The FD/autopilot’s navigation function can be used to guide the aircraft along the course selected on the navigation indicator. Since the navigation display in most advanced avionics cockpits can present indications from a variety of navigation systems, you can use the autopilot’s navigation function to follow a route programmed into the FMS using VOR, global positioning system (GPS), inertial navigation system (INS), or other navigation data sources.
Following a Route Programmed in the FMS
Figure 4-6 demonstrates how to use the navigation function to follow a route programmed into the FMS. With the navigation function engaged, the FD/auto-pilot steers the aircraft along the desired course to the active waypoint. Deviations from the desired course to the new active waypoint are displayed on the navigation indicator. When the aircraft reaches the active waypoint, the FMS computer automatically sequences to the next waypoint in the route, unless waypoint sequencing is suspended.
It is important to note that the normal navigation function provides only lateral guidance. It does not attempt to control the vertical path of the aircraft at any time. You must always ensure the correct altitude or vertical speed is maintained.
When combined, use of the FMS and the FD/autopilot’s navigation function result in an automated form of flight that was formerly limited to very complex and expensive aircraft. This same level of avionics can now be found in single-engine training airplanes. While it is easy to be complacent and let down your guard, you must continuously monitor and stay aware of automated systems status and function and the track of the aircraft in relation to the flight plan and air traffic control (ATC) clearance.
GPS Steering (GPSS) Function
Many autopilots offer a global positioning system steering (GPSS) function. GPSS does all of the same actions as the navigation function, but achieves a higher degree of precision by accepting inputs directly from the GPS receiver. Consequently, the GPSS function follows the desired track to the active waypoint more aggressively, permitting only small excursions from the desired course. On some installations, pressing the autopilot NAV button twice engages the GPSS function.
Following a VOR Radial
The FD/autopilot’s navigation function can also be used to directly track VOR radials. The navigation display must be configured to show indications from one of the aircraft’s VOR receivers. Once you have tuned and identified a VOR station and selected the desired radial, you can select the navigation mode to track the selected radial. Figure 4-7 demonstrates how to use the navigation mode to follow a VOR radial.
When the navigation mode is used to follow a route defined by VOR radials, you must still tune and identify each new VOR facility manually and select the appropriate radials along the way. The autopilot’s navigation function cannot automatically manipulate the VOR receiver. However, some highly automated FMS units tune and identify VORs along a defined route, such as Victor or Jet routes. You should check the FMS documentation and installed options.
Depending on the FMS, the highly automated flight that results when the navigation mode is used to follow a published route from the database uses a different skill set from using the navigation mode to track discreetly tuned VOR radials. Learning how to select preprogrammed routes from the database of airways can be challenging. Programming or tuning discreet VORs en route in turbulent conditions presents different challenges. Either skill set can result in a greater sharing of duties between pilot and technology and an increase in safety.
The heading mode is used to steer the aircraft automatically along a pilot selected heading. Using the FD/autopilot to fly a heading is a simple matter of selecting the assigned heading and then engaging the heading function or, more commonly, accomplished by first engaging the heading mode and gently turning the heading selection knob to the new heading. Gently turning the knob with the mode already engaged allows you to make a smooth change from level to turning flight. Many autopilots make an abrupt bank if engaged when there is a big change to be made in heading or track. The heading function is illustrated in Figure 4-8.
You should note that, when using the heading mode, the FD/ autopilot ignores the pilot-programmed route in the FMS or any VOR radials you set. When in heading mode, the FD/ autopilot will fly the selected heading until fuel starvation.