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Why do aircraft allow sliding / opening windows in cockpits? What purpose does it serve, and why the same can't be done for passenger windows as well?

enter image description here

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I always assumed some airports must have drivethrough mcdonalds for pilots on tight schedules. – Dewi Morgan Jan 16 at 23:18
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Now for the non-serious answer/comment: youtube.com/watch?v=-5viPnzK-S4 "I gave (your keys) to the pilot and he threw them out the window" <-- Ansett Airlines advertisement in New Zealand in the 90s. – Criggie Jan 17 at 0:54
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@Criggie : Thanks for the link.. – Firee Jan 17 at 7:11
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Could you please edit question and source the image ? – kebs Jan 17 at 14:15
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How can you believe that letting passengers open their windows is remotely feasible? – Quora Feans Jan 17 at 17:25
up vote 28 down vote accepted

The opening cockpit windows do serve some purpose, like providing an additional means of evacuation during an emergency.* There have been instances where the crew had used the cockpit windows to escape during hijackings.

There are other purposes like assisting in crew inspection, signaling ground crew, additional ventilation, smoke egress etc (It can also be used for throwing things out, though it is not generally recommended). One main reason (atleast in the past) appears to be to improve the pilot visibility in case the windshield becomes opaque, for example.

The windows are not found in all aircraft (for eg. 787 doesn't have it) and are not very easy to open (note that it opens inwards). From USAToday Ask the Captain:

First there is a locking mechanism, then either a crank which moves the entire window assembly including the frame, or a lever with which to move the window assembly. When the window is closed and locked, it is sealed shut. When open, it does not separate from the frame as the window in your car does.

Actually, FAR 25 Section 773- Pilot Compartment View requires cockpit windows to be openable (or some other means to have clear windshield during precipitation) :

(b) Precipitation conditions. For precipitation conditions, the following apply:

(1) The airplane must have a means to maintain a clear portion of the windshield, during precipitation conditions, sufficient for both pilots to have a sufficiently extensive view along the flight path in normal flight attitudes of the airplane.

(3) The first pilot must have a window that—

(i) Is openable under the conditions prescribed in paragraph (b)(1) of this section when the cabin is not pressurized;

(4) The openable window specified in paragraph (b)(3) of this section need not be provided if it is shown that an area of the transparent surface will remain clear sufficient for at least one pilot to land the airplane safely in the event of—

(i) Any system failure or combination of failures which is not extremely improbable, in accordance with § 25.1309, under the precipitation conditions specified in paragraph (b)(1) of this section.

(ii) An encounter with severe hail, birds, or insects.

In a pressurized cabin, providing windows is not a great idea. (In addition to the increased weight), if anyone manages to open the window, it would be a recipe for disaster (depressurization -> oxygen masks -> land).


*Many aircraft have escape ropes mounted in a compartment above the window on each side of the cockpit as in this A320: A320 cockpit Photo source

And a video of it being used.

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Added a photo and a video link. Hope you don't mind. – TomMcW Jan 16 at 21:57
    
@TomMcW Good one. Thanks.. – aeroalias Jan 17 at 4:38
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I was once riding in the jumpseat of a commercial airliner and the captain seemed to think that these windows were so he could smoke! As soon as the engines were running, so there was pressurization, he opened the window and lit a cigarette. He did the same thing as soon as we landed. With the window opened, I could not smell his cigarette in the cockpit, because the pressurization system did such a good job at removing the smoke. – Adam Jan 17 at 15:44
    
Well now it looks that the openable window does come in handy, wonder why Boeing left it out in the 787. – Firee Jan 18 at 19:45

In the Pilatus PC-12, the pilot's side has a "DV window", a "direct view windows," that is hinged at the back and opens so the pilot has an unobstructed view out forward of the aircraft.

This window is specifically designed to allow the pilot to land an aircraft when the windshield is iced over, and the de-icing equipment is INOP.

DV Window on PC-12

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Other answers provide good broad picture information. Let me add a specific example.

The windows in my work aircraft open, and we have specifically modified them to open fully out of view and remain open. Windows that open allow me to better perform my job. Note that my aircraft is non-pressurized.

One of the things we often do is drop messages to people on the ground. The open window provides a means of jettisoning that message package.

Aerial photography and observation of the ground is in integral part of what we do. Being able to open the window to provide an unrestricted view can give better photo or viewing environment. Conditions such as sun glare render this an important feature.

Finally, being able to open the window on a hot day can be a welcome addition to the ventilation system.

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Very cool. I'd wager your work airplane does not have two engines on each wing like the photo in the question :-) – Dan Pichelman Jan 16 at 16:47
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@Dan You would win that wager :) – Jonathan Walters Jan 16 at 17:19

and why the same can't be done for passenger windows as well?

Anybody who opens even a bit a potential airliner sliding window near his seat creates in the entire plane the conditions existent on the top of Mount Everest, even worse. Nobody wants to stay in such an environment. In conclusion passenger sliding windows serve no purpose.

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That should be a comment, as it doesn't answer the main question which is about cockpit windows and the difference with other windows. That said, good point! – mins Jan 17 at 9:10
    
For any reasonable design, opening the window would be impossible while the cabin is pressurized anyway. The bigger problem is making sure that all of the passengers actually close and securely lock them before the cabin is pressurized and leave them that way. As good as passengers are at following instructions, I'm sure you can see how this would be a problem. Furthermore, it creates ~100 additional potential points of failure, since any window failing to secure would ground the aircraft. And, of course, it also adds a non-trivial amount of weight. – reirab Jan 18 at 1:35
    
The difference between the inside and the outside pressure is max 0.65 atm which means 67 $kgf/dm^2$. With a mechanism like the one used by car windows it is easy to slide a 10 x 10 $cm^2$ window pressed by 67 kgf. Anyway, the biggest issue is that sliding passenger windows serve no useful purpose for the reason I have already explained. – Robert Werner Jan 18 at 6:24
    
@reirab So many doors have to be secured before takeoff, so I am sure securing the windows before takeoff should not be an issue. Plus think of the advantages of openable windows in case of a crash where people die of asphyxia. – Firee Jan 18 at 10:55
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@Firee The cabin doors are secured by trained cabin crew who cross-check each other. The cargo doors are secured by trained ground crew (and accidents still happen.) Passengers are not trained crew and are notoriously bad at following instructions. – reirab Jan 18 at 15:15

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