Materials Technology: Qantas Plane Engine Failure - Case Study Example

Running Head: MATERIALS TECHNOLOGY Qantas plane Accident Student’s Name: Course Code: Lecture’s Name: Date of presentation: Table of Contents Table of Contents 2 Case Study: Qantas Flight 32 3 Background and Literature Review of the case 3 Damage to the plane 4 Analysis of the possible Causes of Failure 5 Description of the Engine 6 Nature of Damage on the EngineNo.2 7 Analysis of Potential Causes of Failure 7 Failure Report Critique 11 Additional Information or Investigation Required 12 Recommendations 12 Part B: Materials Selection 13 Performance Requirements 14 Drawbacks of Inconel material during machining 15 Materials Selection Method 16 Justifications for Material Selection 17 Additional Investigation and Study 18 Suitable Materials for the Stud Pipe 18 Conclusion 18 Bibliography 19 Since the first man used an airplane, air travel has been a complicated and risky as the equipments and operations involved are prone to failure with fatal outcomes. Human error and technical hitch has been attributed to a number of regrettable air accidents in the past. During travel the plane is subject to increased risks due to operating altitude of more than 20, 000 feet above ground. The risk is also magnified if the facilities available such as airports are not in good conditions (Flynn 1961). This paper analyses the report prepared by Australian Transport and Safety Bureau. Case Study: Qantas Flight 32 Background and Literature Review of the case According to Australian Transport Bureau, the particular Airbus aircraft, took off from Changi airport, Singapore and destined to Sydney Australia. On board there were 440 passengers, 24 and five cabin and flight crew. As a norm, immediately after taking off successfully from the airport, the flight crew retracted backs the flaps and landing gear. After stabilizing at 250knots and having attained an altitude of 7,000 ft above sea level, the crew overheard two coincident loud bangs and subsequently the engine No2 inboards indicated some characteristics of failure. The crew experienced a slight yaw and the plane levelled off immediately as per the selected attitude. The autopilot option was no longer active at that point as well as the flight directors. An error message showing “overheat” danger sign in the engine No.2 turbine was displayed on the Electronic Centralized Aircraft Monitor (ECAM) (Australian Transport Safety Bureau 2010). After informing the Singapore Air Traffic Control back in Changi Airport, the flight crew was provided with radar vectors necessary to hold the plane in a defined pattern. After realizing that the damage on this particular engine was major, they discharged two bottles of fire extinguisher but not confirmation was received. They then opted to automatically transfer fuel from the outer wing tanks to the inner tanks. At this type engine No.2 displayed a “failed mode”; while engines No. 1 and 4 had lapsed to a degraded mode and engine No. 3 was working in alternate mode, amongst other warning and error messages. The crew gallantly performed certain actions before descending safely back to Singapore. There were no injuries to the passengers and crew on board (Wikipedia 2012). However, there were some injuries sustained by two peoples on Batam Island, where the incidence occurred. Condition of Engine No.2 upon landing Damage to the plane As a result of failure of the No.2 engine, a number of engine turbine components were liberated overboard at a high speed and landed on the aircraft body. The pieces of the intermediate pressure (IP) turbine struck the edge inboard of the left wing of the engine No.2, leading to structural damage of the leading edge structure. A number of system components were also severed by the thrown-out turbine disc (Australian Transport Safety Bureau 2010). Aircraft Systems The freed debris struck and penetrated the lower structure of the left wing, leading into damage of number of operation systems such as hydraulic system, electrical wiring, flight controls and landing gear, landing edge slat coordination , and fuel leak on both inner tank and feed tank that serves the No. 2 engine as shown in fig. 2 below. Fig.2. Fuselage damage by the liberated turbine debris (Australian Transport Safety Bureau 2010) Analysis of the possible Causes of Failure Technical Details of the damaged Engine No.2 Manufacturing Company Rolls- Royce Plc Year of Manufacture 2008 Hours of Service Prior to Incident 6, 314 Total Engine Cycles 677 Engine Serial Number SN 91045 Description of the Engine The affected engine is a 3- shaft, high by-pass ratio with low pressure turbofan, with high pressure (HP) and intermediate pressure (IP) compressors driven via co- axial shafts by the turbine as shown in the figure 3. Below; Fig 3.Typical No.2 engine structure (Australian Transport Safety Bureau 2010) As