An Off-Airport Landing: Piston Engine Accident Investigation - Assignment Example

Opinion of Off-Airport Landing Event, Cessna 172, N1234ER Name Date Opinion of Off-Airport Landing Event, Cessna 172, N1234ER Executive Summary Before the off-airport landing the pilot had noticed a moderately elevated CHT’s, but associated that to the extended climb gained by the plane. The pilot later noted a gradual reduction of engine power which eventually led to the decrease in engine oil pressure. The engine power eventually dropped to a level that was unable to sustain flight, prompting the pilot to land in a grass field which was close to some several homes. The pilot was able land successfully without incurring any occupant injury or aircraft damage. After landing at the off-airport field, there was need for an investigation to be conducted to establish the exact causes of the off-airport landing. Investigation Summary The aircraft mechanics systematically checked the aircraft engine control system by conducting a thorough inspection of each system. Mechanics removed all the cylinders carefully in order to check all parts from each cylinder by putting all the components in an orderly manner. After checking the Aircraft Engine Log, it was found out that all the AD’s current and had been complied with. The investigation team also found out that the 100 hour inspection had been completed 75 hours prior to the off-airport landing. In addition, the annual inspection was due in 92 days from the day of landing. In relation to the quality of the gasoline there was no water accumulation and 26 gallons were still on board. The carburetor and the remainder of the fuel system were still in operating condition. Oil quantity and quality was also found out to correct an in operating condition. When the spark plugs were investigated, some signs of excessive accumulation were noted. A further inspection of the leak down compression test showed normal results within the manufacturer-specified range except cylinder two. Incorrect piston rings were also found to be included in the S.T.C. kit contents where the piston rings were installed in straight bore cylinders. 3. Opinion & Discussion a. Pro’s and Con’s of the Pilot’s decision to elect an off-airport landing The pilot’s decision for an off-airport landing was instigated by the condition of the aircraft. Any attempt to continue flying with a faulty engine would probably have resulted in fatal crash. In order to avoid such an occurrence, the pilot had to utilize the slightest opportunity of an open field to make a landing. From this, it can be noted that the main pro of an off-airport landing was to prevent a crash while a con is that it could have caused an accident since the safety of the landing venue was not guaranteed. b. What could the pilot have done differently? The pilot should have checked the plane before taking off. This would have ensured that all the causes of the probable engine failure were fixed well in advance. i. Explain how a proper engine logbook entry should read for installation of the S.T.C. kit and the corresponding form 337. A proper engine log book entry should show that the plane has been flight tested for the required hours. The log book entry also declares that a plane is controllable throughout its normal range of speeds. (Trevor 32). All the V speeds, aerobatics, and IFR equipment should be tested and recorded in the log book entry. ii. Findings of advanced magneto timing- how would these be manifested to the pilot by the CHT readings? It is appropriate for the engine magneto timing to be regularly checked. The required timing for a magneto should be 22º BTDC (before top dead center) on the number one cylinder piston. (Trevor 47). Advanced magneto timing of the magnetos leads to pre-ignition or detonation of the combustion gases in the engine and may result in high cylinder head temperatures and eventual power loss. iii. Finding of the non-ferrous metal chips in the oil-what is the most likely source? The non-ferrous metal chips that are found in engine are mainly from aluminum, chrome, tin, or bronze among others. These metals are mainly distinguished by their appearance where bronze has a characteristic of a yellow color while chrome is shiny and very hard. Tin is dull and melts at low temperatures while aluminum fizzes and dissolves when exposed to dilute sodium hydroxide. The source of aluminum in engine oil could be a loose piston-pin plug or burned pistons possibly caused by pre-ignition (Gilles 29). Abnormal wear on piston skirts and piston pin plugs may also be a source of aluminum metal chips in engine oil. Non-ferrous metal chips such as chromium are mainly from the abnormal wear of the chrome –plated piston rings caused by a rough cylinder. iv. What the Pilot could have done differently The “rough-running engine” was probably due to the partially burnt or unburned oil in the engine. Some engines require higher levels of heavy oils in order to prevent the scuffing of the piston and the cylinder wall (Schwaner 36). To ensure the prevention of the damage caused, the mechanic should have checked all the cylinders