All aircraft are affected to some degree by this characteristic,
although they may be inherently stable in all other normal
parameters. This tendency explains why an aircraft cannot
be flown "hands off" indefinitely.

Much research has gone into the development of control
devices (wing leveler) to correct or eliminate this instability.
The pilot must be careful in application of recovery controls
during advanced stages of this spiral condition or excessive
loads may be imposed on the structure. Improper recovery
from spiral instability leading to inflight structural failures
has probably contributed to more fatalities in general aviation
aircraft than any other factor. Since the airspeed in the spiral
condition builds up rapidly, the application of back elevator
force to reduce this speed and to pull the nose up only
"tightens the turn," increasing the load factor. The results
of the prolonged uncontrolled spiral are inflight structural
failure or crashing into the ground, or both. The most common
recorded causes for pilots who get into this situation are:
loss of horizon reference, inability to control the aircraft by
reference to instruments, or a combination of both.

Aerodynamic Forces in Flight Maneuvers

Forces in Turns
If an aircraft were viewed in straight-and-level flight from
the front [Figure 4-28], and if the forces acting on the aircraft
could be seen, lift and weight would be apparent: two forces.
If the aircraft were in a bank it would be apparent that lift
did not act directly opposite to the weight, rather it now acts
in the direction of the bank. A basic truth about turns: when
the aircraft banks, lift acts inward toward the center of the
turn, as well as upward.

Newton's First Law of Motion, the Law of Inertia, states
that an object at rest or moving in a straight line remains
at rest or continues to move in a straight line until acted on
by some other force. An aircraft, like any moving object,
requires a sideward force to make it turn. In a normal turn,
this force is supplied by banking the aircraft so that lift is
exerted inward, as well as upward. The force of lift during a
turn is separated into two components at right angles to each
other. One component, which acts vertically and opposite
to the weight (gravity), is called the "vertical component of
lift." The other, which acts horizontally toward the center
of the turn, is called the "horizontal component of lift," or
centripetal force. The horizontal component of lift is the force
that pulls the aircraft from a straight flightpath to make it
turn. Centrifugal force is the "equal and opposite reaction"
of the aircraft to the change in direction and acts equal and
opposite to the horizontal component of lift. This explains
why, in a correctly executed turn, the force that turns the
aircraft is not supplied by the rudder. The rudder is used to
correct any deviation between the straight track of the nose
and tail of the aircraft. A good turn is one in which the nose
and tail of the aircraft track along the same path. If no rudder
is used in a turn, the nose of the aircraft yaws to the outside
of the turn. The rudder is used to bring the nose back in line
with the relative wind.

An aircraft is not steered like a boat or an automobile. In
order for an aircraft to turn, it must be banked. If it is not
banked, there is no force available to cause it to deviate from
a straight flightpath. Conversely, when an aircraft is banked,
it turns, provided it is not slipping to the inside of the turn.