roberto_vargas_4.2_Case Study Analysis_Accident Research

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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

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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

5 failing in mid-flight. Although some repairs and alterations were done to the Comet to improve the structure after the third accident, an investigation was conducted on the G-ALYP Report ( CIVIL AIRCRAFT ACCIDENT, 1955). The primary cause due to the reports seems to be the windows and emergency escape hatches and their ability to create stress fractures localized around the corners. A few items that I noticed were the rivets they used in these stress areas, the locations of the rivet holes, and the cracks that go through some of them. The rivets seem to have a role in how the fatigue stress grows into the cracks they form. In these areas the countersunk rivets used at the corners of the windows and hatches seem to show the origin of the fatigue and what may have caused the structure to fail. Countersunk rivets are great for aerodynamic applications but are not the best fastener to use in areas known to have high stress. As mentioned in the G-ALYR report para 4.2 cracks entering rivet holes were not stopped during fatigue testing and in someplace could have started in the areas where the skin was countersunk (HER MAJESTY'S STATIONERY OFFICE, 1962). It was noted in the G-ALYP report that the countersunk around the window was added to strengthen the structure around the corners of the window due to DeHavilland knowing it was a highly stressed area ( CIVIL AIRCRAFT ACCIDENT, 1955). The design of the windows themselves causes a lot of the localized stress to form in the areas of the corners which created higher than expected fatigue in those areas. The structural shape of these windows allowed for cracks to form over time until the fuselage broke apart. The flight at Elba with an in-flight break up at 27,000 feet and 40 minutes after takeoff, and the Naples flight at 35,000 feet also 40 minutes after takeoff ( De Havilland DH-106 comet 1, 2023). When tested after the accident the stress around them was more than tested by De Havilland when testing the prototypes.

6 I also believe that the prototype and the way it was tested, set a false condition for the production model of aircraft. The cold working process is when metals go through plastic deformation below their recrystallization temperature ( Metallurgy for dummies, 2023). This leaves the cold work areas with certain properties unlike the original material such as the hardness and tensile strength depending on the degree of cold working accomplished. Oddly enough the prototype comet was tested at twice the load required to ensure compliance. At the time it was not clear if this was known but when the test cycled at twice the pressure it would experience in flight 30 times, and 2000 times at other pressure ranges it invertedly cold worked the area around the square window corners strengthening them in the process ( De Havilland DH- 106 comet 1, 2023). This created a false promise that allowed the aircraft to survive 16,000 cycles and rate the aircraft for a 10,000-cycle life. The production models would not be expected to be subject to these pressures often which in turn would not recreate the condition naturally strengthening the rivet holes and localized stress areas against fatigue. Relevant Human Factors and Organizational Factors Related to the Accident Regarding the human factors of the accidents, a few factors play a part. When the Comet was being produced and flown for passenger travel, there was a competition to see who could produce the world’s first commercial jet airliner to have the comfort of a pressurized cabin experience. De Havilland along with Lockheed, Boeing, and many others were developing pressurized aircraft for all markets (Novell’s, 2018). Becoming the first in many areas of commercial travel to attract the masses and bring aerial travel to everyone. This rush to market and making history without fully understanding the technology they were developing is partly to blame for these accidents. At the time of the Comet's creation, there were no clear guidelines or regulations to define what was safe until shortly after the aircraft was built and flying in the

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7 fleets ( De Havilland DH-106 Comet 1, 2023). During testing, they did exceed the basic requirements for pressurization, but in doing so led to a misleading conclusion. De Havilland also did not subject any of the production models that came out of the assembly to any form of testing to ensure the standards. The testing of the production model would have prevented the company from committing to the standard in place if they knew under normal conditions the cycle life would have been less. Also, it should have been the responsibility of the testing team of an engineer to look at these areas for deformation and unforeseen properties. I am sure they looked for cracks and deformation and typical signs of stress, but what they didn’t see was the cold working of the holes allowing them to survive the rigorous testing. With modern technology this would have been easy to do however if the engineers were looking for signs of the effects of pressurization on an airframe instead of narrowing their focus on whether their design could withstand pressurization enough for flight it might have been found and accounted for. Outcomes of the Accident After the investigation of the original three accidents reported the conclusion arrived at was that more testing was needed to determine the stress applied to the aircraft with the regular use of strain gauges to make calculations more accurate ( CIVIL AIRCRAFT ACCIDENT, 1955). With the suggestion of a new design after testing to address the problems that arise during testing. The testing done at the R.A.E. site was done using a large water tank that the fuselage was pressurized in to simulate loading without the aircraft suffering major damage so they could test the effect more often (HER MAJESTY'S STATIONERY OFFICE, 1962). These tests led to a better understanding which resulted in De Havilland taking every precaution moving forward to cover every possible angle that was suggested caused the accident.

