**6. Conclusions**

This research showed that the airline industry is growing quickly since 2010, as is described in the introduction, based on International Civil Aviation Organization (ICAO) (2018) in its annual report about 2017. Since the financial crisis of 2008 and 2009, the airline industry started growing and has nearly doubled in the number of passengers [83–86]. However, the airline industry has proven to be vulnerable to issues such as oil crises, financial crises, and the 9/11–terrorist attacks, and immediately showed a decline or a stop of growth. This research provided insight into the current development problems of the airline industry on its fastest-growing business model. It shows that an increase in oil prices is still one of the causes of serious airline troubles. However, airlines can counter this with a newer, more fuel-e fficient fleet and successful fuel-hedging practices.

Issues of a whole other type are personnel problems, such as strikes. It is hard to find one exact reason for this issue. In the case of Ryanair, one of the airlines in this research, the reasons given by the pilots are a demand for a better salary, better rosters, and a base location closer to their homes. Furthermore, there are fleet problems, based on unexpected, required aircraft repairs and late deliveries of new aircraft. Airlines are forced to lease aircraft, which is a costly event. The last issue, airport capacity limitation, will a ffect every airline. Whether it is a limited runway capacity, aircraft stand capacity, or noise reduction measures. It will cause airports to run out of available slots for airlines. Given the high growth rate of the industry, the prices for a slot will go up. Airlines can counter this by flying to other airports (for example, secondary airports or by operating a modern, quieter aircraft).

As is visible from the conclusions given above, factors of LCCs can play a role in countering the negative e ffects of the development problems for airlines [11,12,87–95]. Flying to secondary airports of London, rather than the congested London Heathrow, operating a modern fleet, and achieving high load factors can counter e ffects. Machine problems, like engine problems such as Norwegian faced or late aircraft delivery (Primera Air), are di fficult to prevent. It will delay the development of the airline and cause extra costs. Another risk is the stop of investments in an airline; this is, however, more risk for airlines that make a (structural) loss. Only Norwegian seems to have a riskier position on this issue.

The TOPSIS revealed as well that Wizz Air is the only airline meeting all the LCC criteria, whereas Ryanair and easyJet changed their business models during strategy changes. Wizz and Ryanair are both the most successful airlines of the group, with easyJet being in a third place. Subsidiary LCCs as Eurowings, Vueling, and Transavia are performing worse and are flying in general with the oldest fleet. Although ranking low on meeting LCC criteria, airBaltic is profitable and growing. It is implementing a brand new, e fficient fleet and will shift to a single-type fleet. Norwegian is, given its loss and sale of new aircraft, ranking low.

Based on this research, in general, it can be concluded that the LCC business model has the potential for success regarding profitability and growth while having the possibility to counter the effects of some of the development problems in the industry. The e fficient use of materials and a focus on a single type of aircraft can contribute to the success of the airline. Ryanair could counter its major effects of the personnel strikes, since its operating model led to successfulness, whereas Norwegian had to sell aircraft to maintain its existence. A further case study for each airline is recommended to for in depth research of how airlines survive the major issues in the airline industry. This research has led to the possible direction for research in the form of the exact operating model and risk prevention measures of the airlines. The case study of Vueling might give di fferent insights, as it is a subsidiary airline, which is not meeting the majority of the LCC criteria, while it still is the third regarding successfulness within this research. It also has a rather small fleet on order and has one of the lowest seat occupancies amongs<sup>t</sup> the airlines in this research. Further research can also be located in the operating models of FSCs and their success. Case studies can also lead to the improved weighting of the criteria for the MCDM analyses.

**Author Contributions:** Conceptualization, J.S. and G.F.V.K.; methodology, J.S.; software, G.F.V.K.; validation, J.S., G.F.V.K. and V.D.; formal analysis, V.D.; investigation, G.F.V.K.; resources, V.D.; data curation, J.S. writing—Original draft preparation, G.F.V.K.; writing—Review and editing, V.D.; visualization, V.D.; supervision, J.S.; project administration, V.D.; funding acquisition, V.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
