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[email protected] .ac.uk ABSTRACT — At 15.16 on July 19th 1989, Flight 232 travelling from Denver Colorado to Philadelphia Pennsylvania experienced a disastrous failure of the No.2 tail mounted engine. As a result of this event the hydraulic lines that allowed for controlled flight to be possible were damaged making it incredibly difficult to control the aircraft. Pilot Al Haynes and First Officer William Records and an off duty pilot Denny Fitch came to the cockpit to operate the throttle and flight controls. Due to the failure of the engines the throttle was stuck; Fitch had to use all of his body weight to shift the throttle control. The aircraft banked heavily to the right making the lift very unbalanced. Al Haynes instinctively closed throttle to No.1 engine and fired full throttle to No. 3 engine. This went against anything he had ever been taught but gradually brought the plane back level 1. In an attempt to make an emergency landing at Sioux Gateway Airport (Iowa) the aircraft crash landed. At time of the failure of the engine they were 65 miles southwest of the airport. When the aircraft was approaching the 8999ft runway it was travelling at 220 knots, this was 80 knots faster than the safe landing approach speed – meaning a definite crash landing.  At 15:49 the aircraft crashed in a corn field near the runway resulting 111 fatalities and a further 172 Injuries 2.  Introduction – United Airlines Flight 232 used a McDonnell Douglas DC-10-10 aircraft, they were known for having poor safety records. Leading up to 1989 thirteen separate incidents occurred over two decades due to fundamental design flaws and incorrect maintenance. Aircraft flight is accompanied with its obvious risks, unfortunately if a serious fault or failure occurs the consequences are often far more serious and result in high numbers of fatalities. Over the history of air travel the amount of aircraft related fatalities is reducing. This is profoundly due to learning and not repeating mistakes made in the past. This is why reporting and investigating into what caused an event like flight 232 is so important. The analysis of factors allows more clarity on how the event occurred and how to prevent or reduce the risk in the future.By analysing both human and technical factors a study on how an engine explosion cause 111 fatalities will be formed. This will then be followed by a detailed discussion into regulations associated with the aviation industry and if any were breached leading up to the disaster.   ANALYSIS – The disaster was started due to the failure of a General Electric Aircraft Engine (GEAE). No.2 tail mounted engine was a model CF6-6D turbo fan. DC-10-10 aircraft had three engines: No.1 and No.3 on the wings and No.2 on tail. Flight 232 was the first time a problem with the CF6 engine had malfunctioned in its millions hours of service. The stage 1 titanium fan disk inside No.2 engine fragmented causing an engine explosion. The fan disc failure was as due to a fatigue crack which had originated from a defect on the disc bore surface 3. During investigations after the incident the fatigue crack was claimed to be of a critical length and therefore should have been replaced.If there was a surface defect on the disc bore, the quality control on engine parts must have not been sufficient. At the time of the failure the fan disc had completed 41009 hours of service and 15503 cycles. The engine components are designed to not to fatigue over the life span of the entire engine. It may have just been a one off, analysis into the production of the titanium fan disc may give an answer into how the disc fatigue and catastrophically failed. The GEAE processes for producing titanium parts for their engines went through has had two stages. At the start when they melted the titanium it would react with the air, where it did this there would be impurities. Impurities in the metal can lead to cracks over time like that found on the fan disc. To reduce the impurities created during manufacturing a new process called ‘double vacuum’ was introduced. This is when the melting of the titanium is carried out in a vacuum allowed to cool, then the same process repeated again in a vacuum. Double vacuum was used in the manufacture of the fan disc that was in No.2 Engine.Once the double vacuum had been completed it went onto testing and quality control of the ingot produced. Ultrasound was used to detect for impurities. An ultrasound transducer emits ultrasonic waves (frequency > 20 kHz), each boundary that a wave passes through it is partially reflected and received at the transducer. Using simple calculations it can pin point boundary layer and plot on a screen.  v=speed of Electro Magnetic Waves (3×108)?