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Navigation Database Currency (Part Two)

in Navigation

Using the FMS’s flight planning function to check your own flight planning calculations is one example of how advanced cockpit systems can decrease the likelihood of an error. Alternatively, cross-check the computer’s calculations against your own. (Remember the old computer programmer’s adage, “Garbage in, garbage out (GIGO).”)

The flight planning page can also be used to review the route that you entered into the FMS, as illustrated in Figure 3-13. Be sure to check at least four things when reviewing your route.

Figure 3-13. Reviewing the flight route.

Figure 3-13. Reviewing the flight route.

Check the Waypoints

On the flight planning page, compare the sequence of waypoints with that prescribed by his or her clearance. Are any waypoints missing? Did you mistakenly include any extra waypoints in the route? Did you misspell any waypoints? Did the computer mistakenly include any extra waypoints in the route?

Check the Distances

On the flight planning page, you can see that the computer has calculated the distances between the waypoints in the route. These distances can be checked against the en route charts. A common error is to misspell the name of a waypoint and, thus, mistakenly enter a waypoint not appropriate to the planned route (e.g., KHEE versus KHEF). Checking the waypoint distances for unusual numbers is a good way to spot these errors.

Check the Desired Tracks

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.

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.

Figure 3-14. A great circle route.

Figure 3-14. A great circle route.

The shortest route between two points on the surface of the earth is called a great circle route. To find the great circle 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.

Check for Route Discontinuities

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.

Figure 3-15. A route discontinuity and deletion.

Figure 3-15. A route discontinuity and deletion.

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.

Maintaining Proficiency: Aeronautical Knowledge

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.

Coupling the FMS to the Navigation Indicator(s)

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  was selected in the FMS. Every advanced cockpit offers buttons or switches that allow you to choose which navigation indications will be shown on which display or instrument.

This situation becomes complicated in aircraft that contain dual FMS/RNAV installations and redundant selectable displays or instruments. The pilot must learn how to configure each navigation instrument to show indications from each possible navigation source.

Figure 3-16 shows an example of a primary flight display (PFD) navigation indicator that combines a course deviation indicator (CDI) and a radio magnetic indicator (RMI), and allows the pilot to display indications from one of two FMS on either indicator.

Figure 3-16. Coupling the FMS to navigation instruments.

Figure 3-16. Coupling the FMS to navigation instruments. [click image to enlarge]

Common Error: Displaying the Wrong Navigation Source

The annunciations showing which navigation sources are displayed on which navigation instruments are often small, so there is significant potential for displaying a navigation source other than the one you intended to select. The consequences of losing track of which navigation signals you are following can be significant: you may think you are steering along one course when in fact you are steering along a different one.  Be sure to verify these settings prior to departure, and again each time you make changes to any navigation instrument.  Some installations compound this potential with automatic source switching. The most common switching mode is a GPS source to be automatically deselected when the VOR is set to an ILS localizer frequency and a signal is present. Typically, that is not a problem since the pilot intends to switch to the ILS anyway. However, the error arises upon missed approach, when the pilot selects another frequency to follow a VOR missed approach routing. At that point, some units revert back to the previous GPS or other RNAV routing selected instead of the VOR frequency that the pilot just picked. This can result in gross navigation errors and loss of obstruction clearances. In some units, this is a shop programmable or jumper selected option. Check your unit’s features. Always check for correct navigation source selection and cross-check against the published procedure. Be ready and able to fly and navigate manually.

Awareness: Mode Awareness

Mode awareness refers to the pilot’s ability to keep track of how an advanced avionics cockpit system is configured. As shown in Figure 3-16, every advanced avionics system offers an annunciation of which mode is currently set—somewhere in the cockpit! There is no guarantee that you will notice these annunciations in a timely manner. The configuration of these systems must remain part of your mode situational awareness at all times. One strategy is to include “mode checks” as part of your checklist or callout procedures. For example, after programming a route into the FMS, verify that the navigation indicator shows course guidance from the desired source, and that the indication agrees with your estimate of the correct direction and distance of flight.


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