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Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923.
Part II: Rules of Air Warfare#Chapter VII: Search, capture and confiscation. - Art. 51.
Art. 51. Neutral non-military public aircraft other than those which must be treated as private aircraft are only subject to the investigation of their papers.