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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. 18.
Art. 18. The use of tracer projectiles, whether incendiary or explosive, by or against an aircraft is not forbidden.
This rule applies as well to the States which are parties to the Declaration of St.Petersburg of 1866, as to those which are not.