Why are W, S, N, E highlighted on a flight management computer (FMC)? My assumption that they represent West, South, North, and East.

Here is a Boeing FMC:

enter image description here

And here is one from Airbus:

enter image description here

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    $\begingroup$ Correct. North, South, East, West when dealing with longitude and latitude, e.g. creating a waypoint or entering the current position for inertial alignment (the second image). $\endgroup$ – mins Nov 2 '17 at 9:12
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    $\begingroup$ A good question indeed, it's easy to say that it represents the four directions, but why it's done is something else. Whats so time-critical to have those buttons highlighted? $\endgroup$ – Noah Krasser Nov 2 '17 at 10:10
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    $\begingroup$ @NoahKrasser: The only key used when entering coordinates are the rounded numeric keys and these 4 letters, that's enough to make them look different. $\endgroup$ – mins Nov 2 '17 at 10:42

N, E, S, W is definitely used for entering lat/long coordinates. On the Airbus graphic in the question, you can see the pilot has entered their starting latitude and longitude with N and W.

But why are they highlighted? I'm going to guess and say it has to do with the transition between old-style numeric nav computers and "modern" FMS. Old style IRS only had a numeric keypad like this Concorde unit:

old IRS unit

and you can see that N, E, S, W is entered on the 2, 6, 8, 4 keys. So instead of typing "3253.9N" on the CDU you'd hit "325392". You'll still find these numeric keypads today on some forgotten corner of the overhead panel for who knows why, even on a 777.

I think the squares are a cue for the old time pilots that you can't enter a N coordinate by hitting 2, you have to hit the N key, and it also helps them find it.


The highlighted keys denotes the four directions, as mins has mentioned.

I could not find more information about this other than this:

Placement of the letter designator (N, S, E, W) in the string of five characters indicates the value of the longitude one-hundredths digit

  • The letter in the last position indicates longitude is less than 100
  • The letter in the third position indicates longitude is 100 or greater

For Example:

Letter   Lat     Lon
N        North   West
E        North   East
S        South   East
W        South   West

PPrune also said the same.


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