a result of this near accident incident, a sequence of safety actions was undertaken by Airbus, Qantas airline, Rolls-Royce and the European Aviation Safety agency. Nature of Damage on the EngineNo.2 Examination results showed that there was an uncontained failure on the No.2 engine’s Intermediate pressure (IP) turbine. The nozzle guide vanes, blade and turbine disc disintegrated into several sections, tearing the adjacent IP turbine sheath and ultimately severing the thrust reverser of this particular engine (Australian Transport Safety Bureau 2010). Damage was also extended to the outer cowl panels and cold stream duct of the engine. Other damages were examined on turbine blades of stage one LP, nozzle guide vanes of LP turbine and thrust links of engine No.2. Fig 4. Damage on the No.2 Engine Analysis of Potential Causes of Failure The different teams dealing with the investigation have analyzed the failure recorded on Rolls- Royce Trent 900 airbus engine of this particular Airbus A380 plane of Qantas airlines. The investigators have concluded that there was a realistic manufacturing issue with oil pipes of some particular engines. In their preliminary submission into the cause of the engine failure, the Australian transport Safety Bureau gave out a safety recommendation concerning the probable engine failure causes in some engines from Rolls- Royce used in A380 engines In this report, it is shown that there was an accelerated risk of formation and growth of crack, leakage of engine oil and a possible disastrous engine failure due to oil fire as a result of poor manufacturing of the oil stud tube. During operation, this stud tube delivers oil into the structure bearing the high pressure (HP) / Intermediate Pressure (IP). It has been noted that the engine failure was as a result of oil leakage into the buffer zone of the (HP)/ IP structure. This eventually led to the rapid liberation of a piece of the turbine disc; which is suspected to have caused damage to the fuselage and thus electrical and hydraulic system. At the Rolls- Royce factory in Derby, the engine components removed were closely examined and a fatigue cracking was detected within the oil stud pipe. The fatigue cracking had developed within a section of the pipe which had a thinner wall than the rest of the pipe wall. This defect was suspected to have been as result of axial misalignment during the counter- boring process in the internal bore of the pipe (Australian Transport Safety Bureau 2010). This axial misalignment created a localized thinner wall on one side of the stud pipe. According to the assessment, the likely cause of engine failure was the sudden release of oil from the stud pipe into this particular engine part, this in turned cause an oil fire and an “uncontained engine failure” resulted in the subsequent liberation of engine disc elements (Australian Transport Safety Bureau 2010). Fig.5: Fatigue cracking as indicated in the initial ATSB “failure report” (Australian Transport Safety Bureau 2010) Fatigue Cracking as a result of axial misalignment during counter boring of the stud pipe In response to this investigation results, the engine manufacturer Rolls- Royce, safety regulators and the affected airline companies embarked on a closer scrutiny of the affected engines and a times grounding some of the planes whose engines showed similar stud tube problems. Besides, the European Aviation Safety Agency has developed customized engine control software to minimize risks associated with disc failure related to speedy turbines. According to the interim report of ATSB interim report of the investigation, the IP turbine disc suffered failure due to increased speed condition, sending off parts of the turbine discs that struck the wing structure and engine casing. The failure of the turbine disc was set off by a manufacturing oversight in an oil feed tube which resulted in reduction in wall thickness while counter boring. The thin section suffered fatigue cracking in the course of engine operations that caused fire in the internal engine oil thus weakening the adjacent part of the IP turbine disc. A circumferential fracture developed on the turbine disc, thus separating it off from the axial shaft. The abandoned disc burst out after attaining the critical centrifugal speed; penetrating into the engine casing and out into the airframe physical structure and systems. After the release of the initial preliminary “failure report”, a number of tests have been conducted on the fracture surface of the oil stud pipe suspected to have caused oil leakage and thus the oil fire. This has led to an improved understanding of the mechanism of the pipe failure. After carrying out thorough technical reviews it has been established the exact local point of failure is not as indicated in the preliminary initial ATSB report as shown in fig. 5 above. The actual location of