13-2 Figure 13-2.  An amphibious aircraft with retractable wheels (left) and an aircraft with retractable skis (right). Figure 13-1.  Basic landing gear types include those with wheels (a), skids (b), skis (c), and floats or pontoons (d).  Landing Gear Arrangement NOTE: References to auxiliary landing gear refer to the  nose gear, tail gear, or outrigger-type gear on any particular  aircraft. Main landing gear are the two or more large gear  located close to the aircraft’s center of gravity. Three basic arrangements of landing gear are used: tail wheel- type landing gear (also known as conventional gear), tandem  landing gear, and tricycle-type landing gear. Numerous configurations of landing gear types can be  found. Additionally, combinations of two types of gear are  common. Amphibious aircraft are designed with gear that  allow landings to be made on water or dry land. The gear  features pontoons for water landing with extendable wheels  for landings on hard surfaces. A similar system is used to  allow the use of skis and wheels on aircraft that operate on  both slippery, frozen surfaces and dry runways. Typically, the  skis are retractable to allow use of the wheels when needed.  Figure 13-2  illustrates this type of landing gear. 