RNG PULL SCAN PUSH ON BRT PROC CRSR MSG OBS ALT NRST CLR ENT D MENU DTK 049 TK 1.8nm 108 kt LEG APT VOR NDB INT USR ACT FPL SET AUX NAV 1 Desired track to the active waypoint ed track to the active waypoint Track of the aircraft over the ground Distance to the active waypoint 1.8 nm the active waypoint Groundspeed Active waypoint Estimated time of arrival at the active waypoint Figure 3-17. Active waypoint, desired t rack, track, and ETA at  active waypoint. Arriving at the Active Waypoint Desired Track Track Fuel Used and Time Remaining Groundspeed and ETA 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. 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. 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. 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 real‑ time status. These errors can affect both types of systems. The  pilot must determine what equipment is installed. The FMS/PFD/MFD navigation dis play 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 conne cts 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. 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. 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.  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. 3-14