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Is there any particular reason for having the ground represented by brown on an artificial horizon or PFD?

Blue represents sky, brown represents earth, which will be used for easily identifying the aircraft attitude (along with line of origin). Using blue to represent the sky makes conventional sense. But why is brown used? Is there any scientific reason for using brown instead of another color?

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    $\begingroup$ I would also note that this is definitely not unique to PFDs. It's been used on attitude indicators since long before PFDs existed. The digital attitude indicators on PFDs were just designed to look like the analog attitude indicators that pilots were already accustomed to. $\endgroup$ – reirab Nov 24 '15 at 8:50
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The answer to the specific question of "why PFD's (glass cockpits) use this standard" is because their old style gauge cockpits used it before them. The glass cockpits of today are made to be similar to the older gauges so as to avoid confusion in their operation when moving from one type of aircraft to the other. This has been shown to be an issue in the past in the difference between American and Russian attitude indicators as such instrument operation strives to be at least somewhat uniform.

Now as for the more general question "why are attitude indicators Blue/Brown?" there are a few reasons. First off some types of color blindness cause issues with the blue/green mix making it a combination to avoid (some info on color blind flying). Second, and perhaps as important is that blue and brown sit basically on opposite sides of the color wheel making them very contrasty while blue and green sit right next to each other on the wheel. This ensures accurate viewing in most lighting situations.

There are also other attitude indicators like those commonly used in military aircraft which have had different colors

enter image description here (source)

The oldest examples are completely void of any color (you can see why they introduced color)

enter image description here (source)

For what its worth some modern flight displays now include terrain data and are capable of showing many other colors to outline it as can be seen here on this Garmin unit

enter image description here (source)

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If they used green (like grass) you would have asked the same: "Why didn't the use brown (like soil)?"

Blue - brown is easier to distinguish, than blue - green, blue - red or another color-combination you can think of.

Contrast:
The use of this two colors can be explained by the high contrast from blue to brown. The pilot can find out with one quick glance at the PFD how the aircraft is positioned in the air.

Natural colors - pilot instantly knows what it means:
Because those are "natural" colors, the pilot instantly knows that blue means sky and brown means ground. He doesn't have to process the color information in his head just to find out what it means. This would take much longer.

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    $\begingroup$ BTW, most of the geographical places natural colour of soil is black. $\endgroup$ – Lucky Nov 24 '15 at 7:38
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    $\begingroup$ @Lucky care to make some examples? I can't think of a single one. (and in photos from space you don't see vast swathes of black) $\endgroup$ – Federico Nov 24 '15 at 8:20
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    $\begingroup$ @Lucky I don't think Black is a good choice for the ground-part of the PFD... How will you iluminate it at night? Just pointing out the problems ;) $\endgroup$ – jklingler Nov 24 '15 at 8:24
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    $\begingroup$ @Federico Areas around volcanos often have black 'soil' (if it's fair to call volcanic rock 'soil,') to the point that volcanic islands often have black sand beaches. Of course, you're right, though, that the vast majority of soil is not black. $\endgroup$ – reirab Nov 24 '15 at 8:58
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    $\begingroup$ I agree with @jklingler, but some part of ground is brown and some part is black and majority is back soil Federico.. Refer India, North part of Karnataka $\endgroup$ – Lucky Nov 24 '15 at 9:09

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