Heavily forested areas, swamps, city rooftops, and other difficult terrains present obstacles to helicopter landings. Flat areas with deep snow are problems also, as the helicopter's downdraft causes a ground blizzard which obstructs the pilot's vision. U.S.Army Alaska has devised a method to deliver troops without landing: they rappel from a hovering helicopter.

United States Army aviation digest (alternate titles: U.S. Army aviation digest; Army aviation digest; Aviation digest) was published February 1955 -March/April 1995.