Related Rule
New Zealand
Practice Relating to Rule 48. Attacks against Persons Parachuting from an Aircraft in Distress
New Zealand’s Military Manual (1992) states:
It is generally considered to be a rule of customary law that aircrew who have bailed out of a damaged aircraft are to be considered as hors de combat and immune from attack. By [the 1977 Additional Protocol I Art. 42], this is made part of treaty law so that such persons are protected during their descent. Should such a person land in the territory of an adverse Party, he must be given an opportunity to surrender before being made the object of an attack, unless it is apparent that he is engaged in a hostile act. If he lands within his own lines or in territory occupied by his own national authority, he is liable to immediate attack like any other combatant, unless he is wounded and so protected by the [1949 Geneva Convention I].
The ban on shooting down those descending by parachute does not extend to the dropping of agents or parachute troops. 
New Zealand, Interim Law of Armed Conflict Manual, DM 112, New Zealand Defence Force, Headquarters, Directorate of Legal Services, Wellington, November 1992, § 522.
The manual adds: “Airmen abandoning aircraft in distress may not be attacked during their descent. Any such attack would, therefore, be a war crime.” 
New Zealand, Interim Law of Armed Conflict Manual, DM 112, New Zealand Defence Force, Headquarters, Directorate of Legal Services, Wellington, November 1992, § 1704(2)(c), footnote 34.