The FMS provides guidance toward each waypoint in the programmed flight route, and provides information to help you track your progress.
The Active Waypoint
In normal navigation, at any given time, the aircraft is progressing to the next waypoint in the programmed flight route. This next waypoint is called the active waypoint. FMSs typically display the active waypoint on a page dedicated to showing flight progress. While “going to” is the normal function for navigation, nearly all FMSs have the provision to select a point, waypoint, or navaid to navigate “from” that point or position. This can be useful for holding, tracking NDB bearing, grids, etc., and allows the tracking of a bearing with the autopilot engaged and coupled to a navigation source. However, if you are doing an ADF approach, the primary navigation source must be available to support that approach. The page shown in Figure 3-17 indicates that TRACY is the active waypoint. The primary flight display in Figure 2-5 shows the active waypoint (ECA) at the top of the display.
The FMS/PFD/MFD navigation display also shows the desired track to the active waypoint. The desired track is the intended course for the active leg in the programmed flight plan. It is the track that connects the waypoint the aircraft just passed to the active waypoint. On the display in Figure 3-17, the current desired track is the 049-degree course between the SUNOL and TRACY waypoints.
The navigation display shows the aircraft’s track over the ground. The track, which is the result of aircraft heading and winds, tells you which direction the aircraft is actually flying. Winds make it likely that the track and heading will be different. You can get a very good sense of what the winds are doing by comparing the track and heading of the aircraft.
If the aircraft is flying a heading of 090 degrees and the track is 080 degrees, the winds are coming from the south. Notice that having a track indication makes it easy to maintain the desired track. To follow the 049-degree desired track to TRACY upon leaving SUNOL, simply fly the heading that results in a track of 049 degrees. The track display eliminates the traditional method of “bracketing” to find a heading that lets you fly the desired track.
Groundspeed and ETA
The display in Figure 3-17 also shows groundspeed. Again, the navigation display eliminates the need to calculate groundspeed using distance and time. Based on groundspeed and distance from the active waypoint, the navigation page also provides an estimated time of arrival at the active waypoint.
Fuel Used and Time Remaining
Many advanced avionics navigation units offer fuel calculations and fuel state monitoring. Some units automatically load the initial fuel load, while many require the pilot to correctly enter the amount of fuel into the unit as the beginning fuel on board. Some can have installed transducers (sensors) to measure the fuel used, and display fuel used and time remaining at the current consumption rate. Some lower cost units indicate computed fuel consumption values based on fuel burn rates entered by the pilot. This produces an estimate of fuel used and fuel remaining. This estimate is only as accurate as the values entered by the pilot for fuel on board and the consumption rate. Since the pilot is often using the AFM chart data, there is potential for interpretation error. Then, there is the variation error from the factory charts to the specific aircraft being flown. These factors all tend to degrade the accuracy of the fuel calculation based solely on pilot entered data. Other factors such as a fuel burn that is higher than normal, leaks, or other problems are not displayed unless the system actually registers and senses fuel tank realtime status. These errors can affect both types of systems. The pilot must determine what equipment is installed.
Arriving at the Active Waypoint
As the aircraft reaches the active waypoint, there are four new tasks for the pilot: (1) recognizing imminent arrival at the active waypoint; (2) leading the turn to avoid overshooting the course to the next waypoint; (3) making the next waypoint the new active waypoint; and (4) selecting the desired course to the new active waypoint.
All FMS/RNAV computers offer a sequencing mode that greatly simplifies the performance of the first three of these tasks. Sequencing mode provides three services: waypoint alerting, turn anticipation, and waypoint sequencing.
The first service performed by the sequencing mode is waypoint alerting. Just prior to reaching each active waypoint, waypoint alerting advises the pilot of imminent arrival at the active waypoint. Waypoint alerting is illustrated in Figure 3-18.
The second service performed by the sequencing mode is turn anticipation. During waypoint alerting and prior to reaching the active waypoint, the FMS indicates that it is time to begin the turn to fly the desired track to the new active waypoint. The timing of turn anticipation is based on the aircraft’s observed groundspeed and the angle of the turn required to track to the next waypoint. If a standard rate turn is begun when the waypoint alerting indication is presented, the pilot should roll out on course when the aircraft reaches the center of the desired track to the new active waypoint. Turn anticipation is also illustrated in Figure 3-18.
When turn anticipation is used, the aircraft does not fly directly over the active waypoint. Rather, the computer commands a turn that “rounds the corner” to some degree, giving priority to having the aircraft roll out on the new desired track to the new active waypoint. This function is illustrated in the upper illustration in Figure 3-19.
Turn anticipation occurs only when the active waypoint is designated as a fly-by waypoint. A fly-by waypoint is one for which the computer uses a less stringent standard for determining when the aircraft has reached it. By contrast, some waypoints are designated as flyover waypoints. The FMS will not use turn anticipation for a fly-over waypoint; instead, the navigation will lead the aircraft directly over the waypoint (hence the name). A missed approach waypoint is a typical example of a fly-over waypoint. A fly-over waypoint is illustrated in Figure 3-19.