Check for Route Discontinuities Check the Desired Tracks Maintaining Proficiency: Aeronautical Knowledge Coupling the FMS to the Navigation Indicator(s) route that connects two points, imagine a geometric plane  cutting through the earth that passes through the two points  and the center of the earth. On the great circle route from SFO to LGA in  Figure 3-14,  departing SFO, the desired track is a little less  than 90 degrees. Upon arrival at LGA, it appears to be greater  than 90 degrees. The desired track heading is constantly  changing since it is a circle, not a line. If, however, the  difference exceeds several degrees, you need to investigate  further to determine the cause. Some FMS units do not automatically assume that you wish  to fly between each of the waypoints that have been entered  into the flight plan. When there is a question about how to  proceed from one waypoint or instrument procedure to the  next, some units insert a “discontinuity” in the programmed  route. A route discontinuity indicates that the FMS needs  further input from you about how two route segments should  be connected. A route discontinuity is shown in  Figure 3-15. If you wish to proceed directly from the waypoint that appears  before the route discontinuity to the waypoint that appears  after, you can simply delete the discontinuity, as shown in  Figure 3-15. If the route discontinuity is left in the flight plan, the unit  computer will not provide guidance beyond the waypoint  that occurs before the discontinuity. On the flight planning page, you can also see the course that  the computer has calculated between waypoints along the  route. A desired track between two waypoints represents the  shortest path between them. The desired track between two  waypoints may differ from the course seen on the aeronautical  charts. In fact, there may be a difference of several degrees  between the desired track and the airway course. Some of  this difference may be due to the method in which the FMS  accounts for magnetic variation. Some units use an internal  database and interpolate, while others compute all values  from tables. It is easy to use an FMS without performing your own  calculations for courses, headings, times, distances, and fuel  used, but studies have demonstrated that aeronautical skills  that are not practiced regularly quickly fade, regardless of  experience level or certificates and ratings held. Abnormal  and emergency situations (e.g., electrical failure) do occur,  so it is important to maintain proficiency in at least making  “rule of thumb” calculations on your own. Unlike the world as printed on paper charts, the earth is round,  not flat. The shortest distance between two points on the earth  is not a straight line; it is an arc, as shown in  Figure 3-14. The shortest route between two points on the surface of the  earth is called a great circle route. To find the great circle  Every advanced avionics cockpit features one or more  navigation instruments used for course guidance. The  navigation indicator (e.g., a horizontal situation indicator  (HSI) or electronic HSI) may include one or more course  deviation indicators (CDIs), as well as one or more radio  magnetic indicators (RMIs). When automatic course/ en  route/ approach tracking is desired, you must couple (or  connect) the FMS to the autopilot and select “navigation”  as the source for the autopilot versus “heading” source, for  example. With VOR navigation, that was sufficient. Now,  with multiple sources of navigation data available, you must  also ensure that the proper navigation information source  Figure 3-14. A great circle route. Figure 3-13. Reviewing the flight route. 3-11 SFO LGA ALERTS FLIGHT PLAN DTK KSQL DIS TRACY Approach - RNAV 10 051° 18.0 NM KSQL / O27 SUNOL 060° 1.8 NM FLI GHT PL  PL AN Check the sequence of waypoints. Check the distances and desired  tracks between the waypoints. Engage cursor mode on the flight plan page. Use the outer knob to scroll between the waypoints in the route.