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If the pilot has not taken adequate corrective action to
avoid drift during a crosswind landing, the main
wheels’ tire tread offers resistance to the airplane’s
sideward movement in respect to the ground.
Consequently, any sidewise velocity of the airplane is
abruptly decelerated, with the result that the inertia
force is as shown in figure 8-38. This creates a moment
around the main wheel when it contacts the ground,
tending to overturn or tip the airplane. If the windward
wingtip is raised by the action of this moment, all the
weight and shock of landing will be borne by one main
wheel. This could cause structural damage.
Figure 8-38. Drifting during touchdown.
Not only are the same factors present that are attempting
to raise a wing, but the crosswind is also acting on
the fuselage surface behind the main wheels, tending
to yaw (weathervane) the airplane into the wind. This
often results in a ground loop.
GROUND LOOP
A ground loop is an uncontrolled turn during ground
operation that may occur while taxiing or taking off,
but especially during the after-landing roll. Drift or
weathervaning does not always cause a ground loop,
although these things may cause the initial swerve.
Careless use of the rudder, an uneven ground surface,
or a soft spot that retards one main wheel of the airplane
may also cause a swerve. In any case, the initial
swerve tends to make the airplane ground loop,
whether it is a tailwheel-type or nosewheel-type.
[Figure 8-39]
Nosewheel-type airplanes are somewhat less prone to
ground loop than tailwheel-type airplanes. Since the
center of gravity (CG) is located forward of the main
landing gear on these airplanes, any time a swerve
develops, centrifugal force acting on the CG will tend
to stop the swerving action.
If the airplane touches down while drifting or in a crab,
the pilot should apply aileron toward the high wing and
stop the swerve with the rudder. Brakes should be used
to correct for turns or swerves only when the rudder is
inadequate. The pilot must exercise caution when
applying corrective brake action because it is very easy
to overcontrol and aggravate the situation.
If brakes are used, sufficient brake should be applied
on the low-wing wheel (outside of the turn) to stop the
swerve. When the wings are approximately level, the
new direction must be maintained until the airplane has
slowed to taxi speed or has stopped.
In nosewheel airplanes, a ground loop is almost always
a result of wheelbarrowing. The pilot must be aware that
even though the nosewheel-type airplane is less prone
than the tailwheel-type airplane, virtually every type of
airplane, including large multiengine airplanes, can be
made to ground loop when sufficiently mishandled.
WING RISING AFTER TOUCHDOWN
When landing in a crosswind, there may be instances
when a wing will rise during the after-landing roll. This
may occur whether or not there is a loss of directional
Wind Force
Center of
Gravity
Force Resisting
Side Motion
Inertia Force
Weight
Airplane Tips
and Swerves
CG Continues Moving in
Same Direction of Drift
Touchdown
Roundout
Roundout
Figure 8-39. Start of a ground loop.
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control, depending on the amount of crosswind and the
degree of corrective action.
Any time an airplane is rolling on the ground in a
crosswind condition, the upwind wing is receiving a
greater force from the wind than the downwind wing.
This causes a lift differential. Also, as the upwind wing
rises, there is an increase in the angle of attack, which
increases lift on the upwind wing, rolling the airplane
downwind.
When the effects of these two factors are great enough,
the upwind wing may rise even though directional
control is maintained. If no correction is applied, it is
possible that the upwind wing will rise sufficiently to
cause the downwind wing to strike the ground.
In the event a wing starts to rise during the landing roll,
the pilot should immediately apply more aileron pressure
toward the high wing and continue to maintain
direction. The sooner the aileron control is applied,
the more effective it will be. The further a wing is
allowed to rise before taking corrective action, the
more airplane surface is exposed to the force of the
crosswind. This diminishes the effectiveness of the
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AIRPLANE FLYING HANDBOOK 飞机飞行手册上(103)
