https://www.eda.admin.ch/dam/eda/en/documents/publications/EuropaeischeAngelegenheiten/Schweiz-und-EU_en.pdf
http://www.kas.de/wf/doc/kas_450-2190-2-30.pdf?170424105349
http://www.aviationlaw.eu/wp/wp-content/uploads/2013/09/Freedoms-of-the-Air-Explained.pdf