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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 IV: Hostilities#Espionage. - Art. 27.
Art. 27. An individual, found on board a belligerent or neutral aircraft, can be considered as a spy only when, during a flight acting clandestinely or on false pretences, he obtains or endeavours to obtain information within the belligerent jurisdiction or in the zone of operations of a belligerent, with the intention of communicating it to the hostile party.