Related Rule
Practice Relating to Rule 7. The Principle of Distinction between Civilian Objects and Military Objectives
Section E. Attacks against civilian means of transportation
Croatia’s LOAC Compendium (1991) provides that “civilian aircraft escorted by enemy military aircraft” and “civilian aircraft that refuse to modify their routes, land or alight on water if so ordered and after warning” are proper targets in the air. The manual adds that “civilian aircraft that do not violate the airspace of a belligerent” are protected aircraft. 
Croatia, Compendium “Law of Armed Conflicts”, Republic of Croatia, Ministry of Defence, 1991, p. 44.