Airplane Windows

Why are airplane windows so small?

AND

Why are airplane windows double glazed?

AND

Why are airplane windows Oval?

Answer in One Word: Safety

Now, the long verion

Let’s start with a quick science lesson. As airplanes fly above 12,500 – 14,000 ft (or 3800 – 4300m), the pressure outside decreases and the tightly packed air in the cabin start to exert tremendous pressure on the fuselage (body) of the aircraft. This is called cabin pressurization. Cabin pressurization is needed in order to create a safe comfortable environment for passengers and crew while flying at a high altitude. This is achieved by pumping conditioned air into the cabin. The conditioned air is bled off the engines at the compressor stage, which is then cooled, humidified and mixed with recalculated air and distributed to the cabin. Typically, for an aircraft flying at 39,000ft, the cabin altitude (i.e., the cabin environment) is maintained at around 8000ft. Newer airliners like the Boeing 787 are trying to lower the cabin altitude to about 6000ft.

The small windows of an airliner are needed to withstand the forces of a pressurized cabin. If an airliner had large windows, the force of the cabin pressure exerted on it would be greater and that increases the likelihood of breaking the window – causing a rapid loss of cabin pressure which could be fatal! The rule of thumb here is, the higher an airplane flies, the smaller the windows.

So why are airliner windows oval? Shape shouldn’t matter as long as the windows are small right?

WRONG!!!

Actually the shape of the window does play a role in safety. This lesson was learnt after a series of tragic accidents of the De-Havilland DH-106 ‘Comet’. The De-Havilland Comet is a British built aircraft which is credited as the first  commercial jetliner. The Comet flew for the first time on 27th July 1949 and entered commercial service with British Overseas Airways Corporation (BOAC) on 2nd May 1952. There were a total of four variants of the Comet- The Comet 1, Comet 2, Comet 3 and Comet 4. The original model, Comet 1, had square windows that were riveted into the fuselage.

In 1954, a series of accidents where the De-Havilland Comet disintegrated mid-air, a few minutes after take-off (the famous ones being BOAC flight 781 and South African Airways flight 201), grounded the entire fleet. The investigations revealed that there was a major flaw in the design of the aircraft. The corners of the square windows had a high stress concentrations. The constant cycle between the pressurized (i.e., during the climb, cruise) and the de-pressurized state (i.e., during descent, on ground) put an enormous strain on the airplane’s fuselage gradually weakening it, causing metal fatigue. The metal fatigue of the stress concentrations at the corner of the windows, led to the rupture of the fuselage causing the airplane to disintegrate mid-air.  In response to the investigation, De-Havilland designed ‘Comet 2’ with oval windows thus eliminating stress concentrations. The oval windows can still be seen today on all pressurized commercial airliners.

Fun Fact:

An airliner’s window consist of two to three layers of acrylic.

 

For more information, please visit:

Cabin Pressurization: http://en.wikipedia.org/wiki/Cabin_pressurization

De Havilland Comet: http://en.wikipedia.org/wiki/De_Havilland_Comet

BOAC flight 781: http://en.wikipedia.org/wiki/BOAC_Flight_781

South African Airways flight 201: http://en.wikipedia.org/wiki/South_African_Airways_Flight_201

Boeing 787’s new window design: http://usatoday30.usatoday.com/money/biztravel/2006-10-30-boeing-air-usat_x.htm

Don’t forget to follow me on twitter: @av8radi (link opens in a new tab)

Until my next post… Happy Landings!!!

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