misaligned axial counter bore and fatigue cracking is understood as indicated on the oil stud pipe shown in fig 6 below (Australian Transport Safety Bureau 2010); Fig. 6: Revised location of the misaligned counter boring and fatigue cracking After the incident, Roll-Royce plc initiated the removal from service of all the Rolls- Royce engines whose oil pipes were not manufactured to standard of minimum wall thickness limit of 0.5mm. However, there were no measurement records of some pipes from the FW48020 standard. This meant that the company was unable to ascertain whether the said pipes were manufactured to the set specification. Failure Report Critique Technically the failure report produced by Australian Safety Bureau is logically incorrect. The approach used is pre judgment on the assumption that the accident was only caused by engine failure. Although initial indicators point out on the plane developed damage on its engine; it is too early to rule out any other cause. For instance, the plane could have suffered a terrorist strike! The approach should not have shown outward inclination to one approach. It should have used a checklist to rule out other probable causes such as flaws in structural and instrumentation systems. Although the final report is not yet accessible, both preliminary and interim reports from ATSB point finger on the reduced thickness of the oil tube, which caused oil fire in the turbine. Intensive investigations have proved the presence of fatigue cracking on an area with misaligned axial boring. The results of the investigation were crucial. First of all, the Airbus 380 with Roll- Royce was grounded for proper examination to be carried out. Secondly, the accident drew interest to the importance of the manufacturing oil pipe. After the incident and the inspection results, significant steps need to be taken to improve the reliability of this engine oil delivering stud tube and thus overall engine reliability in form of design philosophy, material selection, consideration of the impacts of the growth of a fatigue crack, and controlling the manufacturing processes such as the countering boring procedures, as well as instituting proper and fool proof inspection procedures (McEvily 2002). Additional Information or Investigation Required In carrying out their investigation Australian Transport Safety Bureau (ATSB) should have first looked at the historical deveklopment of the Rolls- Royce Engine, this would include looking at the the past record of maintenace and repair of the engine. In failing to do so, the reports fails to address the underlying root cause of the accident. We do not find instances where The account by Australian Transport Safety Bureau (ATSB) collected and analysed data in order to make conclusions from. In order to make reports from ambigious and subtle conmditions, it is prudent to correct data from the evidence presented, analyse through the use of approved methods then make conclusions. The cvonclusions make however, do no give an outright verdict but rather give a most probable phenomena. In trhis initial report, the Australian Transport Safety Bureau (ATSB) has hinted that airframe inspection, residual testing as well as worklioa, and flight simulations tests are in progess. This statement clearly shows that a lot is still not yet done and consequently a lot of crucial information is not reflected in the report. Recommendations The step taken by the concerned airlines to suspend the flights of Airbus A380 with Rolls- Royce engine is very important in order to ensure safety of the potential passengers. Instead of condemning the entire engine, it is prudent for the concerned parts to carryout proper tests and research on suspect parts of the engine. For instance the mentioned oil “stud” tube. Alternative material for manufacturing the oil pipe should be looked at in terms of performance in those extreme operating conditions. The alternative material should be easy to machine. Proper control systems should be installed in the Trent 900 series engines in order to the engine to regularly cross check itself in anticipation of any problem. This should include installing of trouble-shooting programs aimed at diagnosing any potential technical hitch. Even in cases where fire occurs, the turbine blades which experience a lot of stress cycles in their life span; they should be designed in such a manner that these stresses are way below the designed fatigue strength levels of the selected material. The objective should be such that these components even when subjected to adverse conditions such as oil fire, fatigue cracks and fractures should never develop within the design life span, since if a crack develops on the turbine, it will increase uncontrollably to a critical size due to the centrifugal forces and thus disintegrate as was the case with this particular Airbus A380. The crack may develop