The landing gear is the undercarriage assembly that supports an aircraft when it is on the ground and consists of wheels, tires, brakes, shocks, axles and other support structures. Virtually all jet aircraft have a nose wheel with two tires, plus two or more main gear assemblies with as many as 16 tires. The landing gear is usually raised and lowered hydraulically and fits completely within the lower fuselage when retracted. Aircraft tires are filled with nitrogen rather than air because nitrogen does not expand or contract as much as air during extreme temperature changes, thus reducing the chances of a tire blowout. Engines The exact number of engines on an airplane is determined by the power and performance requirements of the aircraft.
Most jet airplanes have two, three or four engines, depending on aircraft size.
Some have the engines attached to the rear of the fuselage. Many have them mounted on pylons, hanging below the wings. Some have a combination of both, with an engine under each wing and one on top of the fuselage at the rear of the plane. The power produced by the engines is controlled by the pilots, either directly or indirectly, through computerized controls. All large airliners are designed to fly safely on fewer than all engines.