3-4 3rd Ed. Sep 30, 06
S–TEC
If the Course Deviation Indication (CDI) is at full scale (100%) needle deflection
from center, then the autopilot will establish the aircraft on a 45° intercept
angle relative to the selected course. Even if CDI needle deflection is less than
100%, the autopilot may still establish an intercept angle of 45°, provided that
the aircraft's closure rate to the selected course is sufficiently slow. Otherwise,
the intercept angle will be less than 45°.
As the aircraft approaches the selected course, the autopilot senses the
corresponding rate at which CDI needle deflection approaches center (closure
rate), in order to initiate the aircraft's turn onto the course at the proper point, and
thereby prevent overshoot. The point at which this turn begins is variable, being
further from the course at faster closure rates, and closer to the course at slower
closure rates. Although closure rate is principally a function of groundspeed, the
distance of the aircraft from the VOR station also has an effect. Nevertheless,
the turn will always begin between 100% and 20% CDI needle deflection.
During this stage of the intercept sequence, the autopilot operates at maximum
gain and sensitivity to closure rate. In addition, it limits the aircraft's turn rate to
90% of a standard rate turn, although for some higher performance (turboprop)
aircraft this is 75%.
When the aircraft arrives at 15% CDI needle deflection, the course is captured.
At that instant, a step reduction in autopilot gain occurs, so that the Course
Pointer (HSI) or Heading Bug (DG) has sufficient authority to complete the
intercept. In addition, the sensitivity to closure rate is reduced. The overall
authority of the autopilot during this stage of the intercept sequence is called the
CAP condition, which is acknowledged only on the optional Remote Annunciator,
as shown in Fig. 3-3.
Fifteen seconds after course capture, a second step reduction in autopilot gain
occurs, to limit the aircraft's turn rate to 45% of a standard rate turn, although for
some higher performance (turboprop) aircraft this is 37.5%. In addition, the
sensitivity to closure rate is reduced again. The overall authority of the autopilot
during this stage of the intercept sequence is called the CAP SOFT condition,
which is acknowledged only on the optional Remote Annunciator, as shown in
Fig. 3-4.
Thirty seconds after course capture, the autopilot establishes the required
crosswind correction angle.
Seventy five seconds after course capture, a third step reduction in autopilot
gain occurs, to limit the aircraft's turn rate to 15% of a standard rate turn,
although for some higher performance (turboprop) aircraft this is 12.5%. In
addition, the sensitivity to closure rate is reduced once more. This is
acknowledged by the extinguishment of the CAP annunciation on the optional
Remote Annunciator, as shown in Fig. 3-5. It marks the end of the intercept
sequence, and the beginning of tracking. The overall authority of the autopilot
during tracking is called the SOFT condition.
The pilot should make speed, distance, and time considerations during the 75
second period from course capture to the beginning of tracking, to account for
the aircraft's position. For example at 115 kts, a distance of 2.4 nautical miles
will be traveled in 75 seconds.
If it should happen that the Course Pointer (HSI) or Heading Bug (DG) is already
within 5° of the selected course, and CDI needle deflection is less than 10%,
then the autopilot will immediately establish the SOFT condition upon
engagement of the navigation mode.
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