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unless the pilot takes a prompt corrective action. Proper pitch
and power coordination is critical in this region due to speed
instability and the tendency of increased divergence from
the desired speed.
Large Airplanes
Pilots of larger airplanes with higher stall speeds may find the
speed they maintain on the instrument approach is near 1.3
VSO, putting them near point C [Figure 2-10] the entire time
the airplane is on the final approach segment. In this case,
precise speed control is necessary throughout the approach. It
may be necessary to temporarily select excessive, or deficient
thrust in relation to the target thrust setting in order to quickly
correct for airspeed deviations.
most common device used on general aviation light aircraft
is the vortex generator. Small strips of metal placed along
the wing (usually in front of the control surfaces) create
turbulence. The turbulence in turn mixes high energy air from
outside the boundary layer with boundary layer air. The effect
is similar to other boundary layer devices. [Figure 2-12]
Small Airplanes
Most small airplanes maintain a speed well in excess of 1.3
times VSO on an instrument approach. An airplane with a
stall speed of 50 knots (VSO) has a normal approach speed
of 65 knots. However, this same airplane may maintain 90
knots (1.8 VSO) while on the final segment of an instrument
approach. The landing gear will most likely be extended at
the beginning of the descent to the minimum descent altitude,
or upon intercepting the glide slope of the instrument landing
system. The pilot may also select an intermediate flap setting
for this phase of the approach. The airplane at this speed has
good positive speed stability, as represented by point A on
Figure 2-10. Flying in this regime permits the pilot to make
slight pitch changes without changing power settings, and
accept minor speed changes knowing that when the pitch is
returned to the initial setting, the speed returns to the original
setting. This reduces the pilot’s workload.
Aircraft are usually slowed to a normal landing speed when
on the final approach just prior to landing. When slowed to
65 knots, (1.3 VSO), the airplane will be close to point C.
[Figure 2-10] At this point, precise control of the pitch and
power becomes more crucial for maintaining the correct speed.
Pitch and power coordination is necessary because the speed
stability is relatively neutral since the speed tends to remain
at the new value and not return to the original setting. In
addition to the need for more precise airspeed control, the pilot
normally changes the aircraft’s configuration by extending
landing flaps. This configuration change means the pilot must
be alert to unwanted pitch changes at a low altitude.
2-10
Figure 2-13. Forces In a Turn.
Turns
Like any moving object, an aircraft requires a sideward force
to make it turn. In a normal turn, this force is supplied by
banking the aircraft in order to exert lift inward, as well as
upward. The force of lift is separated into two components
at right angles to each other. [Figure 2-13] The upward
acting lift together with the opposing weight becomes the
vertical lift component. The horizontally acting lift and its
opposing centrifugal force are the horizontal lift component,
or centripetal force. This horizontal lift component is the
sideward force that causes an aircraft to turn. The equal and
opposite reaction to this sideward force is centrifugal force,
which is merely an apparent force as a result of inertia.
The relationship between the aircraft’s speed and bank angle
to the rate and radius of turns is important for instrument
pilots to understand. The pilot can use this knowledge to
properly estimate bank angles needed for certain rates of
turn, or to determine how much to lead when intercepting
a course.
Rate of Turn
The rate of turn, normally measured in degrees per second,
is based upon a set bank angle at a set speed. If either one of
these elements changes, the rate of turn changes. If the aircraft
increases its speed without changing the bank angle, the rate
of turn decreases. Likewise, if the speed decreases without
changing the bank angle, the rate of turn increases.
Changing the bank angle without changing speed also causes
the rate of turn to change. Increasing the bank angle without
changing speed increases the rate of turn, while decreasing
the bank angle reduces the rate of turn.
For example, a pilot is on an instrument approach at 1.3
VSO, a speed near L/DMAX, and knows that a certain power
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Instrument Flying Handbook仪表飞行手册上(34)
