A Great Circle is the shortest distance between 2 points on a sphere. Because most world maps are flat but the Earth is round , the route of the shortest distance between 2 points on the Earth will often appear curved when viewed on a flat map, especially for long distances. If you were to simply draw a straight line on a flat map and measure a very long distance, it would likely be much further than if you were to lay a string between those two points on a globe. Because of the large distance between Podkamennaya Tunguska Airport and Adisucipto International Airport, the route shown on this map most likely appears curved because of this reason. Try it at home! You'll see that it will travel the same route of the red line on this map!