Why do airplane windows have two panes of glass?

Question

Why do airplane windows have two panes of glass?

Answer 1

It is mostly for thermal insulation, as the outer layer is in direct contact with the very cold outside air (it can be -50 or -60 at altitude). Without the second pane, humidity in the cabin would be condensing, perhaps even frosting over the window.

Answer 2

Well, that depends on what type of airplane (aeroplane) and where the windows are located. if you are referring to a passenger airliner, then the passenger windows are, as Vincent G correctly states, for insulation. They are also made multi-layered in the interests of redundancy in safety as well.

They are not made out of glass, however; they are made from acrylic (for example, "Perspex").

If the windows on the airliner are located in the cockpit, though, they are usually made up of multiple layers of high-impact glass, layers of plastic, heating elements (to keep them defogged) and sheets of material to keep the rain from interfering with the pilot's vision.

Answer 3

I suppose one answer could be incase one of the pieces, in or out, gets cracked or hit by something that might crack it. If there was only one piece of glass and it cracked and while flying at a high altitude the pressure difference would be so great that things would be sucked out the window and it could be a person or a part of that person until the hole got filled or untill the pressure was the same in and out..ok

Answer 4

to increase the isolation

Answer 5

Likely, a single pane of glass capable of doing the job (transition between cold outside air and warm moist inside air) would be too heavy.  Airplane parts must be as light as practicable (unlike parts on a steam locomotive, etc.).  The outer pane is also part of the streamlined body to reduce drag.  The inside pane should be flush with the interior panels to avoid sharp corners that passengers might hit in turbulent air (or a crash scenario?).