roberto_vargas_4

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Embry-Riddle Aeronautical University *

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335

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Mechanical Engineering

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Feb 20, 2024

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1 Case Study Analysis 4.2: Accident Research on Flight 38 Roberto L. Vargas Embry Riddle University BSAS 335: Mechanical and Structural Factors in Aviation Safety Dr. Thomas L. Holmes Jr November 12, 2023
2 Introduction On January 17, 2008, A Boeing 777 landed short several hundred feet from the runway in Heathrow International Airport. Primary Causal Factors of the Accident Due to the evidence, it appears that the aircraft that went down in Rome was due to structural failure of the fuselage with the one in India it is unclear if the accident was caused due to structural overload due to turbulence since they tried to fly through a storm. It seems that near the squarish windows and emergency Escape hatches were vulnerable to localized high stress how stress acts around more rigid shapes like squares and rectangles versus ovals and circles on the fuselage (Groh, 2017). This structural flaw with the pressurization of the cabin climbing in altitude caused the fuselage to fail and rupture at the localized areas of the windows where it would crack after just a few thousand cycles below its maximum cycle life (HER MAJESTY'S STATIONERY OFFICE, 1962).
3 Fig 1. (Groh, 2017) Contributing Factors to the Accident Some of the contributing factors that led to the overall failure of these areas of the fuselage could be traced back to the design of the window edges, The use of countersunk rivets in high-stress corners of the windows and emergency escape hatches, and the method of testing used in the prototype. These factors along with insufficient testing on a new technology can contribute to the failure as well (Her Majesty's Stationery Office, 1962). The window design was one of the big indicators that the area around the window when pressurized would build up stress at the corners of the windows (Groh, 2017). This localized stress increases as the plane is pushed and pulled in different directions due to forces like lift, drag, interior pressure, temperature ranges, and exterior forces like gusts and turbulence acting on the aircraft. The stress could not easily move around the rigid corners of the windows and
4 hatches collecting there like a roadblock on a highway. As the aircraft experiences more and more cycles of pressurization and depressurization the push and pull would over time create fatigue cracking that over time grows until the material fails. The countersunk rivets were an area of failure for the cracks to continue growing as the aircraft continued flying. It is known in the Structural mechanical world that countersunk rivet heads or “flush heads” are not strong with bonding two pieces of material like the skin to the mail aircraft structure ( Solid shank rivet, 2023). When preparing the surface of a sheet metal rivet hole for a flush head rivet, the area where the head goes is either countersunk with a cutting tool or dimpled with a special dimpling machine. This process reduces the bonding potential of that area leaving it more vulnerable than if you were to use button head rivets instead. Another factor that contributed to these accidents was how DeHavilland tested the Prototype and how it affected the results of their pressure testing of Comet 1. When testing the prototype, the aircraft was over-pressured to 2 P which is twice the operational pressure of the cabin in flight. When this happened this caused the rivet holes during the pressurization to coldwork which had the effect of strengthening the areas around the holes allowing the test to last the 16,000 cycles they recorded during testing ( De Havilland DH-106 comet 1, 2023). This effect did not apply to the production models of the aircraft and did not have the cold work effect giving it this property. Structural and Mechanical Factors Related to the Accident The factors that led up to each of these accidents look mainly structural due to the cause most likely to be pressurization failure of the fuselage. This is indicated by the test The Ministry of Aviation ran in 1962. This document directly annotates the factors that led to the fuselage
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