1. The World of Air Transport in 2018https://www.icao.int/annual-report-2018/Pages/the-world-of-air-transport-in-2018.aspx
2. Covid-19 Impact on the European Air Traffic Networkhttps://www.eurocontrol.int/covid19
3. Global Air Traffic—Scheduled Passengers 2004–2021, Statistahttps://www.statista.com/statistics/564717/airline-industry-passenger-traffic-globally
4. Delta is Hiring 12,000 through 2020 as Airline Expands Operationshttps://www.cnbc.com/2019/10/10/delta-is-hiring-12000-through-2020-as-airline-expands-operations-ceo-says.html
5. 2019: The Year United Airlines eXpanded into New Marketshttps://simpleflying.com/2019-the-year-united-airlines-expanded-into-new-markets/