Ground loop (aviation)
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aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot air ...
, a ground loop is a rapid
rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
of a
fixed-wing aircraft A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinc ...
in the
horizontal plane In astronomy, geography, and related sciences and contexts, a '' direction'' or ''plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point. Conversely, a direction or plane is said to be hor ...
( yawing) while on the ground.
Aerodynamic Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
forces may cause the advancing
wing A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expres ...
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 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 undercarriage consisting of two main wheels forward of the center of gravity and a small wheel or skid to support the tail.Crane, Dale: ''Dictionary of Aeronautical Term ...
, due to the
centre of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
being positioned behind the main wheels. It may also occur with
tricycle landing gear Tricycle gear is a type of aircraft undercarriage, or ''landing gear'', arranged in a tricycle fashion. The tricycle arrangement has a single nose wheel in the front, and two or more main wheels slightly aft of the center of gravity. Tricycle g ...
if excessive load is applied to the nosewheel, a condition known as
wheel-barrowing Wheel-barrowing is a problem that may occur in an aeroplane with a tricycle gear configuration during takeoff or landing. As the aeroplane gains speed during takeoff the wing generates an increasing amount of lift although not enough to raise the ...
. 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 Taxiing (rarely spelled taxying) is the movement of an aircraft on the ground, under its own power, in contrast to towing or pushback where the aircraft is moved by a tug. The aircraft usually moves on wheels, but the term also includes aircra ...
,
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 ...
, 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 t ...
. Ground loops can damage the
undercarriage Undercarriage is the part of a moving vehicle that is underneath the main body of the vehicle. The term originally applied to this part of a horse-drawn carriage, and usage has since broadened to include: *The landing gear of an aircraft. *The ch ...
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 Pan Am Flight 121 was a scheduled Pan American World Airways flight from Karachi to Istanbul. On the evening of June 18, 1947, the Lockheed L-049 Constellation serving the flight, known as the ''Clipper Eclipse'' (previously ''Clipper Dublin''), ...
, 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. 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 China Airlines Flight 605 (callsign "Dynasty 605") was a daily non-stop flight departing from Taipei at 6:30 a.m. and arriving at Kai Tak Airport in Hong Kong at 7:00 a.m. local time. On November 4, 1993, the plane went off the runwa ...
. 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