Accident Investigation for Aircraft Reciprocating Engines Conducted by ATSB - Report Example

Accident investigation report for Aircraft reciprocating engines conducted by ATSB A reciprocating engine is an engine that uses more than one piston to convert pressure into a rotating motion. This engine is also known as piston engine. In most of the cases, reciprocating engines uses burning gasoline or diesel for getting the pressure. There can be more than one piston. Each piston is located inside a cylinder. A mixture of fuel an air is introduced into the cylinder and ignited. The hot gas is expanded which pushes the piston away. A connecting rod and a crankshaft are used to convert the linear movement of the piston into a circular movement. Reciprocating engines are also known as internal combustion engines. Although not common now, steam was also used as a power source for reciprocating engines. In such cases high pressure steam is used to drive the piston. But nowadays piston engines are replaced by turbines. Pistons are now used only in automobiles because of the requirement of a high level of torque (Reciprocating engine – Definition. 2009). There have been cases of accidents with respect to carriers having reciprocating engines. In a specific case involving a reciprocating engine powered low capacity aircraft used for transporting passengers in Australia, a detailed investigation was conducted by the ATSB. The investigation revealed that from the period ranging from January 2000 to December 2005, twenty cases of failures in reciprocating engines were associated with aircrafts having reciprocating engines. All these problems were associated with powertrain structural failures. The failures due to these were in flight engine shutdown, engine failure, forced landing, engine failure which was combined with in flight engine fracture of the upper mounts of the engine an last not but the least, structural failures of both the engines to ditching during night flying. For the safe operation of the aircraft, it is of at most importance that the reciprocating engines are totally reliable. The study conducted by the ATSB, revealed some interesting facts, it showed that the powertrain structural failure was not restricted to one engine model or one engine manufacturer or one single powertrain component. The failures associated with engine can be grouped into melting of combustion chamber melting breakup of bearing, and fatigue cracking of powertrain components. More analysis of each category revealed that the reliability of powertrain component is affected by the development of shockwaves during the time of combustion. Moreover the response of the bearing towards boundary lubrication as well as out of plane alternating loads, the increase the component alternating stress magnitudes, creation of stress concentrating features in components during the time of operation of engine etc are also considered factors that affect the performance and reliability of reciprocating engines. The study also cleared that the above mentioned factors can act single-handedly sometimes but on major occasions they combine together to create problems. The study also proved that the airworthiness assurance system could have been ineffective for stopping the occurrence of powertrain component structural events. All, corrective actions could be taken only after analyzing the detailed feedback and having a broad view of the interacting systems collectively (Romeyn. 2007).Below are the detailed extracts from the ATSB regarding accident investigation it conducted during the period ranging 2000 to 2005 the ATSB used different investigation methods to make out a clear report on the accidents. First of all an understanding about the aircraft safety critical systems was developed prior to the study. Secondly an evaluation of available evidences that were associated with failure of powertrain components were collected against a background of component failure plan control. Thirdly an analysis of powertrain component events in an individual manner as well as in a group done by experts in this field was conducted. Moreover an analysis of the airworthiness assurance system during the period ranging from 2000 to 2005 was done. As pointed earlier in the report, the ATSB found the following: The failure of powertrain structurals that was conducted in this study was dominated by melting of combustion chamber component, breakup or movement of plain bearing, as well as the instigation and enlargement of fatigue cracking in the components that are intended to have a life that is not limited by exhaustion. A thorough analysis brought into light that the instigating factors were not associated to one reciprocating model itself or one component. The reliability of powertrain components are affected by the change in the process of combustion, that is from flame propagation all through the fuel air mixture to the auto ignition of several parts of the mixture. The consequence of detonation is connected to the power of detonation which is reliant on the quantity of end gas that experiences auto ignition. Light to medium detonation can effect in some type of mechanical damage. Moreover heavy detonation results in the melting of the aluminum alloy components in the combustion chamber. The crashes of aircrafts having reciprocating engines were mainly due to these reasons. All the engine failure occurrences studied makes it clear that likelihood of detonation increases due to leaning at climb power settings. It also became clear that the fuel air mixture settings would result in the deposition of a non volatile lead compound on the surfaces of the combustion chamber. This increases the chances of detonation. The report also stated the following facts. The connecting rods provide the means for transferring the reciprocating motion of the pistons of the engine to the rotary or circular motion of the crankshaft. Plain Bearings are used for connecting both ends of the crankshaft. During the process of flight the left engine of an aircraft stopped and the propeller got feathered. Upon inspection after immediate landing of the aircraft it was understood that the number 1 cylinder was separated from the crankcase. Moreover the connecting rod and the housing also got damaged. Upon scrutiny it was found that the nature of the failure was constant with the increase of fatigue in the housing. It was understood that during operation of the engine, the crankshaft is subject to continuing and alternating stresses that is created by the discontinuous gas pressure loads from each cylinder as well as the inertia loads that are associated with the reciprocating actions of the pistons and circular movement of unbalanced masses. In another fatal accident involving an aircraft where the plane crashed into the sea, the engine was recovered from underwater. During examination, it was found that the number 6 connecting rod was separated from the crankshaft journal. Such types of