Everything in an aircraft you see is for a reason. Nothing is there coincidentally. And one such thing is the shape and design of the windows.
With increase in commutation through flights, the aeroplanes started flying at higher altitudes, since lesser air density meant lesser drag which directly affected fuel efficiency. Also turbulence due to atmosphere is less. But some modifications had to be done to make the conditions safe for the passengers at high altitudes. So, a pressurized cabin was introduced into the flights. This pressurized cabin keeps the interior of the flight at a higher pressure compared to the low atmospheric pressure. This initiates an expansion of the flight due to which a stress field is formed.
The shape of the window plays an important role in the formation of this field. In square windows used before, when the stress field line flowed and encountered the window, it changed its path sharply around the corners increasing the stress at the corners significantly. This high stress caused the formation of cracks in the windows leading to three tragic crashes. Later investigation and analysis proved the reason for the cracks, leading to change the shape of windows to oval or circular where the stress field lines bend smoothly without resulting in high stress concentration.
Another curious thing about the aircraft design is a tiny hole located at the bottom of the windows. The window is actually made of three panes. There is a gap between the outer pane and the inner pane in which air is filled to maintain a steady temperature gradient to prevent condensation of vapour on the middle pane. Now, this air, due to high pressure might crack the window open. This is where the little hole comes into picture. The hole located on the outer pane balances out the pressure difference between the atmosphere and the air in between the panes.
The third pane? That is the inner pane which is just added as an extra safety measure so that the passengers cause no damage to the middle pane.
So, the next time you see anything in an aircraft, remember there’s loads of engineering behind it.