[1] Airline Timetable Images (5/26/2016, 8:12:01 AM) <http://www.timetableimages.com/>
[2] 航空路 - Wikipedia (2/4/2017, 7:56:54 AM) <https://ja.wikipedia.org/wiki/%E8%88%AA%E7%A9%BA%E8%B7%AF>
Google search: 航空路