=wavelength of ultrasoundf=frequency of ultrasounds=distance travelled to and from boundaryt=time between emission and wave being received If there is only one boundary (titanium) it will appear as a solid however if there is an impurity the screen will show a solid with miss areas indicating an impurity. Depending on the amount and sizes of these impurities a decision is made to either pass the material to be used or not. The impurities are caused due to hard alpha inclusion. Hard alpha inclusion are hard, low density regions of a titanium alloy that make the material more brittle. The regions are made up of a high concentration of nitrogen and oxygen matter 7. When the fan disc then inevitably shattered, fragments were fired in all directions. These fragments then split the three individual isolated hydraulic systems.  These systems then lost a significant amount of their fluid. With the vitally important hydraulic systems failing to perform and on engine destroyed this made controlling the aircraft close to impossible. When the hydraulic systems were designed they were isolated from one another in the case that if one broke there were two more still functioning. However it was discovered that although the three hydraulic systems were physically separated from one another, they were all within close proximity to each other; this meant it was a zonal target for the spray of sharp fragments.A fan disc is classified as a critical component and it has a number of key functions. The blades that make up the bypass fan are fixed into this disk – if this disc fractures the engine must be able to with stand damage cause by the loose blades. Secondly the fan disc connects the bypass fan onto the low pressure shaft, this shaft is then connected onto the remaining low pressure components inside the engine. Turbofan engines consist of two sections of high and low pressure. They both contain similar parts apart from the low pressure section contains a bypass fan. The bypass fan is what can be seen when looking at a turbo fan straight on. Engine thrust relies upon the bypass fan bringing in large volumes of air which is then used in the compression chambers. Vapour fuel is then added and ignited in the compression chamber causing thrust out the back of the engine. The bypass fan is also what protects the engine from airborne hazards e.g. birds and extreme weather conditions. Finally the fan disc is required to affix the nose cone on to the engine which allows for aerodynamic air flow into the face of the engine. Due to the vast responsibilities of the fan disc, it is considered to be the most dangerous engine component failure a pilot will have to tackle 5. Although it may be obvious just to assume that the engine failure was purely to blame for the disaster, it would be ignorant to ignore the possibility that human factors had a part to play in the disaster. Firstly when an engine is designed it has to pass rigorous testing procedures, for the engine to be commissioned No.2 tail mounted engine passed these tests.Before every flight, checks and maintenance is carried out to ensure the aircraft is completely safe to fly. If something is found to need replacing then it will not fly until the relevant maintenance is performed. This then raises serious concerns into how a fatigue crack of critical length was not noticed by the crew carrying out the pre-flight checks. Personnel are responsible for these checks, they will follow a list of checks before every flight. The crack being missed could mean two things.·        Personnel who were in charge of the checks didn’t follow set guidelines and missed a critical failure.·        Organisation in charge of creating the pre-flight checks missed fundamental checks of the guidelines.On board flight 232 were 285 passengers and 11 crew members. When the incident occurred Fitch (off duty pilot) went into the cabin to help gain control of the aircraft, leaving the remaining crew members to prepare the passengers for an emergency landing. The crew should have been trained into dealing with a crash landing and how to reassure panicking passengers. Jan Brown was head flight attendant, she has been described as doing everything correct and in followed what she had been trained to do. She briefed her crew one by one in passing to avoid raising any suspicions to the passengers. Closer to the airport she carried out the difficult task of briefing the passengers which required children to be put on the floor rather than on their parents. In 1989 there was no regulation regarding infant safety in flight, the flight attendants provided cushions and blankets for protection padding for the infants 2. Adults were instructed to adopt the brace position and prepare for what Al Haynes described as the roughest landing they have will ever experience 1. As you can see from figure 3 the flight down to