Related Rule
United States of America
Practice Relating to Rule 7. The Principle of Distinction between Civilian Objects and Military Objectives
Section E. Attacks against civilian means of transportation
With respect to civil aircraft, the US Air Force Pamphlet (1976) states:
If identified as a civil aircraft, air transport in flight should not be the object of attack, unless at the time it represents a valid military objective such as when there is an immediate military threat or use. An unauthorized entry into a flight restriction zone might in some conflicts be deemed an immediate military threat. Wherever encountered, enemy civil aircraft are subject to instruction in order to verify status and preclude their involvement … Civil aircraft on the ground, as objects of attack, are governed by the rules of what constitutes a legitimate military objective as well as the rules and principles relative to aerial bombardment. As sources of airlift they may, under the circumstances ruling at the time, qualify as important military objectives. Civil aircraft entitled to protection include nonmilitary state aircraft and a state owned airline. The principle of law and humanity protecting civilians and civilian objects from being objects of attack as such, protects civil aircraft in flight, because civil aircraft are presumed to transport civilians. Such an aircraft is not subject to attack in the absence of a determination that it constitutes a valid military objective. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 4-3.
The US Air Force Commander’s Handbook (1980) states: “Civilian vehicles, aircraft, vessels … may be the object of attack if they have combatant personnel in them and if collateral damage would not be excessive under the circumstances.” 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 2-2.
The US Naval Handbook (1995) provides:
Civil passenger vessels at sea and civil airliners in flight are subject to capture but are exempt from destruction. Although enemy lines of communication are generally legitimate military targets in modern warfare, civilian passenger vessels at sea, and civil airliners in flight, are exempt from destruction, unless at the time of the encounter they are being utilized by the enemy for a military purpose (e.g., transporting troops or military cargo) or refuse to respond to the directions of the intercepting warship or military aircraft. Such passenger vessels in port and airliners on the ground are not protected from destruction. 
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), § 8.2.3(6).
The US Naval Handbook (2007) states:
Civilian passenger vessels at sea and civil airliners in flight are subject to capture but are exempt from destruction. Although enemy lines of communication are generally legitimate military targets in modern warfare, civilian passenger vessels at sea, and civil airliners in flight, are exempt from destruction, unless at the time of the encounter they are being utilized by the enemy for a military purpose (e.g., transporting troops or military cargo) or refuse to respond to the directions of the intercepting warship or military aircraft. Such passenger vessels in port and airliners on the ground are not protected from destruction. 
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, § 8.6.3.
Following investigations by the Secretary-General of the International Civil Aviation Organization (ICAO) into the shooting down of two civil aircraft by the Cuban air force on 24 February 1996, a debate took place on 26 July 1996 in the UN Security Council, during which the United States stated: “Cuba violated the principle of customary law that States must refrain from resorting to the use of weapons against civil aircraft in flight – a principle that applies whether the aircraft are in national or international airspace”. According to the United States, an attack against a civilian aircraft in flight violates elementary considerations of humanity. 
United States, Statement before the UN Security Council, UN Doc. S/PV.3683, 26 July 1996, p. 3.