Ever since the Wright Brother's first powered flight, humans have strove to fly higher. Beyond the thrill of looking out your cock-pit and seeing the world pass by like a distant fairy tale, flying high provides many benefits. It allows military pilots to escape pursuit from less capable aircraft and avoid anti-aircraft fire from the ground. Flying above the turbulent lower atmosphere means a more comfortable flight, while also providing a economic advantage due to the reduced air density.
But humans cannot survive at altitude. Altitude sickness, hypoxia and decompression sickness create a triple threat that can ultimately kill the occupants of a high flying aircraft. Early pilots wore flight suits and oxygen masks to counter-act these effects. but full cabin pressurization offered superior comfort, which was essential for commercial aircraft. Here are just a small selection of the most innovative planes that help usher in this new era of travel.
1940 - Boeing 307 Stratoliner – First Cabin Pressurised Plane
The incentive to develop a pressurised cabin began to rise as commercial travel became more popular; strapping yourself into a firm fitting mask does not fit into our image of the luxurious lifestyle of 1950's jet setters, thus 1940 saw the introduction of the worlds first commercial pressurised airliner, the Boeing 307 Stratoliner.
It was the epitome of luxury, in-fact multi-millionaire playboy Howard Hughes was the first customer to receive a Stratoliner. He had originally planned to break his world record circumnavigation with his Stratoliner, but his plans were abandoned with the outbreak of World War 2. He instead transformed the plane into a veritable flying penthouse.
The war also hampered the 307's success. Only 10 307s were made before Boeing was forced to focus on the production of bombers. TWA'S Stratoliner fleet was drafted into the US military to provide transport for the troops, but the cabin pressurisation equipment was removed. The military saw it as an unnecessary luxury, which just added weight to the aircraft.
1942 - Boeing B-29 Superfortress - First Pressurised Bomber
The military toyed with the notion of a pressurised cabin, but few aircraft made it passed the experimental stage as a pressurised cockpit adds complexity and weight to the design, so engineers of the time chose the simplicity of the pressurised suit and gas mask, which is still a feature of modern day jet fighters.
The crew of the Boeing B-29 Superfortress were the first military personnel to enjoy the comfort of cabin pressurisation. A bomber poses a unique problem to engineers attempting to implement cabin pressurisation. At some point in the flight the bomb doors are presumably going to be opened, which would result in de-pressurisation of the entire aircraft. The engineers at Boeing avoided this problem by separating the occupied sections of the plane, leaving the bomb bay unpressurised. These crewed sections were joined by small tunnels which the crew could crawl through to travel between the cockpit and the 2 gunner sections, as shown.
1952 De Havilland Comet – First Jet Airliner
Cabin Pressurisation was not without its share of toothing problems. The introduction of jet engines required planes to fly even higher in order to make the fuel hungry engines economically viable. The first jet powered airliner, the De Havilland Comet, entered service in 1952 and initially proved to be a massive success, but just one year into service catastrophe struck. Three Comets suffered fatal mid-flight disintegrations and the entire fleet was grounded until the cause was identified. For a more visual explanation of the problem, my video on the subject is linked below.
As a plane increases in altitude the external atmospheric pressure lowers to a greater extent than the internal cabin pressure. This creates a pressure differential that causes the fuselage to expand ever so slightly. Engineers accounted for this, but the effects of repeated pressure cycles over time were not well known at the time. Over thousands of cycles and metal begins to fatigue becomes and cracks can form at high stress locations.
The effects of stress concentration were also not well understood at the time. Stress concentration occurs when the flow of stress is interrupted. Square windows, in contrast to modern oval windows, provide a significant barrier to the smooth flow of stress. Because of this stress peaks at the sharp corner of the window, and this is exactly where investigators determined the origin of failure to be.
These combined phenomenon proved to be fatal. Today all airliners feature oval windows to avoid this stress concentration and comprehensive fatigue testing is required before a plane can be approved by the FAA. We often learn the most from our failures, this is particularly true for advancements in the field of engineering. These events were instrumental in the advancement of our understanding of material mechanics.
Another small related fact I wanted to add: Boeing 787 Dreamliner windows are bigger because the fuselage is made from composite materials, which do not suffer from fatigue stress to the same extent as metal.