Sioux Gateway airport was very much out of control, the initial banking after the failure caused a flight path to the right of 20 miles radius and even when the aircraft levelled the path spiralled towards the airport increasing and decreasing in elevation as full control was not possible.DISCUSSION – Aircraft manufacturing has to comply with many federal aviation regulations, these cover all part of the aircraft’s life span. Regulations manage and control the many variables involved with flight, from the design of the aircraft to the pre-flight checks carried out before every flight. Regulations set the basic standard which ensures safety and quality within the aviation industry. However standards can be dropped which can unfortunately produce severe repercussions.McDonnell Douglas the manufacture of the aircraft had a non-legally binding agreement with the FAA (Federal Aviation Administration) that the FAA would provide sufficient maintenance checks of their aircrafts. Although United Airline employees would perform the checks they were strictly following the guidelines set by the FAA. The United Airlines flight 232 wasn’t the first incident for this agreement to be breached. In 1979 a DC-10 aircraft also crashed in Chicago, investigations concluded that the FAA failed to provide satisfactory safety standards 8. On inspection after the NTSB (National Transport Safety Board) issued 29 safety recommendations. Of the 29 in total, 23 were issued towards the FAA. These recommendations are issued so that the areas of concern can be analysed and improvements can be made. Due to the number of variables which all contributed to the disaster the safety recommendations covered: aircraft control systems, cabin procedures, engine uncontained failure 3. From the national transport safety board report it is clear that the pre-flight checks and maintenance were not the responsibility of United Airlines but in fact the FAA’s. On the 15th September 1989, McDonnel Douglas released design developments of the DC-10’s hydraulic systems. These developments allowed for sustained flight controls should all three hydraulic systems get damaged. Three separate amendments were released.1.      Electrically operated shut-off valve and check valve in the third system return hydraulic line.2.      A sensor switch in the third system hydraulic reservoir.3.      A cockpit warning light to warm crew that shut off valve has been activated.This necessary development was required to be implemented onto all models of the DC-10 aircraft within twelve months. The DC-10-10 was subject to certification requirements, these included Part 25 Amendment 1-22 and a Special Condition (SC) 25-18-WE-7, which were dated 07/01/70 10. As part of this regulation it was outlined that the aircraft design must include features to lessen the hazardous damage caused during an engine failure. A special condition was issued due to §25.903(d) (part of Code of Federal Regulations) being in process of being reviewed. McDonnell Douglas responded to the special condition by an assessment of the hydraulic system design. This addressed the issue of the hydraulic systems being in close proximity to one another and in a vulnerable location. However at the time of the assessment there was no official regulation/FAA guideline on minimizing hazards during an uncontained engine failure. Since then CFR 25.903 has been updated to certification 14 CFR 33.19, which relates to durability of engines 10. This now results in any engine being built must be able to self-contain the rotor debris without causing any further damage to any other component of the aircraft.Detailed investigations were then focused on the fan discs used in the CF6 engines. The records kept by GEAE showed that eight fan discs were made out of the same titanium ingot. Six fan discs were in service and were all recalled for the investigation. The six discs had serial numbers MPO 00382 to MPO 003882. The results of the study concluded that two discs MPO 00388 and 00382 contained defects. These defects were unable to be identified by the standard ultrasound testing, although the defects were minute over a number of cycles they could potentially result in a similar failures to what happened to flight 232. The manufacturing process needed to be modernised. Firstly instead of the two vacuum process, a third vacuum stage was added to the process to further reduce the amounts of defects inside the titanium ingot. Secondly a new testing procedure was needed which could detect defects of smaller magnitude. The solution was macroetch inspection, this provides a cross-sectional view of a weld or material: showing different grain boundaries and also defects in inspection sample 12.  