The concept of flight has fascinated people for hundreds of years: in the 15th century, Leonardo da Vinci theorized several ideas for flying machines; in the late 1780s two Frenchmen flew the first hot air balloon; and the Wright brothers flew the first airplane in 1903. As the global economy is becoming more and more connected, the aviation industry is now one of the fastest transportation sectors. Commercial airlines carried more than 4.5 billion passengers in 2018.