability. The substantial increase in publications on sustainability since the COVID-19 pandemic reflects its growing importance in the industry. The study's comprehensive framework provides a valuable resource for researchers, executives, and policymakers. The findings confirm the importance of both traditional value drivers (financial performance, operational efficiency, market conditions) and emerging factors (sustainability, customer relationships). The study highlights the need for airlines to adopt sustainable practices and effectively manage customer relationships to enhance their long-term value.

This study contributes to the literature by providing a comprehensive review of firm value determinants in the airline industry, highlighting the growing importance of sustainability. The findings suggest that airlines need to integrate sustainability initiatives into their strategies to enhance firm value and resilience in the face of crises. Future research could focus on improving the comprehensiveness of keyword searches, expanding database coverage, developing more sophisticated models for stock price prediction, and conducting more detailed research on the impact of government interventions during crises.

The study's limitations include potential incompleteness of the keyword search, the restriction to Scopus and Web of Science databases, and the exclusion of certain studies. Future research should address these limitations by employing more comprehensive search strategies, expanding the scope of databases, and potentially incorporating qualitative methods to gain richer insights. The relatively weak collaboration network among authors in the field also suggests an area for future development.