I'm working on a navigation display for a simulation and I want to know if someone can explain me how the navigation display displays the "curved lines" for fly-by waypoints.I know how to calculate the distance needed for turn anticipation and the turn radius but how would a real ND draw this ? I attached a screenshot for the ADMAG2X departure for gatwick from a simulator navigation display but why does the turn start after "KKW02" and "KKW02" is no overfly waypoint ? :)

enter image description here

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    $\begingroup$ That's a screenshot from a home simulator. Couldn't it just be an inaccurate one wrt no overfly wpts? $\endgroup$
    – user14897
    Commented May 25, 2019 at 16:03
  • $\begingroup$ I think it should be accurate because it's from the FSlabs A320 which is a pretty accurate software and all the other displays from other software look the same but maybe I can find real world screenshot :) $\endgroup$ Commented May 25, 2019 at 16:08
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    $\begingroup$ That would be great, if you do please use the edit button to add it. Welcome aboard. $\endgroup$
    – user14897
    Commented May 25, 2019 at 16:11
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    $\begingroup$ You mention that the KKW02 is not an overfly waypoint, but rather a "fly-past" waypoint. Are you sure that this is how it's coded in the database behind what's being depicted? There are places a fly-over waypoint makes sense, and first waypoint off a runway is often coded that way. $\endgroup$
    – Ralph J
    Commented May 25, 2019 at 16:29
  • $\begingroup$ But KKW02 IS a fly-over waypoint! tinyurl.com/admag2x $\endgroup$ Commented May 25, 2019 at 17:31

1 Answer 1


First, the ND doesn't calculate anything. It just renders the navigation data sent to it from the FMS. Yes, it can pan and rotate and scale the nav data. It also displays the compass rose with heading or track from the IRS/AHRS and various data fields around the edge.

The green line on your ND is the lateral path computed by the FMS. The source data for the path is the flight plan which is a list of waypoints and associated leg types. The FMS builds the path by connecting the waypoints as defined by the leg types (ref ARINC 424).

As you noted, it also builds transitions between each of the legs. They can be fly-over or fly-by transitions. The 'normal' transition is a fly-by. Within procedures defined in the nav database, some waypoints will be coded as fly-over. Others will be fly-over due to the specific leg type. Some legs end at fixes, others end at a 'floating' waypoint such as an altitude.

For altitude terminated legs the FMS can predict roughly where it will be, but ultimately the end of the leg is only known with certainty when the altitude is reached at which time you just did a fly-over of the 'fix'.

In your example, KKW02 is a fly-over because it is altitude terminated. Takeoff legs are typically coded in the database as a VA (Heading to Altitude) or CA (Course to Altitude) leg, commonly terminating at 400 feet above the runway altitude.

The FMS builds the transitions based on the actual geometry of the inbound and outbound legs, bank angle limits from the aircraft manufacturer, aircraft speed, altitude, and what it knows of the wind. The resulting transition will be an arc that is tangent to both the inbound and outbound legs. For large course changes like the one shown in your example, it may require two arcs.

Thus the path is made up of a series of straights and arcs. The straights are defined by their start and end positions (latitude/longitude). Arcs have the start and end positions (lat/lon) as well as the radius of the curve (NM) and the center of the arc (lat/lon).

The display gets the aircraft position and track from the FMS/GPS/IRS and heading from the IRS. Based on the pilot's selected display range the display plots out the path on the display.

RTCA DO-283 Minimum Operational Performance Standards for Required Navigation Performance for Area Navigation covers the generation of the path. RTCA DO-257, Minimum Operational Performance Standards for the Depiction of Navigation Information on Electronic Maps is the standard for NDs. How the manufacturers actually accomplish it is up to them and they don't share that information.

  • $\begingroup$ I suggest simultaneously to display the MCDUs to check the data related to every point on the flight plan therefore for KKW02. $\endgroup$
    – user40476
    Commented May 27, 2019 at 11:46
  • $\begingroup$ Thank you for the long explanation but I'm actually searching for the explanation how the ND draws the Geometry.Will the curved path always be drawn between 2 points or is just the inbound course and outbound curve important for the curve ? I'm trying to understand the laws for the drawing of the geometry so maybe trigonometry and all that stuff :) $\endgroup$ Commented May 27, 2019 at 16:03
  • $\begingroup$ The ND just reproduces the plan generated by the FMS (flight management system). Another transcription of the same data is available on the MCDU for every point of the flight point. Analysis of the displayed data on the MCDUs is very helpful. $\endgroup$
    – user40476
    Commented May 27, 2019 at 17:40

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