and the piston before the aircraft took off. v. Speculate the most likely source of the deposits found on tops of all pistons (other than cylinder two) The deposits found on top of all the pistons other than cylinder two were caused by partially burnt fuels. When the combustion of oil is incomplete, oxide deposits accumulate at the piston causing some black coloration that can be in a form of soot. If not regularly checked, the oxide deposits can cause the sticking of the piston ring and can eventually disrupt the flow of the exhaust system and hence causing some engine damage (Gilles 21). The lighter oils evaporate more quickly after being subjected to the piston temperature. This is however different with the heavy engine oils that take time to evaporate and hence leave residues on the pistons. When detergent additives are used in water-cooled oils and can lead to ash-like deposits at the pistons. Ash is a non-combustible residue of lubrication oil or fuel and the components of the detergent additives may also be part of the ash deposits. It therefore means that the deposits may also be in the form of metallic derivatives such as calcium, magnesium, and barium. These deposits may impair the efficiency of an engine (Gilles 24).The ash deposit can affect the foul spark plugs and may create hot cylinder spots that may cause destruction at pre-ignition. Ash deposits are normally grey in color while the carbon deposits are black and sooty. vi. Speculate on the cause of the damage to the piston from cylinder two. Was this damage likely to cause detonation or pre-ignition? The damage to the piston as illustrated in diagram four was probably due to lack of enough oil metering to the cylinder wall for the control of the cylinder barrel temperature. The oil metering past the piston ring belt ought to be sufficient in order to establish an oil film even in severe conditions. An oxide layer forms around the cylinder wall when an aircraft is parked and is wiped off by the piston rings after the ignition of the engine (Mills 63). This damage is likely to have been caused by detonation which had led to the spontaneous combustion of fuel initiated by the spark plug. The detonation may have caused the overheating of the piston and hence causing the abrasion and the scuffing. vii. What is the relationship between the damage to the piston in photo 4 and the exhaust valve in photo 5? What type of stress likely caused this damage? A damaged piston may cause the burning of the exhaust valve as the burning fuel mixture escapes past the valves. This causes the burning of the valve seal and may eventually cause loss of power in the cylinder. The type of stress that is likely to have caused this type of damage to the piston was axial stress. The tension on the piston in this type of stress is applied along its axis. The inertia forces in the running gear causes tension in the connecting rod and hence the piston changes direction between the exhaust and inlet strokes. viii. Why is the cylinder 2 darker than all other cylinders? What does this discoloration indicate about cylinder 2? The discoloration of the cylinder is due to the high temperatures from the spark plug. This is the reason why the cylinder two is darker than all the other cylinders as high temperatures burn and leave residues that form the discoloration. ix. Discuss what is meant by scuffing a round piston at the “four corners”. What is a cause of the amount of heat generation required to make this wear pattern visible? The four-corner piston scuffing failure can be experienced on any number of pistons within an engine. It is caused by an extremely high abnormal combustion temperature. The hot temperatures then travel down to the piston and concentrate highly in the pin area (Schwaner 47). As the area continues to be heated up, it also increases in diameter. In the instances when the temperature is high enough, the size of the piston enlarges and exceeds the diameter of the bore size and hence causes the scuffing. Since the piston is smaller in diameter before the engine is started, the scuffing occurs after the expansion of the piston beyond the required limits. The expansion is caused by the excessive temperature. x. Define the role of piston from a heat transfer standpoint Piston plays a major role in heat transfer as it conducts heat from the conduction gases to the cooling oil. It also transfers heat to the pistons rings and the piston skirts. The heat is transferred from the piston dome to the piston rings. From the piston rings the heat is then transferred to the cylinder oil film and then to the cylinder barrel. The heat is then transferred from the cylinder barrel into the atmosphere. Since the combustion temperatures present at the piston are higher than the piston’s melting point, the rings absorb the heat energy at the piston. The heat is then transferred from the rings into the cylinder walls and eventually into the atmosphere. Piston Heat Transfer xi. Define and explain what detonation is and what it causes to happen inside the combustion chamber during the combustion event. Detonation is the instantaneous explosive