Everyday thousands of people travel through airplanes all around the globe but most people aren’t aware of why the windows are rounded. The current generation of airplane travelers think that the windows has always been the shape it is today, i.e. oval but this is not the case. Before 1950s aircraft did have square windows. The evolution from square to round/oval windows isn’t simply for aesthetics, there is also a science behind this. This is the physics of Fracture Mechanics.
In 1903, wright brothers made the first flight and it wasn’t until 1952 the first jet airliner made its first maiden flight. The De Havilland DH 106 Comet was a great success since it could fly up to 35000 ft because of its pressurized cabin. The Higher Flight altitude meant that the density of air is low. This results in less drag on the structure which leads to higher speed. Bad weather could be avoided at higher altitude. The aircraft was considered feat of British engineering at that time. Even queen Elizabeth made a special flight on 30 June 1953 but the fame did not last long when two DH 106 Comet broke apart in midair killing 56 people. BOAC Flight 781 after suffering explosive decompression crashed into the Mediterranean killing all 35 people on board and South African Airways Flight 201 disintegrated in the night sky at around 35,000 feet (11,000 m), killing everyone (21 people) on board.
An extensive investigation was conducted on both crashes.The findings of the investigation proved that the mid air disintegration was due to their square windows. De Havilland had infact conducted many tests during the production of the aircraft and the company had even proved the aircraft couldn't fail under pressurized cabin. Everyone at that time believed the pre-production test to be enough for safety ,since de Havilland had conducted lots of experimental tests.The Calculations during the design had been made for an average stress which found the stress to be less than half the ultimate strength of the material. The calculation seemed enough for the flight safety and De Havilland did not calculated further stress values and relied on testing as the main evidence for the safety of the Comet. The company missed one thing, it did not take account of stress concentration in the corners.
Figure 1: De Havilland DH 106 Comet with square windows
Stress concentration is the accumulation of stress in a body due to sudden change in its geometry. When there is a sudden change in the geometry of the body due to cracks sharp corners, holes and decrease in the cross section area, then there is an increase in the localized stress near these cracks, sharp corners, holes, and decreased cross section area. The body tends to fail from these places where the stress concentration is more. So to prevent a body from getting failed, the stress concentration should be avoided or reduced.
Figure 2: Stress concentration on the corners due to abrupt change in geometry
The investigation found that the stress concentration was high because of the square shape of the windows and window frame.The abrupt change is geometry on the corners disrupts the stress flow .This is not the case with modern rounded windows.Due to higher stress on the corners, cutouts in the structure there is always a chance of crack initiation on those areas. The aircraft structure is under various loads. All the loads are not of constant magnitude and are in cyclic nature. This results in fatigue load. Fatigue load results in crack growth over a large span of cycle. But since fracture mechanics was not understood in 1954, the crack propagation theory was not considered during design of the aircraft. The failure of the pressure cabin was due to fatigue crack growth from defects which were probably present from the construction of the aircraft and had not been a problem in earlier designs of aircraft, as the required cabin pressure had been lower. That this problem was not detected by the rigorous testing undertaken by de Havilland was probably due to an unfortunate set of circumstances with regard to the order in which the tests were performed, and could not easily be foreseen at the time.
Figure 3: Stress concentration of square and oval windows
Figure 4: Computer animation of BOAC Flight 781 Midair disintegration
After these high profile crashes, the comet wasn’t able to recover its old glory and in 1997 the original Comet I made its final flight. The failure of comet took the aircraft manufacturers by surprise. After withdrawing the comet 1 from service extensive fatigue test were conducted on the aircraft. This helped the aircraft manufactures to identify the design flaws and critical areas in the aircraft. Although the accidents of comet 1 were tragic ,it opened a new frontier for safety of future aircrafts.
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