damages are clear indicators that big end housing was fractured when the engine was operating ant normal speeds. Further studies revealed that there was fatigue cracking. It became clear that in every aircraft engine, the engine cylinder assembly and the piston is the reaction vessel where the combustion takes place. There can be stresses in the cylinder chambers caused by gas pressures and thermal strains. Thermal strains can damage or create fatigue cracking in areas of the cylinder head. Another important component in the reciprocating engine is the piston. A piston helps to create a movable surface inside the combustion chamber which is transmitted to the crankshaft through the help of a connecting rod. Aluminum alloy is used for manufacturing piston crown so that heat is resisted. Detailed studies conducted by experts revealed that sometimes overheating will cause even the aluminum alloy to melt this happens when the quantity of heat that is produced during combustion exceeds the allowable amount or if the rate of the heat transfer to parts of the piston is increased. In cases involving crashing of aircrafts loaded with reciprocating engines in Australia, another significant result of detailed investigation conducted by the ATSB was that problems can also occur in the connecting rod of the reciprocating engine. The rod provides a link between the piston and the crank shaft. Piston motion often creates stresses on the connecting rod as a result there are chances of fatigue fracture in the connecting rod. Fatigue fracture can occur if the engine operation limitations are exceeded or if the stress concentrating features are created in the housing during the time of manufacture. Fatigue fracture the connecting rod will create serious problems to the reciprocating engine. With respect to aircraft using reciprocating engine, such problems are too serious which can force the aircraft to land in an emergency situation. The accident report also stated that fatigue fracture can also occur on the crankshaft. The crankshaft is an important component to transfer the reciprocating motion to rotating motion. Alternate stresses on the crankshaft are created by varying forces acting on a crankshaft. It was found on investigation of some parts of crashed aircrafts that crankshaft fatigue fractures occur when engine operation limits are exceeded, creation of stress concentrating features at critical locations and when the fatigue strength of the steel alloy is low. Some aircrafts that have crashed during flights were also examined thoroughly and it was found that failure of powertrain plain bearing also occurs in reciprocating engines. Plain bearings are used for transmitting force between the piston, connecting rod, the crankshaft and the crankcase. Plain bearings operate smoothly with the creation of hydrodynamic oil film lubrication. Plain bearings are treated to be a failure when seizure occurs between the bearing surfaces. There will be breaking in the bearing surfaces which in turn destroy the hydrodynamic lubrication. Failure of the bearing may occur through fatigue cracking, excessive wear or friction or if the bearing inserts are displaced from their housings. Engines are damaged due to the destruction of bearings. This reason could be fatal for the life of the aircraft. Detonation could also affect the reliability of powertrain components through pressure increase, particularly the term detonation. Increased pressure results in increased transfer of heat to the piston as well as cylinder heads. Problems in detonation could affect engine power. Engine power is a critical aspect for aircrafts. Variations in engine power can cause serious accidents and grounding of the aircrafts. Many instances were reported on the case of variability or engine power due to problems in detonation. The report also stated that aircrafts having reciprocating engines have been handicapped due to the phenomenon of pre-ignition which is always a limiting factor. Usually in high compression engines, there would be accumulated deposits of combustion leased fuels; these heated deposits would normally cause non spark ignition sites. This could be either before spark ignition or after spark ignition. Some aircrafts were also damaged due to breaking up of the surface of the reciprocating engine. There are different factors that are associated with the surface breakup, for example fatigue cracking of the surface of the bearing, change from hydrodynamic to boundary lubrication during high power operation of the engine is some attributed to this fault. Hydrodynamic oil film lubrication is a very important aspect in the process of efficient transmission of engine power. For an aircraft engine which has an established bearing design, the control of the temperature of the oil film inside the bearing is a critical factor in maintaining hydrodynamic lubrication. Moreover bearing clearance is an important aspect that affected the very nature of lubrication of bearing. If the clearance is too small then the oil film temperature increases leading to boundary lubrication. The report by the ATSB concluded that aircrafts using reciprocating engines are threatened by failures of engine parts, namely structural failure of the power components, combustion chamber assemblies, connecting rods and pistons along with crankshafts. It is possible to provide propulsion system redundancy in such aircrafts that use reciprocating engines. This is done through designs that employ two independent wing mounted propulsion systems in the aircraft. However it has been proved that if the failure of one propulsion system of the aircraft, the other propulsion system will not be capable of providing the thrust necessary in all phases of the flight. Due to the failures and drastic consequences associated with the failures of the reciprocating engine, mainly the powertrain structural failure, the reliability of powertrain components should have a very high degree of importance (Aircraft reciprocating engine failure. 2007). Conclusion: Although many problems are associated with reciprocating engines in aircrafts, it is still recommended by aeronautical expert’s world wide. That is why most small aircrafts are now equipped with such type of engines. Moreover continuous research in this are have somewhat helped to rectify some mistakes and also helped to bring modernization to reciprocating engines (Reciprocating Engines. 2009). Reference: Aircraft reciprocating engine failure: An analysis of failure in the complex engineering system. (2007). Reciprocating Engines. (2009). (online). Available: http://ma3naido.blogspot.com/2009/12/reciprocating-engines.html Reciprocating engine – Definition. (2009). (online). Available:http://www.wordiq.com/definition/Reciprocating_engine Romeyn. A. (2007).Aircraft Reciprocating-Engine Failure: An Analysis of Failure in a Complex Engineered System. (online). Available: http://www.atsb.gov.au/publications/2007/B20070191.aspx Read More