combustion of the air-fuel charge in a cylinder due to the presence of the high temperatures in the engine (Schwaner 57). It causes a very high pressure spike in the combustion chamber. Detonation is not always destructive as there are some engines that can sustain long periods of detonation. xii. Does the detonation pressure spike occur before or after the normal firing of the spark plug? The detonation pressure spike in the combustion chamber occurs for a short duration. A normal burn is experienced at the combustion chamber as the pressure rises normally (Trevor 101). A very sharp spike is then experienced after the detonation occurs. The spike always occurs after the firing of the spark plug. xiii. What are some typical causes of detonation? Detonation is influenced by chamber design, engine timing, compression ratio, and the cylinder pressure. The availability of too much spark ignites the burning process and hence causes the spontaneous combustion of the gases. In order to reduce the impact of detonation, the chamber should be made to burn faster as this reduces the spark advance. xiv. What are some typical indicators of detonation? Detonation can be detected through some indicators such as the pinging sound made by engines especially when aircrafts are flying at low speeds or are carrying heavy loads (Mills 42). It may however be difficult to hear the pinging sound if the interior of the aircraft is well insulated. xv. Compare and Contrast detonation to pre-ignition Pre-ignition is basically, the ignition of the fuel charge prior to the spark plug firing. It is mainly caused by an overheated spark plug tip and the carbon deposits in the combustion chamber. Pre-ignition is likely to occur in a glowing spot in the chamber where charge can be ignited easily while the piston is at the compression stroke (Trevor 85). During the pre-ignition process the engine is tasked to compress hot mass of expanded gas. As result there is overloading in the engine that causes overheating in most of its parts. Engine can therefore be subjected to damage very quickly as the pre-ignition is difficult to notice. Unlike in detonation where the spark plug ignites a sharp pressure spike, with pre-ignition the ignition of the charge takes place before the firing of the spark plug. In pre-ignition, huge amount of pressure persists for a long time and there is no noise due to the absence of the sharp pressure spike. Pre-ignition is therefore hardly detectable when it occurs and is only noticed after it causes some engine failure due to the excessive heat and pressure. xvi. Relatively speaking, can engine survive detonation or pre-ignition longer? An engine can cope with detonation for a considerable amount of time but there are no engines that continue surviving when pre-ignition occurs (Schwaner 68). The extreme heat and pressure of the pre-ignition at times causes a hole in the middle of the piston and this is its main indicator. Xvii. Could this S.T.C. have had anything to do with the demise of this engine? What effect would finding number 16 have on all cylinders? The supplemental type certificate (S.T.C) could have caused the demise of the engine if it was issued without proper inspection by the Experimental Aircraft Association (Schwaner 72). The continuous operation on an engine depends on the simultaneous function of all the cylinders and therefore finding number sixteen will enhance the functions of all the other cylinders. Conclusion The pilot had noticed an increase in the CHT initial stages before making a decision about off-airport landing. Mechanics had removed all the cylinders carefully in order to check all parts from each cylinder by putting all the components in an orderly manner. After checking the Aircraft Engine Log, it was found out that all the AD’s current and had been complied with. The investigation team also found out that the hundred hour inspection had been completed seventy five hours prior to the off-airport landing. In addition, the annual inspection was due in ninety two days from the day of landing. In relation to the quality of the gasoline there was no water accumulation and twenty six gallons were still on board. The carburetor and the remainder of the fuel system were still in operating condition. Oil quantity and quality was also found out to correct an in operating condition. The excessive accumulation of the ‘oil fouling’ at the spark plugs was due to a poor seal of piston rings. There is no mention that the pilot heard any pinging sound from the engine and hence the engine failure may not be described as detonation. The damage that was noted in the cylinder two was a suggestion that the failure was pre-ignition as this form of engine failure happens almost immediately. Works Cited Gilles, Simon. Automotive Engines: Diagnosis, Repair and Rebuilding. Boston: Routledge,2010. Mills, Simon. Condition monitoring and diagnostics of machines. New York: Routledge. Schwaner, John. Sky Ranch engineering: operation, failure, and repair of piston aircraft engines. London: Cambridge, 2006. Trevor, Hunt. Oil Analysis Handbook. New York: Coxmoor Publishing, 2008. Read More