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determine if the glidepath will result in an undershoot
or overshoot, but, taking into account float during
roundout, the pilot will be able to predict the touchdown
point to within a very few feet.
For a constant angle glidepath, the distance between
the horizon and the aiming point will remain constant.
If a final approach descent has been established but 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 (aiming
point 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.
A runway, obviously, 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.
If the airplane continues down the glidepath at a
constant angle (stabilized), the image the pilot sees
will still be trapezoidal but of proportionately larger
dimensions. In other words, during a stabilized
approach the runway shape does not change. [Figure
8-10]
If the approach becomes shallower, however, the
runway will appear to shorten and become wider.
Conversely, if the approach is steepened, the runway
will appear to become longer and narrower.
[Figure 8-11]
The objective of a stabilized approach is to select an
appropriate touchdown point on the runway, and
adjust the glidepath so that the true aiming point and
the desired touchdown point basically coincide.
Immediately after rolling out on final approach, the
pilot should adjust the pitch attitude and power so that
the airplane is descending directly toward the aiming
point at the appropriate airspeed. The airplane should
Distance Traveled in Flare
Touchdown
Aiming Point (Descent Angle Intersects Ground)
Figure 8-9. Stabilized approach.
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be in the landing configuration, and trimmed for
“hands off” flight. With the approach set up in this
manner, the pilot will be free to devote full attention
toward outside references. The pilot should 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.
In this way, the pilot will be more apt to perceive a
deviation from the desired glidepath, and whether or
not the airplane is proceeding directly toward the
aiming point.
If the pilot perceives any indication that the aiming
point on the runway is not where desired, an adjustment
must be made to the glidepath. This in turn will move
the aiming point. For instance, if the pilot perceives that
the aiming point is short of the desired touchdown
point and will result in an undershoot, an increase in
pitch attitude and engine power is warranted. Aconstant
airspeed must be maintained. The pitch and power
change, therefore, must be made smoothly and
simultaneously. This will result in a shallowing of
the glidepath with the resultant aiming point moving
towards the desired touchdown point. Conversely,
if the pilot perceives that the aiming point is farther
down the runway than the desired touchdown point
and will result in an overshoot, the glidepath should be
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
glidepath be detected early, so that only slight and
infrequent adjustments to glidepath are required.
3°Approach Angle
4000' x 100' Runway
1600' From Threshold
105' Altitude
Same Runway, Same Approach Angle
800' From Threshold
52' Altitude
Same Runway, Same Approach Angle
400' From Threshold
26' Altitude
Figure 8-10. Runway shape during stabilized approach.
Too High
Proper Descent Angle
Too Low
Figure 8-11. Change in runway shape if approach becomes
narrow or steep.
8-9
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AIRPLANE FLYING HANDBOOK 飞机飞行手册上(84)
