|CHAPTER 4. Automated Flight Control
The approach function is similar to the navigation mode, but flies the selected course with the higher degree of precision necessary for instrument approaches and allows glideslope tracking in the vertical dimension. Most autopilots feature a separate button that allows you to engage the approach function, as shown in Figure 4-14. (NOTE: Usually, this mode is not used with most GPS receivers. The GPS approach RNP (required navigation performance) of 0.3 induces the necessary flight tracking precision. This mode is used only if specifically stated as a command in the avionics handbook for that equipment in that aircraft).
Like the navigation function, the approach mode can be used to execute precision and nonprecision approaches that rely on types of ground-based navigation facilities (e.g., VOR, VOR/DME, and localizer approaches).
Coupled ILS approaches make use of the autopilot’s glideslope function. Figure 4-15 shows the procedure for one type of autopilot.
Note that you cannot directly arm or engage the glideslope function. The autopilot must usually be engaged first in the approach and altitude modes. When the FD/autopilot begins to sense the glideslope, the glideslope function will automatically arm. When the aircraft intercepts the glideslope, the glideslope function engages automatically, and uses the aircraft’s pitch control to remain on the glideslope. It is important to note that, generally, the glideslope function can capture the glideslope only from below or on glideslope.
RNAV Approaches With Vertical Guidance
Coupled RNAV approaches with vertical guidance work in the same way as coupled ILS approaches. Lateral and vertical guidance commands are generated by the FMS/NAV and sent to the FD/autopilot. The same approach and glideslope functions of the autopilot are used in the same way to carry out the lateral and vertical guidance and control of the aircraft. This process is transparent to the pilot. Most “VNAV” functions do not qualify as approach vertical functions and many FMS/GPS units inhibit that function during approaches.
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