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Landing is the last part of a flight, where a flying animal, aircraft, or spacecraft returns to the ground. When the flying object returns to water, the process is called alighting, although it is commonly called "landing" and "touchdown" as well. A normal aircraft flight would include several parts of flight including taxi, takeoff, climb, cruise, descent and landing. This article describes the last portion of flight as the plane, bird, or rocket touches the ground. Landing occurs after descent.
While inflight, the four major forces acting on the object are; lift, thrust, gravity and drag. Flying is accomplished by generating enough lift to offset gravity to stay in the air. See the picture of the wing describing the four forces.
To land, the airspeed and the rate of descent are reduced to where the object descends at a slow enough rate to allow for a gentle touch down.
Each different type of flying object generates lift in a different manner. Airplanes, birds and flying insects use a wing. A bird generates thrust and lift by flapping its wings, and aircraft generate thrust with some form of an engine. The air passing over the wing of an aircraft generates lift. A helicopter uses rotary wings to generate lift and changes the angle of the rotor to generate thrust. Rockets or Vertical Jet engines are also commonly used on speciality aircraft to generate Lift. Air balloons use a lighter than air gas to generate buoyancy or lift.
The term landing is also applied to people or objects descending to the ground using a parachute. These objects are considered to be in a controlled descent instead of actually flying. A parachute works by capturing air inducing enough drag that the object that is falling hits the ground at a relatively slow speed. There are many examples of parachutes in nature including the seeds of a dandelion. People who intentionally land using a parachute are called parachutists.
Sometimes, a safe landing is accomplished by using multiple forms of lift, thrust and dampening systems. The lunar lander used a rocket, landing gear and the legs of the astronauts to land on the moon. Several Soviet rockets including the Soyuz have used parachutes and airbag landing systems to dampen the landing on earth.
Aircraft usually land at an airport on a firm runway or helicopter landing pad, generally constructed of asphalt concrete, concrete, gravel or grass. Aircraft equipped with pontoons are able to land on water. Aircraft also sometimes use skis to land on snow or ice.
For aircraft, landing is accomplished by slowing down and descending to the runway. This speed reduction is accomplished by reducing thrust and/or inducing a greater amount of drag using flaps, landing gear or speed brakes. As the plane approaches the ground, the pilot will execute a flare to induce a gentle landing.
A flare is performed by rotating the wings where the rate of descent will be reduced often by adopting a nose-up attitude. The attitude is held until the undercarriage touches the ground, and the controls are either held until all wheels touch the ground or gently adjusted (in the case of tail-draggers) to ensure the nose-wheel or tail-wheel lightly touches the runway.
In a small plane, with little crosswind, it is considered a "perfect" landing when contact with the ground occurs as the forward speed is reduced to the point where there is no longer sufficient airspeed to remain aloft. The stall warning is often heard just before landing indicating that this speed and altitude have been reached. The effect causes a very light touch down for the pilot and passengers.
In large transport category (airliner), aircraft pilots land the aircraft by "flying the airplane on to the runway." The airspeed and attitude of the plane are adjusted for landing. The airspeed is kept well above stall speed and at a constant rate of descent. A flare is performed just before landing and the descent rate is significantly reduced causing a light touch down. Upon touchdown, spoilers (sometimes called "lift dumpers") are deployed to dramatically reduce the lift and transfer the aircraft's weight to its wheels, where mechanical braking, such as an autobrake system, can take effect. Reverse thrust is used by many jet aircraft to help slow down just after touch-down, redirecting engine exhaust forward instead of back. Some propeller planes also have this feature, where the blades of the propeller are re-angled to push air forward instead of back.
Factors such as crosswind where the pilot will use a crab landing or a slip landing will cause pilots to land slightly faster and sometimes with different attitudes to ensure proper handling and safety of the plane. Other factors affecting a particular landing might include some or all of the following partial list; the plane size, wind, weight, runway length, obstacles, ground effects, weather, runway altitude, air temperature, air pressure, air traffic control, visibility, avionics, and the overall situation, et cetera.
For example landing, a multi-engine turboprop military (C-130 Hercules) under fire in a grass field in a war zone, requires different skills and precautions than landing a single engine plane (Cessna 150) on a paved runway in uncontrolled airspace, which is different from landing an airliner (Airbus A380) at a major airport with the support of air traffic control.
Pilots follow a course of training to develop the experience to routinely land in each situation. Professional pilots have extensive training, experience, and certification on the types of planes they are flying.
This text uses material from Wikipedia, licensed under CC BY-SA