Ground loop (aviation)
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aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' include fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air aircraft such as h ...
, a ground loop is a rapid
rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...
of a
fixed-wing aircraft A fixed-wing aircraft is a heavier-than-air aircraft, such as an airplane, which is capable of flight using aerodynamic lift. Fixed-wing aircraft are distinct from rotary-wing aircraft (in which a rotor mounted on a spinning shaft generate ...
in the
horizontal plane Horizontal may refer to: *Horizontal plane, in astronomy, geography, geometry and other sciences and contexts *Horizontal coordinate system, in astronomy *Horizontalism, in monetary circuit theory *Horizontalidad, Horizontalism, in sociology *Hor ...
( yawing) while on the ground.
Aerodynamic Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...
forces may cause the advancing
wing A wing is a type of fin that produces both Lift (force), lift and drag while moving through air. Wings are defined by two shape characteristics, an airfoil section and a planform (aeronautics), planform. Wing efficiency is expressed as lift-to-d ...
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 their early gliding experiments, the
Wright Brothers The Wright brothers, Orville Wright (August 19, 1871 – January 30, 1948) and Wilbur Wright (April 16, 1867 – May 30, 1912), were American aviation List of aviation pioneers, pioneers generally credited with inventing, building, and flyin ...
referred to this action as ''well-digging''.


Looping phenomenon

In powered aeroplanes, the ground loop phenomenon is predominantly associated with aircraft that have
conventional landing gear Conventional landing gear, or tailwheel-type landing gear, is an aircraft Landing gear, undercarriage consisting of two main wheels forward of the Center of gravity of an aircraft, center of gravity and a small wheel or skid to support the tail ...
, due to the centre 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 different from the aircraft's direction of motion, a sideways force is exerted on the wheels. If this force is in front of the centre 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.


Contributing factors

Ground loops occur when the aircraft is moving on the ground—either taxiing,
landing 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", "touchdown" or " spl ...
, or during
takeoff Takeoff is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For aircraft traveling vertically, this is known as liftoff. For aircraft that take off horizontally, this usually involves starting with a tr ...
. Ground loops can damage the undercarriage and wingtips of an aircraft. Several extreme incidents of ground loop have resulted in fatalities. In the case of the 1947 crash of Pan Am Flight 121, Captain Michael Graham, one of the surviving passengers, said that the landing would have been successful had an engine on the port wing not dug into the ground, dragging the plane in that direction in a ground loop and breaking it in two. 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. A controlled ground loop may also be used as a rudimentary form of emergency braking while landing, "in case one is still rolling too fast to stop." According to Robinet, "The pilot would merely hold the right brake (in this case...no place on the left) harder than the left and wishes the airplane around in a tight turn on the ground. Another way of putting it, the airplane swaps ends. This is a ground loop." Gliders commencing a takeoff behind a tow plane are vulnerable to ground looping during cross-wind conditions because the slipstream from the propeller of the tow plane generates more lift on the downwind wing of the glider than on the upwind wing. If the flight controls are unable to overcome the rolling tendency at this low speed, the upwind wingtip will contact the ground and initiate a ground loop; the glider pilot must release the tow rope to abandon the takeoff. Gliders with a large main wheel and a tail wheel or tail skid are particularly susceptible to this form of ground looping during cross-wind takeoffs because of the large angle of attack on the wing. Gliders with a nose wheel or nose skid cause the wing to present a lower angle of attack at the commencement of the take off roll and are much less susceptible to this form of ground looping. Tow plane pilots are taught to delay applying full power until the glider is moving fast enough that its tail is off the ground, reducing the angle of attack on the wing.


Intentional looping

Pilots may decide to execute a ground loop deliberately, usually as a last resort before hitting an immovable object, as in the case of China Airlines Flight 605. In such cases, energy may be dissipated by damaging the wings of the aircraft to protect the occupants seated in the fuselage.


References

{{reflist Aviation risks Gliding technology