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Almost all control towers I've seen have tilted glass panels:

enter image description here
Amsterdam-Schiphol tower (EHAM), source.

  • What is the reason behind this choice?
  • How is the tilt angle determined?
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    $\begingroup$ This should me moved to Architecture SE, which does not exist, so it will stay here. $\endgroup$ – qq jkztd Oct 25 '17 at 16:11
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    $\begingroup$ Related physics.stackexchange.com/questions/303815/… :) $\endgroup$ – Stelios Adamantidis Oct 25 '17 at 16:12
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    $\begingroup$ I do not think this should be moved, anymore than other aviation related questions that have simple physics answers, like why fighter jet windows have metal in them or why runways use asphalt instead of cement. My criteria for determining if it belongs on this site is usually "would people here be interested in it" not "would another, possibly nonexistent SE be a better fit" $\endgroup$ – Cody P Oct 25 '17 at 16:26
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    $\begingroup$ If anyone comes up with an official answer (such as from the FAA) on why these windows are slanted, could you please add that information to the Physics.SE question Stelios linked? I'm the creator of the "accepted" answer on that. I later felt that CandiedOrange's answer was better, so I'm still curious if there's any sources. An official statement might help clear that up... neither of us could find a citable source. $\endgroup$ – Cort Ammon Oct 25 '17 at 19:22
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    $\begingroup$ @CortAmmon Per this, refer to “Study of reduction of glare, reflection, heat and noise transfer in air traffic control tower cab glass” by J. Michael Clinch $\endgroup$ – bishop Oct 26 '17 at 0:46
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I'm pretty sure that has to do with reflections especially at night.

If you have ever stood between two mirriors you will understand the infinity mirror.

enter image description here

Light bounces back and forwards between the mirrors so when you look into them it appears like the scene goes on forever.

Window glass reflects a certain amount of light, like a mirror. In a control tower, where windows are all around you, if the surfaces of the windows are parallel you get that infinite mirror effect.

During daylight hours this can be seen as glare, but at night when you are trying to see that navigation light on Flight 594 amid a sea of runway and taxi-way lights, having your vision further confused by a myriad of reflections from the back window would be a bad thing. Further, since the room is a glass box, light can bounce all the way round the room. It would be quite possible to look off to your right and think so you see an unidentified aircraft approaching, but be actually seeing some truck's lights on the hill a couple of miles away over your right shoulder.

Tilting the windows outwards means the reflection angle is towards the ceiling, which is generally painted black, and the effect goes away.

Further, tilting out means you can no longer see your own reflection, or reflections from other lit workstations in the window. Ever tried looking out of a window at night into the dark and only see your own ugly mug staring back at you, then tried to move your head to the side to see around yourself. LOL

Of course, for all this to work, the ceiling should be dark.

A few other reasons include, less affected by rain, fewer refraction effects, and external reflections. Bonus feature, less greenhouse effect and AC bills. Downside... darn hard to clean.

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    $\begingroup$ inside the tower the reflection is towards the ceiling, and outside the tower it is towards the ground, which should prevent glare from the sun hitting approaching aircraft. $\endgroup$ – Owen Oct 25 '17 at 17:23
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    $\begingroup$ @Owen yes that may be an effect too, but since the aircraft are moving and the tower isn't the plane could still move into the reflection path. Even then though, one would think the reflections from the myriad of terminal windows would be a much greater problem. But still, you have a valid point. $\endgroup$ – Trevor_G Oct 25 '17 at 17:26
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    $\begingroup$ Related: Investigations of Characteristics of the Pentagonal Tower Cab $\endgroup$ – mins Oct 26 '17 at 7:35
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    $\begingroup$ The (downward) tilted windows are common to ships as well. One of the reasons for its use is to avoid reflecting direct sunlight to the surrounding. $\endgroup$ – sampathsris Oct 26 '17 at 9:06
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    $\begingroup$ @Krumia, tilting the window down just moves the point to which sunlight is reflected closer, but does not remove it. However, the reason with internal reflections is applicable to ships as well—the crew often needs to look for a small light on the horizon in otherwise pitch black night, so they want to avoid reflections from lights in instruments on the bridge in their view. $\endgroup$ – Jan Hudec Oct 26 '17 at 13:18
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The glass is the most transparent and easiest to view through when it is strictly perpendicular to the line of view. Otherwise it scatters the passing light, adding "fog" that makes remote objects more difficult to see and identify. Following Lambert's cosine law, radiant intensity observed from an diffusely reflecting surface is directly proportional to the cosine of the angle θ between the direction of the incident light and the surface normal". At the right angle, cos(90) = 0, so. This should be much more noticeable when the glass is not perfectly clean (see also here).

