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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. - Art. 20.
Art. 20. In the event of an aircraft being disabled, the persons trying to escape by means of parachutes must not be attacked during their descent.