at a very fast rate such that periodic inspections carried out regularly on the engines may never detect the cracks in time to ward off a catastrophe (Hoo 1993). In this case the components will be designed using the safe- fail design approach. In safe- life design, a component is made in as a statically determinant structure that is aimed at lasting throughout the design lifetime of the component, without failure. To check any premature failure, regular inspections need to be carried out throughout its in-service lifetime. Part B: Materials Selection Material selection is a process followed in designing a physical component. In this case, the main objective is to minimize cost while attaining the performance goals. Proper techniques of material selection include defining carefully the utilization requirements of the component in terms of thermal, electrical, mechanical and chemical qualities (Perrin and Garner 1985). The process begins with defining carefully the main requirements. Manufacturing process involved in the process play a significant role in the selection of the appropriate material; based on its workability and tolerance needed. Performance Requirements Aircraft and jet engines operate under very intense conditions such as under high temperatures speeds, and stresses. This condition requires the materials that can sustain the dimensional stability at such intensive conditions. Engines also differ with make, purpose and type of airplane. However, generally all are made of materials that withstand high temperature effects and residual tensile forces (Flynn 1961). Currently Roll- Royce uses nickel based super alloy to make these oil stud tubes. The material has a superb mechanical strength and excellent creep resistance properties as high pressure and temperature, oxidation and corrosion resistance, and high surface and dimensional stability. The base alloying element for Ni- super alloy is cobalt, nickel, or nickel iron. The material is primarily used in the manufacture of components in power industries and aerospace; such as hot components of jet engine, and turbine blades. Superior Properties of Nickel based Alloys Super alloys are usually used in the gas turbine engines in areas where they are subjected to intense temperatures and where the materials are expected to exhibit high strength, superior resistance to temperature creep, and show increased fatigue life. These materials obtain this strength at high temperatures due to solid solution strengthening. In this type of strengthening, the material forms thermal barrier coating upon exposure to oxygen; this barrier protects the rest of the material (www.nidi.org. n.d.). Additional elements such as chromium and aluminum are responsible for the provision of the corrosion and oxidation resistance. In majority of this turbine engines, the temperature can be as high as 2000C above the melting point of this superalloy materials. The commonly used alloys are Mar- M 247 and Inconel 713 (ASM Aerospace Soecification Metals inc. 2012). Although these materials exhibit superior properties, they also have considerable shortcomings due their method of manufacture. Their production method leads to formation of polycrystalline alloys which have an unacceptable creep level. Currently Rolls- Royce Company is using Inconel in the manufacture of its jet engine parts. Actually Inconel is a trade name for austenitic nickel- chromium superalloys. This material often exhibits increased strength over a broad range of temperature, where other materials would succumb to creep (Rolls-Royce plc 2012). This is achieved through precipitation strengthening or age hardening in which small quantities of niobium are added to nickel to create intermetallic compound or gamma prime. This formed material inhibits creep and slip which occur at increased temperature levels since they form tiny cubic crystals. Drawbacks of Inconel material during machining Inconel material is shows some difficulty to machine and shape due to its inherent work hardening. Immediately after the first cutting or machining process, the material plastically deforms on subsequent passes. This means that materials such as Inconel 718 can only be machined through a belligerent but slow machining and by using a hardened tool thus minimizing the number of cuts (www.nidi.org. n.d.). This means that the material is costly to manufacture with as advanced and costly methods such as selective laser melting are often applied. Materials Selection Method The main objective of conducting material selection procedure is to come up with the best material for a particular application based on the functionality and economic cost consideration. Prior to any project undertaking, it is prudent to carry out material selection analysis through a systematic checklist with regard to functionality, workability, and cost considerations. This involves