Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 48. Attacks against Persons Parachuting from an Aircraft in Distress
The UK Military Manual (1958) specifies:
It is lawful to fire on airborne troops and others engaged, or who appear to be engaged, on hostile missions whilst such persons are descending from aircraft, in particular over territory in control of the opposing forces, whether or not that aircraft has been disabled. It is, on the other hand, unlawful to fire at other persons descending by parachute from disabled aircraft. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 119, footnote 1(b).
The UK LOAC Pamphlet (1981) states that the duty to give quarter “prohibits shooting at persons who are escaping from disabled aircraft. On the other hand, members of hostile airborne forces descending by parachute are legitimate military targets.” 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 4, p. 12, § 2(b).
The UK LOAC Manual (2004) states:
Parachutists
12.67. No “person parachuting from an aircraft in distress shall be made the object of attack during his descent”. A person who has parachuted from an aircraft in distress must, upon reaching the ground in territory controlled by an adverse belligerent, be given an opportunity to surrender before being attacked unless it is apparent that he is engaged in a hostile act. Airborne troops do not fall under this protection.
12.67.1. The destruction or an attempt at destruction of the aircraft, any part of its equipment or related documents would constitute a hostile act.
Airmen in Enemy-held Territory
Downed airmen
12.68. On land, a “downed” airman from an aircraft in distress must be afforded a reasonable opportunity to surrender before the attack upon him is resumed. Clearly if the “downed” airman is incapacitated he is hors de combat and the general rule will apply. The attack may be resumed immediately if he offers violence, attempts to escape or if, suffering no incapacity, he is in territory controlled by his own forces. The pilot who has crash-landed his aircraft and is attempting to complete its destruction or the destruction of any part of his or its equipment is committing a hostile act and may be attacked immediately.
Combat rescue of downed aircrew on land
12.69. The use of, for example, military assets to rescue aircrew who have been “downed” on territory under the control of the enemy is a combat activity. It is therefore legitimate for an enemy in such circumstances to attack the rescuers or by some other means to impede or prevent the rescue activity. However, that would not apply in the case of medical personnel, units or transports collecting the sick or wounded, see chapter 7.
12.69.1. The mere fact that a rescue service saves enemy personnel as well as its own does not entitle it to immunity from attack. Once taken prisoner, such enemy personnel should be accorded all the rights of a prisoner of war, at least until their status has been determined. Arguably, any communication by downed aircrew with their national authorities constitutes a hostile act, which would at that point justify an attack upon that aircrew. Such justification may also arise from other conduct indicating that the downed individual is seeking to continue the fight.
Combat rescue at sea
12.70. A “downed” airm[a]n at sea falls within the definition of “shipwrecked” so that he must not be attacked if he refrains from hostile acts.
12.70.1. Although it can be argued that search and rescue by aircraft used exclusively for rescuing airmen “downed” at sea in areas either not controlled by friendly forces or not occupied by enemy forces should be regarded as a protected activity under the provisions of Geneva Convention II 1949, it has been the general practice not to afford such protection. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 12.67–12.70.1.