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
Ecuador’s Naval Manual (1989) 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. 
Ecuador, Aspectos Importantes del Derecho Internacional Marítimo que Deben Tener Presente los Comandantes de los Buques, Academia de Guerra Naval, 1989, § 8.2.3(6).