If this assumption is true, the slanting angle should be such that the glass is perpendicular to the most usual line of sight into the ground from above.

As other answer suggests, avoidance of the direct reflection (this is is not a diffused scattering) may be another reason.

Railway control towers, where present, also have the similarly slanted windows.

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    $\begingroup$ I considered that too but dismissed it. Reasoning: Although a valid phenomenon, it would bias the windows for a downward view. and significantly impact the view of aircraft in the air. Since controllers must view both, such a bias would be counter productive. It would be better to bias somewhere in the middle. $\endgroup$ – Trevor_G Oct 25 '17 at 20:07
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    $\begingroup$ @Trevor is correct. The most important direction for air traffic controllers to look is in the sky which is generally towards the horizon or slightly up. Using this logic the windows should be normal and not slanted. Thus this can't be the reason for slanted windows. $\endgroup$ – slebetman Oct 26 '17 at 2:36
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    $\begingroup$ @slebetman. I think you're making a very big assumption that ATC manage traffic in the air by visual means. Just maybe radar might be used more? Just how far can you see anyway? On the ground however, visual confirmation that a taxing aircraft hasn't crossed a runway it shouldn't have, is more likely. Often aircraft will be instructed to "hold short" of a runway until a landing aircraft has touched down and exited the runway. $\endgroup$ – Penguin Oct 26 '17 at 8:09
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    $\begingroup$ This is great reasoning for a railway control tower, but airplanes fly. $\endgroup$ – Octopus Oct 26 '17 at 17:31
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    $\begingroup$ @Octopus The primary reason is still to keep dirt/rain away and so as to reflect any light originating from computer monitors upward onto the ceiling, reducing internally-generated visual disturbances at night. The windows would be mainly used to visualize aircraft on the ground; aircraft post-takeoff are (largely) monitored by electronic means. Human visual judgement of distance and velocity isn't precise enough alone, one aircraft leave the tarmac. $\endgroup$ – apraetor Oct 26 '17 at 20:04
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As FAA (Order 6480.7D) used to put it: Positive tilt outward eliminates reflections from consoles and provides shading at high sun angles.

Another reason for tilting the Visual Control Room (VCR) windows is less deposits and less need for cleaning. This is particularly important to avoid rain impacting on the controller's ability to see from the tower.

In fact, many other typical planning and design requirements for control towers (and for airports in general) are based on visibility, such as the use of dark ceiling tiles in the VCR, restricting the use of solar farms in and around airports, or the highly controllable VCR room and desk lighting, among others.

The tilt angle is determined by the appropriate authority, which depends on the country but in the USA is the FAA. From my experience, tilt angle is usually set between 15 degrees and 20 degrees.

Another fun fact is that the increased sun shading provided by the window tilt helps keep the VCR cool. Keep in mind the VCR is essentially a greenhouse!

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Machicolations ;-)

I'm not building control towers, but I think one reason would be to allow for better viewing of what happens on the ground near the tower.

It may not be needed so much with an airport control tower as a railroad control tower (and most are using screens anyway), but there is also the comfort for passengers that a control tower looks like what they expect: Up there there are people looking down on the airport and making sure we are all safe.

If the windows were vertical or slanted upwards, it would look "wrong", and signal that the people in the tower weren't interested in little me on the ground.

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    $\begingroup$ "there is also the comfort for passengers that a control tower looks like what they expect" sounds like Catch-22 logic to me. $\endgroup$ – Ambo100 Oct 26 '17 at 22:08

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