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A stabilized approach
is one in which the pilot establishes and maintains
a constant angle glide path towards a predetermined
point on the landing runway. It is based on the pilot’s
judgment of certain visual clues and depends on the
maintenance of a constant final descent airspeed and
configuration.
An airplane descending on final approach at a constant
rate and airspeed is traveling in a straight line toward
a spot on the ground ahead. This spot is not the spot
on which the airplane touches down because some float
occurs during the round out (flare). Neither is it the
spot toward which the airplane’s nose is pointed
because the airplane is flying at a fairly high AOA,
and the component of lift exerted parallel to the Earth’s
surface by the wings tends to carry the airplane forward
horizontally.
The point toward which the airplane is progressing is
termed the “aiming point.” It is the point
on the ground at which, if the airplane maintains a
constant glide path and was not flared for landing,
it would strike the ground. To a pilot moving straight
ahead toward an object, it appears to be stationary.
It does not appear to move under the nose of the aircraft
and does not appear to move forward away from the aircraft.
This is how the aiming point can be distinguished—it
does not move. However, objects in front of and beyond
the aiming point do appear to move as the distance is
closed, and they appear to move in opposite directions.
During instruction in landings, one of the most important
skills a pilot must acquire is how to use visual cues
to accurately determine the true aiming point from any
distance out on final approach. From this, the pilot
is not only able to determine if the glide path results
in either an under or overshoot but, taking into account
float during round out, the pilot is able to predict
the touchdown point to within a few feet.
For a constant angle glide path, the distance between
the horizon and the aiming point remains constant. If
a final approach descent is established and the distance
between the perceived aiming point and the horizon appears
to increase (aiming point moving down away from the
horizon), then the true aiming point, and subsequent
touchdown point, is farther down the runway. If the
distance between the perceived aiming point and the
horizon decreases, meaning that the aiming point is
moving up toward the horizon, the true aiming point
is closer than perceived.
When the airplane is established on final approach,
the shape of the runway image also presents clues as
to what must be done to maintain a stabilized approach
to a safe landing.
Obviously, runway is normally shaped in the form of
an elongated rectangle. When viewed from the air during
the approach, the phenomenon known as perspective causes
the runway to assume the shape of a trapezoid with the
far end looking narrower than the approach end and the
edge lines converging ahead.
As an airplane continues down the glide path at a constant
angle (stabilized), the image the pilot sees is still
trapezoidal but of proportionately larger dimensions.
In other words, during a stabilized approach, the runway
shape does not change.
If the approach becomes shallow, the runway appears
to shorten and become wider. Conversely, if the approach
is steepened, the runway appears to become longer and
narrower.
The objective of a stabilized approach is to select
an appropriate touchdown point on the runway, and adjust
the glide path so that the true aiming point and the
desired touchdown point basically coincide. Immediately
after rolling out on final approach, adjust the pitch
attitude and power so that the airplane is descending
directly toward the aiming point at the appropriate
airspeed, in the landing configuration, and trimmed
for “hands off” flight. With the approach
set up in this manner, the pilot is free to devote full
attention toward outside references. Do not stare at
any one place, but rather scan from one point to another,
such as from the aiming point to the horizon, to the
trees and bushes along the runway, to an area well short
of the runway, and back to the aiming point. This makes
it easier to perceive a deviation from the desired glide
path and determine if the airplane is proceeding directly
toward the aiming point.
If there is any indication that the aiming point on
the runway is not where desired, an adjustment must
be made to the glide path. This in turn moves the aiming
point. For instance, if the aiming point is short of
the desired touchdown point and results in an undershoot,
an increase in pitch attitude and engine power is warranted.
A constant airspeed must be maintained. The pitch and
power change, therefore, must be made smoothly and simultaneously.
This results in a shallowing of the glide path with
the aiming point moving towards the desired touchdown
point. Conversely, if the aiming point is farther down
the runway than the desired touchdown point resulting
in an overshoot, the glide path is steepened by a simultaneous
decrease in pitch attitude and power. Once again, the
airspeed must be held constant. It is essential that
deviations from the desired glide path be detected early
so that only slight and infrequent adjustments to glide
path are required.
The closer the airplane gets to the runway, the larger
and more frequent the required corrections become, resulting
in an unstable approach.
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