Ground loop (aviation)
In aviation, a ground loop is a rapid rotation of a fixed-wing aircraft in the horizontal plane while on the ground. Aerodynamic forces may cause the advancing wing to rise, which may then cause the other wingtip to touch the ground. In severe cases (particularly if the ground surface is soft), the inside wing can dig in, causing the aircraft to swing violently or even cartwheel.
In powered airplanes, the ground loop phenomenon is predominantly associated with aircraft that have conventional landing gear, due to the center of gravity being positioned behind the main wheels. It may also occur with tricycle landing gear if excessive load is applied to the nosewheel, a condition known as wheel-barrowing.
If the aircraft heading is slightly different from the aircraft's direction of motion, a side force is exerted on the wheels. If this force is in front of the center of gravity, the resulting moment rotates the aircraft's heading even further from its direction of motion. This increases the force and the process reinforces itself. To avoid a ground loop, the pilot must respond to any turning tendency quickly, while sufficient control authority is available to counteract it. Once the aircraft rotates beyond this point, there is nothing the pilot can do to stop it from rotating further.
Ground loops occur when the aircraft is moving on the ground—either taxiing, landing, or during takeoff. Ground loops can damage the undercarriage and wingtips of an aircraft. Several extreme incidents of ground loop have resulted in fatalities.
Ground loops may occur when landing on muddy ground, wet pavement, or frozen surfaces, especially if there are puddles or patches. They may also occur when an aircraft departs a paved surface: for example, after an engine failure in multi-engine airplanes produces asymmetric thrust. Another common cause is failure of a tire or wheel brake, causing a loss of directional control. They also commonly occur without outside influence, due entirely to pilot error.
In gliders, ground loops can also occur because there is only a single main wheel near the center of gravity, and because the long wings are close to the ground. Any tendency to touch a wingtip to the ground while moving must be quickly counteracted. The leverage from the long wings can apply great stress to the rear fuselage, sometimes breaking the tail boom, especially on T-tail gliders.
Pilots may decide to execute a ground loop deliberately, usually as a last resort before hitting an immovable object. In this case it may be a better option to dissipate energy by damaging the wings of the aircraft to protect the occupants seated in the fuselage.
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