When you turn on "TERR ON ND" in a A320, you see terrain info on you navigation display (ND), but where does its source come from ?

  • if data comes from FMGS stored inside its memory, then how come it cannot display it in all bounds of the display (see picture) ?

enter image description here

  • if data comes from the nose radar, then how is it possible to see oceans when you're on the ground in an airport, say in JFK for example ? The radar beam would never be able to get to the water directly.
  • $\begingroup$ how come it cannot display it in all bounds of the display (see picture) ? uh? I've seen the picture, but I don't get what you mean with "all bounds of the display" $\endgroup$
    – Federico
    Commented Sep 12, 2016 at 8:55
  • $\begingroup$ I mean like below the aircraft for example (and on the sides) $\endgroup$
    – Matt
    Commented Sep 12, 2016 at 9:11
  • $\begingroup$ aircraft are non-holonomic systems, they can't move sideways and they can't pull back mid-air, so way waste precious screen real-estate for stuff that does not influence you? $\endgroup$
    – Federico
    Commented Sep 12, 2016 at 9:16
  • 3
    $\begingroup$ an A320 is not a fighter, its manoeuvrability is limited. you can be sure that they evaluated this when deciding exactly which angle to display $\endgroup$
    – Federico
    Commented Sep 12, 2016 at 9:22
  • 2
    $\begingroup$ Regarding the area displayed: The full 180° are only covered by the recent versions of the EGPWS. the reason is likely that calculating relative altitudes is a heavy workload for the EGPWS CPU, and with new chips, more pixels can be computed in a decent time. See how slowly the display is refreshed in this video. $\endgroup$
    – mins
    Commented Sep 12, 2016 at 11:21

1 Answer 1


The terrain data is contained in a database within the EGPWS (Honeywell TM) or the Terrain Awareness and Warning System (TAWS) as defined in the regulations -- can't use a trademark in the regs.

The discussion in your link about "blockiness" when zooming in is a result of the database grid references. To save memory space, the resolution of the world-wide terrain data varies based on terrain variation and it's relative hazard to flight operation. There's greater resolution (smaller grid) near airports and in mountainous areas.

The A320 does have airborne radar that is capable of displaying "weather" which really means precipitation. It can't be displayed at the same time as the terrain to avoid confusion.

Neither the EGPWS or the radar data are in the FMGS. Their data is just underlayed on the navigation display. (EGPWS does receive data from the FMGS to support it's warning functions.)

TAWS/EGPWS function is described in TSO-C151c

  • $\begingroup$ It just seemed confusing when you look at how the terrain display updates itself: it doesn't update visually by top to bottom, but instead, by radials of a circle (just like weather radar)... $\endgroup$
    – Matt
    Commented Sep 16, 2016 at 12:57
  • 3
    $\begingroup$ That's a consequence of designing the system to the least common denominator. When Allied Signal (now Honeywell) designed the EGPWS in the mid 90's, a major criteria was that it had to be retrofitable into the majority of the fleet. A big issue was the availability of a display for the data. Since almost all had a dedicated weather radar display the decision was to 'share' the display. That meant using the same physical interface to the display (via an external switch) and data format. As a result, the terrain data is drawn in the same manner as the radar displays the weather. $\endgroup$
    – Gerry
    Commented Sep 16, 2016 at 17:12

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