Finally professional ethics must be discussed when such an event transpires. These ethics dictate the behavior and actions of the engineering profession. The first fundamental duty outlined in the code of ethics is to uphold a paramount standard of safety 13. It was concluded that the safety regarding children on board wasn’t ethical or up to standard. Since the happenings of flight 232 the FAA have updated their guidelines on infant restraint safety. Currently, infants less than two years old can be held on the lap of their parents. The safety board still believes that this isn’t the safest solution and wish for infants under two year to have a special seat. However FAA argues that the valuable time used to distribute these restraints could be better spent during an emergency. Lastly the FAA currently requires infants (older than 2 years) to have a lap seatbelt similar to an adult. These changes all occurred due to the safety board recommending A-90-78 and A-90-79 which addressed the child restraint methods 2. CONCLUSION – United Airlines Flight 232 undoubtedly could have been prevented. After gathering information from a variety of sources I deduce the cause of the disaster was the critical engine failure of No.2 tail mounted engine which severely damaged the flight controls. However the inability to carry out sufficient maintenance before the flight commenced and not noticing a fatigue fracture of critical length allowed for an engine failure to be possible. Had the crack been seen, the aircraft would have never taken flight and landed safely with no fatalities. This conclusion is justified by the in-depth report performed by National Transport Safety Board which claims inadequate inspection of the aircraft by United Airlines engines repair facility resulted in a critical fatigue crack going unnoticed 9. Although United Airlines were responsible for the checks, the FAA set the guidelines for these checks therefore I also conclude that the FAA must accept significant responsibility for the disaster. Personally I believe there was no on board human factors which contributed to the disaster. The NTSB issued a recommendation relating to time management in an emergency situation. Pilots made every effort to land the incapacitated aircraft with the exceptionally limited control. Flight attendants prepared passengers for an emergency landing. Without the heroic response of all the crew the number of fatalities would have been significantly more.        REFERENCES –1.      Popular Mechanics. (2017).The Crash of United Flight 232. online Available at: http://www.popularmechanics.com/flight/a10478/the-final-flight-of-united-232-16755928/ Accessed 07 Dec. 2017. 2.      Federal Aviation Administration (2005) United Airlines, Available at: https://www.faa.gov/about/initiatives/maintenance_hf/library/documents/media/human_factors_maintenance/united_airlines_flight_232.mcdonnell_douglas_dc-10-10.sioux_gateway_airport.sioux_city.lowa.july_19.1989.pdf  Pages(54, 70) Accessed: 12/12/17. 3.      Federal Aviation Administration. United Airlines, Available at: http://lessonslearned.faa.gov/ll_main.cfm?TabID=3=17=2 Accessed: 08/12/17.4.      NASA. Turbojet and Turbofan Systems, Available at: https://history.nasa.gov/SP-468/ch10-3.htm Accessed: 06/12/17. 5.      Paul Scott (03/12/17) What is a Fan Disk?, Available at: http://www.wisegeek.com/what-is-a-fan-disk.htm Accessed: 04/12/17. 6.      National Transport Safety Board (1990) Aircraft Accident Report, Available at: https://web.archive.org/web/20110104033524/http:/www.airdisaster.com/reports/ntsb/AAR90-06.pdf Accessed: 10/12/12. 7.      Thermoelectric detection of hard alpha inclusion in Ti-6AL-4V by magnetic sensing – ScienceDirect. 2017. Thermoelectric detection of hard alpha inclusion in Ti-6AL-4V by magnetic sensing – ScienceDirect. Available at: http://www.sciencedirect.com/science/article/pii/S0925838806002829. Accessed 09 December 2017. 8.      Fielder, J.H.F (1992). The DC-10 Case: A Study in Applied Ethics, Technology, and Society. Social Science: SUNY Press, page 10. 9.      National Transport Safety Board, United Airlines Flight 232 McDonnell Douglas DC-10-10, Available at: https://www.ntsb.gov/investigations/accidentreports/pages/AAR9006.aspx Accessed: 11/12/12. 10.   Federal Aviation Administration. Relevant Regulations / Policy / Background, Available at: http://lessonslearned.faa.gov/ll_main.cfm?TabID=3=17=5 Accessed: 11/12/12. 11.   United Airlines Flight 232, Available at: https://readtiger.com/wkp/en/United_Airlines_Flight_232 accessed 13 December 2017. 12.   Malcolm, Macro Etch (2011), Available at: http://www.mig-welding.co.uk/w/index.php?title=Macro_Etch accessed 13 December 2017. 13.   National Society of Professional Engineers (2017) NSPE Code of Ethics for Engineers, Available at: https://www.nspe.org/resources/ethics/code-ethics Accessed: 13/12/17.

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