General worldwide: Chicago Convention aka ICAO convention (currently under auspices of UN), Annex 8 [2] and Annex 6 [3].

For USA [4], Title 14 of Code of Federal Regulations, Chapter I Subchapter C, Part 39

For EU [5] Article 76 (6) of the Basic Regulation (EU) 2018/1139

[1] https://www.icao.int/publications/pages/doc7300.aspx [2] https://ffac.ch/wp-content/uploads/2020/09/ICAO-Annex-8-Airw... [3] https://ffac.ch/wp-content/uploads/2020/09/ICAO-Annex-6-Oper... [4] https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C... [5] https://www.easa.europa.eu/en/document-library/regulations#b...