Related Rule
Practice Relating to Rule 29. Medical Transports
Section B. Respect for and protection of medical aircraft
Belgium’s Law of War Manual (1983) states:
Medical aircraft are immune from attack during the flights agreed upon beforehand between belligerents. They may not fly over enemy controlled or occupied territory without authorization. They must obey each order to land … No authorization is necessary to fly over territory controlled by one’s own forces. Medical aircraft are still protected above contact zones, but the risk of sustaining damage are bigger in the absence of an agreement. 
Belgium, Droit Pénal et Disciplinaire Militaire et Droit de la Guerre, Deuxième Partie, Droit de la Guerre, Ecole Royale Militaire, par J. Maes, Chargé de cours, Avocat-général près la Cour Militaire, D/1983/1187/029, 1983, p. 49.