Navigation Modes (GPS, VOR, LOC, BC)
When making a VOR or LOC approach, a valid GPS position is required.
The selected navigation receiver must have a valid VOR or LOC signal or active GPS course for the flight director to enter Navigation Mode.
When intercepting a flight plan leg, the flight director gives commands to capture the active leg at approximately a 45° angle to the track between the waypoints defining the active leg. The flight director does not give commands fly to the starting waypoint of the active leg.
When making a backcourse approach, set the Selected Course to the localizer front course.
Pressing the NAV Key selects Navigation Mode. Navigation Mode acquires and tracks the selected navigation source (GPS, VOR, or LOC). The flight director follows FMS roll steering commands when GPS is the selected navigation source. When the navigation source is VOR or LOC, the flight director creates roll steering commands from the Selected Course and deviation. Navigation Mode can also be used (instead of using Approach Mode) to fly non-precision RNAV (GPS) and Localizer approaches where glidepath/glideslope capture is either not required or desired. Navigation Mode can also be used to fly LOC Backcourse approaches (as described later in this section).
Pressing the NAV Key arms or activates the Navigation Mode corresponding to the CDI navigation source selected. An armed Navigation Mode annunciation appears in white below the active lateral FD mode.
Navigation Mode will engage/activate when avionics activation criteria are met. Navigation Mode activation is dependent on factors such as airspeed, intercept angle, and closure rate. If activation criteria are met upon NAV Key press, then navigation mode will activate immediately.
When the selected navigation source is LOC and the NAV Key is pressed, either Localizer Navigation or Backcourse Navigation Mode (annunciated as ‘BC’) may be selected. Which mode is selected, though, depends on aircraft heading. While heading is within 105° of a tuned localizer front course, LOC Mode will be armed (and subsequently activated upon localizer intercept). Conversely, while aircraft heading exceeds 105° of the localizer front course (i.e., heading is within 75° of the localizer back course), BC Mode will be armed (and subsequently activated upon localizer backcourse intercept).
Armed LOC Mode may switch to armed BC Mode (and vice versa) as the aircraft maneuvers toward the final approach course (e.g., while being vectored to final by ATC). Once either mode becomes activated (i.e., the LOC front- or backcourse is captured), the FD lateral mode will no longer switch based on aircraft heading.
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A different VOR or LOC is tuned while in VOR or LOC Navigation Mode (VOR/LOC Mode reverts to armed).
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The navigation source is manually switched.
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During a LOC/ILS approach, the FAF is crossed after the automatic navigation source switch from FMS to LOC but while FMS Navigation Mode is still active.