8 At this time of the accidents, there was no Airworthiness Directive issued due to the uncertainty of the accident cause and it being in the infancy of Commercial aviation ( De Havilland DH-106 comet 1, 2023). The Comet 1 was still in circulation but in very limited numbers until the Comet 1A was released as the redesign of the original improved the fuselage strength, and other changes were made. The Comet 1XB was made with an Ovel window to mitigate the fatigue stress of the window on the skin of the aircraft ( Comet 1 SN diagram animation, 2021). Risk Mitigation or Reduction Strategies Due to the way the Comet was tested a produced as the first commercial jet airliner, the first thing I would point out is the need to compete. De Haviland did their research and testing at the time to prove that it was safe to the public as much as to the organizations that regulated air travel as publicity to promote their product. Skipping the long-term research required to ensure the safe use of a pressurized cabin, most of the aviation world at this time was unfamiliar with what would happen. Regulators started publishing documentation governing standards over pressurized vessels, but at this time Comet 1 was already flying ( De Havilland DH-106 Comet 1, 2023). The engineers testing the Prototype should have realized at some point that the structure they designed was strengthened in fatigued areas due to the nature of testing. I believe the test was not carried out with a more scientific approach with controls or an open mind. They had a focus on seeing if the fuselage could survive multiple cycles and they missed the effects of testing beyond what was required. They should have had a control like two fuselages test at the same time to simulate. One would be at the standard pressure that would be experienced in flight and the other could have been to test the stress-loading effects of irregular pressurization. This

9 could have allowed the designer, in the beginning, to see how many cycles the aircraft could withstand and would have found that 2 P was Cold working the fuselages localized stress areas like the windows and hatches. From the images I saw in the testing done in R.A.E., I noticed the use of closely packed countersunk rivets supporting the corner of the fatigue areas. Countersunk heads are used as bonding fasteners but are not as strong as other fasteners like button-head rivets. They significantly weaken the skin’s ability to bond to the main structure. Especially in high fatigue areas where flexing and deforming could occur. Engineers today avoid using rivets that could weaken the ability to bond when high-stress loads are present. Another thing I did not see in my research or understood after reading the report was if cold-working the holes around the windows and hatch worked so well why didn’t they use this process to fix and improve the design? It seems that the cold work process that was accidentally achieved worked well enough for the prototype to survive 16,000 cycles. It could have been refined on later models and used to extend the life of this aircraft. Conclusion After reading it I understand why Comet 1 had structural failure well below the cycle life of the airframe. The combination of narrow focus on the problems they aimed to solve and the company’s eagerness to show the aircraft’s safety inadvertently created false assumptions. With the culmination of not testing the production model to ensure their results and knowing that the areas around the windows were susceptible to high stress yet did not find an alternative until a few models later. With more meticulous testing and proper research on how pressure affects the airframe, it would have been caught sooner. This case study brings into view the information and experience I’ve learned my whole career. I believe these accidents help change the standards and

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10 help us understand the effects better. Shaping our understanding and teaching others to be aware of so they do not occur in the future. References Blogger. (2023, January 4). Solid shank rivet - aircraft structural fasteners . Aircraft Systems. https://www.aircraftsystemstech.com/2018/12/solid-shank-rivet-aircraft-structural.html De Havilland DH-106 comet 1 . Federal Aviation Administration. (2023, March 7). https://www.faa.gov/lessons_learned/transport_airplane/accidents/G-ALYV Federal Aviation Administration . (2021, January 4). Comet 1 SN diagram animation . YouTube. https://www.youtube.com/watch? time_continue=9&v=QgjoN15kfAk&embeds_referring_euri=https%3A%2F%2Fif- cdn.com%2F&source_ve_path=Mjg2NjY&feature=emb_logo Groh, R. (2017, May 9). The dehavilland comet crash . AeroCert Online. https://www.aerocertonline.com/the-dehavilland-comet-crash/ HER MAJESTY’S STATIONERY OFFICE. (1962). Behaviour of Skin Fatigue Cracks at the Corners of Windows in a Comet I Fuselage . FAA. https://www.faa.gov/sites/faa.gov/files/2022-10/G-ALYR_Report_0.pdf Metallurgy for dummies . Metallurgy for Dummies Cold Working Processes Comments. (2023). https://www.metallurgyfordummies.com/cold-working-processes.html MINISTRY OF TRANSPORT AND CIVIL AVIATI. (1955, February 1). CIVIL AIRCRAFT ACCIDENT Report of the Court of Inquiry into the Accidents to Comet G-ALYP on 10th January 1954 and Comet G-ALYY on 8th April, 1954 . Federal Aviation Administration. https://aviation-safety.net/database/record.php?id=19540110-1 Novells, R. (2018, May 25). When did pressurized cabins on commercial airliners become a reality – May 25, 2018 . Robert Novell’s Third Demension Blog. https://www.robertnovell.com/when-did-pressurized-cabins-on-commercial-airliners- become-a-reality-may-25-2018/