As with many things concerning the design of an aircraft, the final arrangement of various parts is based upon a series of compromises.
An aircraft designer’s life would be so much easier if there were no windows at all, but so far the consensus seems to be that we should have them.
Britain lost the initiative in jet airliner manufacture when the development of the de Havilland Comet in the 1950s suffered a setback through a series of crashes, in part because metal fatigue around the windows led to structural failure.
While windows remain an accepted part of aircraft design, they have since been kept as small as possible. These days they are typically 33 centimetres high. The window has to have three panes: two pressure panes and one interior pane to prevent passengers getting at and damaging the vital ones.
The panes are contained in a window unit which is fastened and sealed to the aircraft structure.
It is, of course, much heavier and costlier than the thin sheet of aluminum it replaces, and the structure of the aircraft needs to be reinforced to support it. All this extra weight means fewer passengers or less cargo can be carried, so it reduces airlines’ potential revenues.
Windows also present a maintenance problem. As well as getting scratched and broken, they are a source of air leaks from the cabin and they also suffer from condensation and icing.
The position of the windows varies depending on the aircraft but generally designers try to place them with their centre line a little below the eye level of seated passengers.
On the ground this is perhaps too low, but in flight it gives an oblique view of the ground.
Little would be gained by positioning the window higher. Because the seats are placed at the widest part of the circular or oval fuselage, the windows would end up angled upwards some 10 or 15 degrees. The only view the passenger would then have in flight would be of the sky.
Also, if the top of the window were above eye level there would be a constant problem from the sun’s glare and dazzle. Passengers would just end up pulling down the blinds, which would negate the benefit of having a window in the first place.
It would be useful to have them deeper, but, as I have already said, the weight penalty makes this impractical.
It also has to be remembered that every civil aircraft flying today was designed at least ten years ago, and some actually started life on the drawing board 40 years ago.
During this time people have changed and seat design has changed. When these aircraft were developed, the structural design, including the position of the windows, was fixed and the window line has traditionally been used as a convenient breaking point to bring pieces of fuselage shell together.
This position having been determined, and production lines then set up with the correct tooling, it would be enormously costly to change it.
In the meantime, people have been getting bigger.
Designers have to use what are known as “Dreyfuss criteria” to determine seat sizes. These criteria are constantly changing, but a designer will typically make a plane’s seats big enough to accommodate 95 per cent of American males.
If you are particularly tall, this is going to make the window seem lower for you, and people are generally taller than they used to be.
Finally, the present trend in air travel is away from luxurious, spacious layouts to high-density seating. In these circumstances, where the seat pitch is reduced to accommodate as many passengers as possible, the seat base has to be higher to provide leg-room for the person sitting behind.
This also makes the relative window position lower still than was originally intended.
The windows on aircraft are so small to make them safe. The first major jet airliner, the de Havilland Comet, had large, rectangular picture windows through which the passengers had a great all-round view. But after a few years in service, the aircraft started to break up during flight.
To find out why, de Havilland put a new Comet into a tank of water and then pressurised and depressurised it repeatedly to simulate the conditions of flight.
After the equivalent of two years’ worth of pressurisation cycles, which actually only took a few weeks in the water tank, the airframe was found to fail in the top corner of one of the large windows, which caused a catastrophic break-up in flight.
The windows had to be redesigned and small, round windows set low in the fuselage were created.
This solved the problem and the position of the windows remains the same today.