Related Rule
United States of America
Practice Relating to Rule 48. Attacks against Persons Parachuting from an Aircraft in Distress
The US Field Manual (1956) provides:
The law of war does not prohibit firing upon paratroops or other persons who are or appear to be bound upon hostile missions while such persons are descending by parachute. Persons other than those mentioned in the preceding sentence who are descending by parachute from a disabled aircraft may not be fired upon. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 30.
The US Air Force Pamphlet (1976) states:
When an aircraft is disabled and the occupants escape by parachutes, they should not be attacked in their descent … However, persons descending from an aircraft for hostile purposes, such as paratroops or those who appear to be bound upon hostile missions, are not protected. Any person descending from a disabled aircraft who continues to resist may be attacked. Downed enemy airmen from aircraft in distress are subject to immediate capture and can be attacked if they continue to resist or escape or are behind their own lines. Otherwise they should be afforded a reasonable opportunity to surrender.
If downed in their own territory, they remain lawful targets, as combatants, unless rendered hors de combat by sickness, wounds or other causes … If downed in the attacker’s territory and subject to capture, the advantages of capture outweigh any minimal advantage secured by attack. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 4-2(e), including footnote 14.
The US Soldier’s Manual (1984) provides:
Individuals parachuting from a burning or disabled aircraft are considered helpless until they reach the ground. You should not fire on them while they are in the air. If they use their weapons or do not surrender upon landing, they must be considered combatants. Paratroopers, on the other hand, are jumping from an airplane to fight. They are targets and you may fire at them while they are still in the air. 
United States, Your Conduct in Combat under the Law of War, Publication No. FM 27-2, Headquarters Department of the Army, Washington, November 1984, p. 6.
The US Rules of Engagement for Operation Desert Storm (1991) instruct: “Do not engage anyone who … is an aircrew member descending by parachute from a disabled aircraft.” 
United States, Desert Storm – Rules of Engagement, Pocket Card, US Central Command, January 1991, reprinted in Operational Law Handbook, International and Operational Law Department, The Judge Advocate General’s School, United States Army, Charlottesville, Virginia, 1995, pp. 8-7 and 8-8, § A.
The US Naval Handbook (1995) states:
Parachutists descending from disabled aircraft may not be attacked while in the air unless they engage in combatant acts while descending. Upon reaching the ground, such parachutists must be provided an opportunity to surrender. Airborne troops, special warfare infiltrators, and intelligence agents parachuting into combat areas or behind enemy lines are not so protected and may be attacked in the air as well as on the ground. Such personnel may not be attacked, however, if they clearly indicate in a timely manner their intention to surrender. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 11.6.
The Annotated Supplement to the US Naval Handbook (1997) states that Article 42(1) and (2) of the 1977 Additional Protocol I codifies the customary rule set out in Article 20 of Part II of the 1923 Hague Rules of Air Warfare. It adds:
Firing a weapon is clearly a combatant act.
A downed airman, who aware of the presence of enemy armed forces, attempts to evade capture, will probably be considered as engaging in a hostile act and, therefore, subject to attack from the ground or from the air. However, mere movement in the direction of one’s own lines does not, by itself, constitute an act of hostilities. 
United States, Annotated Supplement to the Commander’s Handbook on the Law of Naval Operations, prepared by the Oceans Law and Policy Department, Center for Naval Warfare Studies, Naval War College, Newport, Rhode Island, November 1997, § 11.6, footnotes 39 and 40.
The US Naval Handbook (2007) states:
Parachutists descending from disabled aircraft may not be attacked while in the air unless they engage in combatant acts while descending. Upon reaching the ground, such parachutists must be provided an opportunity to surrender. Airborne troops, special warfare infiltrators, and intelligence agents parachuting into combat areas or behind enemy lines are not so protected and may be attacked in the air as well as on the ground. Such personnel may not be attacked, however, if they clearly indicate in a timely manner their intention to surrender. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 8.2.3.1.
In the Dostler case before the US Military Commission at Rome in 1945, the accused, the commander of a German army corps, was found guilty of having ordered the shooting of 15 American prisoners of war in violation of the 1907 Hague Regulations and of long-established laws and customs of war. The accused relied on the defence of superior orders based, inter alia, on the Führer’s order of 18 October 1942. This order provided that enemy soldiers participating in commando operations should be given no quarter, but added that these provisions did not apply to aviators who had baled out to save their lives during aerial combat. 
United States, Military Commission at Rome, Dostler case, Judgment, 12 October 1945.
In 1987, the Deputy Legal Adviser of the US Department of State, referring to Article 42 of the 1977 Additional Protocol I, affirmed: “We support the principle that persons, other than airborne troops, parachuting from an aircraft in distress, not be made the object of attack.” 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 425.
In 1991, in response to an ICRC memorandum on the applicability of IHL in the Gulf region, the United States pointed out that its practice was consistent with the prohibition to attack a pilot parachuting from an aircraft in distress and that the protection applied to all air crew rather than to the pilot only. 
United States, Letter from the Department of the Army to the legal adviser of the US armed forces deployed in the Gulf region, 11 January 1991, § 8(J), Report on US Practice, 1997, Chapter 2.8.