first collecting a list of available material and finally arranging them in order based on their applicability for a particular purpose. Optimum choice should be accompanied by quantitative justification and where necessary computer analysis software should be utilized (The american University in Cairo 2012). In a case like this Pugh method can be used to obtain the best material to be used in making the oil “stud” pipe. Pugh method makes use of decision matrices aimed at getting the best particular material. This process finally achieves the selection of the material with the best fit properties pre- determined prior to the selection procedure. Pugh Decision Chart ((McEvily 2002)). Other than stress analysis, in recent days other fields such as fatigue research, fracture mechanics, non destructive testing, and corrosion resistance have emerged. Fracture resistance capabilities of materials have been improved. For instance, in the metallurgical field, advanced alloy design has led to superior metal processing techniques and heat treatment thus leading to production of high quality materials that can considerably resist failure as a result of induced stress (McEvily 2002). Justifications for Material Selection Alternative Materials A variety of niobium base alloys are amalgamated with protective coating of metal to offer a material that has similar properties as Ni- based alloys. The created material system allows for a wide range of alloy performance characteristics and properties for the manufacture of structural components used in jet engines. Typical niobium base alloys are basically composed of 32-48 percent Niobium, 8- 16 percent titanium, 2-12 percent chromium: giving it the necessary protective coating against environmental attack. This super alloy can be well applied in both rotating and static components of the jet engine (www.nidi.org. n.d.). Additional Investigation and Study Unlike the Inconel super alloy that is currently used by Rolls- Royce Company to manufacture stud pipes, the niobium base alloy is formable, workable for engine structures such as shafts, pipes, and turbine cases. It is worth noting that the particular oil pipe should suggest a machining flaw. This means that the method of counter boring used should be re checked again. Suitable Materials for the Stud Pipe A variety of niobium base alloys are amalgamated with protective coating of metal to offer a material that has similar properties as Ni- based alloys. The created material system allows for a wide range of alloy performance characteristics and properties for the manufacture of structural components used in jet engines. Typical niobium base alloys are basically composed of 32-48 percent Niobium, 8- 16 percent titanium, 2-12 percent chromium: giving it the necessary protective coating against environmental attack (McEvily 2002). Conclusion In spite of the technological advances recorded in manufacture of components, incidences of failure have been occurring unabated and in some cases often accompanied by considerable economic and human loss. Although the report by Australian Transport Safety Bureau (ATSB) is sketchy in its analysis, it is worth appreciating the effort done in unravelling the mystery. Other than replacing the Inconel material used, it is worth first looking at the manufacturing method used in making the oil stud pipes. Bibliography ASM Aerospace Soecification Metals inc. Nickel and Cobalt Alloys. 2012. http://www.aerospacemetals.com/nickel_alloy.html (accessed May 26, 2012). Australian Transport Safety Bureau. In-flight uncontained engine failure overhead Batam Island, Indonesia. Preliminary, Canberra City: Commonwealth of Australia 2010, 2010. Flynn, John. “Human factors that Cause Aircraft Accidents.” Flying, March 1961: 45 - 62. Hoo, Joseph J. Creative Use of Bearing Steels, Issue 1195. Philadelphia: ASTM International, 1993. McEvily, Arthur J. Metal Failures: Mechanisms, Analysis, Prevention. New York: John Wiley & Sons, 2002. Perrin, J S, and F A Garner. Effects of Radiation on Materials: Twelfth International Symposium ..., Volume 1. Baltimore: ASTM International, 1985. Rolls-Royce plc. Trent 900. 2012. http://www.rolls-royce.com/civil/products/largeaircraft/trent_900/ (accessed May 22, 2012). The american University in Cairo. “Chapter 9 The Materials Selectionm Methods.” The american University in Cairo. 2012. https://docs.google.com/viewer?a=v&q=cache:M216LVchEc8J:faculty1.aucegypt.edu/farag/presentations/Chapter9.pdf+&hl=en&pid=bl&srcid=ADGEESgE1VPDkPUCNi7y-CFfpZl6XqqLmkPglNPMWFrdEB7gCqthmL6h3bZ3z_fASNxjLgzmbhg2NC0GdI3ZRo-Z9kHwA_AZOIDPxZmmWlNbdITvLsvIQ6jt_kb_ (accessed May 25, 2012). Wikipedia. Qantas Flight 32. 18 May 2012. http://en.wikipedia.org/wiki/Qantas_Flight_32 (accessed May 19, 2012). www.nidi.org. Nickel and Cobalt Alloys. http://www.aerospacemetals.com/nickel_alloy.html (accessed May 26, 2012). Read More