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Table of Contents

22.3.2 Oil coking

 Oil coking

Oil coking is a frequent and serious problem. Usually the main cause is a high local temperature in the oil system.Typival regions are ducts for scavenge oil and vent lines inside struts with of hotgas flow at the outside (Fig. "Problems of bearing chambers near hot parts" and Fig. "Oil coaking endangers main bearings"). Here oil coke can assemble till the blocking of the flow cross-section. Walls of the main bearing chambers in the hot part can accumulate coke as well during operation, as also during stand still. This is the case when the necessary heat insulation is locally not sufficient by design or is damaged. Local coke deposits in b earing chambers can also indicate a temporarily limited oil fire (Fig. "Intershaft oil fire").
Oil coke does not only form during operation. Just after shut down, when a cooling flow no more exists, heat radiation and heat conduction can trigger an overheating with the formation of coke. In this case also the fresh oil pipes are concerned. A further problem is the coke formation at labyrinth seals and main bearings during stand still. Here the heat comes from the relatively massive hot turbine wheels through the shaft to the bearings (heat soaking, Fig. "Importance of resting time at idle").
A further danger comes from peeled off coke. This can be as well fine particles, as also bigger laminar particles. They clogg/block flown through cross sections like oil nozzles/jets or sieves/screens. Does so markedly oil deficiency in the bearings occur, the danger of an extensive failure gets acute. Gets coke on the races of anti friction bearings, a drop of the bearing lifetime by fatigue pittings must be expected (Fig. "Fatigue pittings at bearings").
Accumulates coke in labyrinth gaps or between concentric shafts, this can trigger rub processes with overheating of the neighbouring components, especially the shafts (Fig. "Overheating by rub causef of coke formation"). The fracture of a shaft lets fear the overspeed and bursting of the turbine wheel.
It can be expected, that the proneness for the formation of oil coke rises with the operation time by aging processes and chemical reactions with metals of the oil system.

Fig. "Formation of depositions in hot oil systems" (Lit. 22.3.2-3, Lit. 22.3.2-4 and Lit. 22.3.2-12): Many influences are based on the formation of deposits, respectively oil coke. They can act together in a complex manner. The understanding of the formation mechanisms and the precipitation, depending from the operation conditions, is a requirement for targeted remedies. So if possible from the build up of layers, it can be concluded at the deposition rate and with this at the formation process (Fig. "Deposition with layer formation"). Also the hardness of the deposits can be helpful to find the cause. These findings, together with the appearance (Fig. "Formation conditions shown by coke deposits" and Fig. "Formation mechanisms and oil coke features"), already allow to some degree substantiated conclusions at important formation parameters. This can be assured with microscopic structure investigations (Ill. 22.3.2-2, Lit. 22.3.2-3). In the following such effects are discussed in detail.

 Formation of depositions in hot oil systems

Diagram A: Already at normal temperatures oil changes its properties during typical operation time periods. Acids are developed (parameter: total acidity number = TAN), evaporation of volatile components and molecules are dissociated or polymerise (combine) to larger molecules. Thereby the density and viscosity increases. A visual feature of such changes of the oil is a dark discolouration. It is markedly accelerated during the contact with a condsiderable more than 200 °C hot surface. Supporting acts the contact with air (oxidation). Dust particles from the intake air, especially together with adsorbed salt (sea atmosphere) support the process. They get with the leakage air of the labyrinth seals (e.g., at the bearing chambers) into the oil. There they act like condensation nuclei for the coke formation in the oil. These changes again promote the proneness of the oil to form deposits. So, if oil is already „aged“ , it tends even more to coke formation. With this, the frequency of oil changes or fill-up/refilling decreases the coke formation.

Diagram B: Above 200 °C a jump of the viscosity change, as deterioration feature of the oil occures. Therefore operation conditions like start frequency and maeuvers, which act at the power output and with this at the temperature level in the aeroengine, play a disproportionate role. Drops the oil flow for example during certain flight attitudes in the military use (e.g., upside-down flight, Lit. 22.3.2-2) this promote the rise of the temperature.

Diagram C: Not only the surface temperature of the oil wetted area plays an important role for the formation of coke, this is also true for the mode how the oil is deposited and the lapse of time. First the contact with an oil layer (Ill. 22.3.2-2, static conditions) with sprayed on droplets or fast changing wetting (dynamic conditions) is distinguished. As to see, the amount of deposition, depending from the temperature, is very different. Under static conditions there is a pronounced maximum in the region of high operation temperatures. But at extreme temperatures the deposition rate drops again. In contrast, under dynamic conditions it increases exponentially.

Diagram D: In contrast to the temperature depending coke accumulation („C”), the behaviour almost inverts, depending fom the air stream. The more intense it is, the larger is the amount of deposits under dynamic conditions und the less at static conditions (see explanation in Ill. 22.3.2-2).

Diagram E and F: Astonishingly under dynamic conditions with increasing spray rate (amount of sprayed on oil related to the time), the deposit mass drops (see explanation in Ill. 22.3.2-2). The contrary is the behaviour under static conditions.

Diagram G: If the time dependence of the oil coke formation is considered, this rise in all cases at the beginning considerably linear, to drop then again. The hardness of the deposits also increases clearly linear with the time. However the curve flattens at a certain point in time.

 Formation conditions shown by coke deposits

Fig. "Formation conditions shown by coke deposits" (Lit. 22.3.2-1, Lit. 22.3.2-4 and Lit. 22.3.2-12): Coke deposits can represent a high danger potential. To counteract this specific, it is necessary to identify the development causes; respectively the formation conditions as detailed as possible. Thereby helps the observation, that obviously certain parameters (Fig. "Formation of depositions in hot oil systems") are in connection with characteristic features of the deposits (Fig. "Formation mechanisms and oil coke features"). How this happens show specific coking tests (middle sketches).

A“: Hot wall impinged by oil vapour (layered structure see „III” in Fig. "Formation mechanisms and oil coke features"). For example these conditions dominate in the leakage flow out of a bearing chamber at the turbine side between shafts; with wall temperatures of about 300 °C . During stand still similar conditions can arise under „heat soaking“ in bearing chambers or pipe lines/scavenge lines. This can also be expected in ventlines during operation. Are these oriented vertical this does promote the process by the draining of the oil film.

B”: There are quite regions like the walls of bearing chambers or bearing cages in an aeroengine, where oil droplets spray on a hot wall (layer structure see „II“ in Fig. "Formation mechanisms and oil coke features"). They can originate from oil jets and its back splashing. Also during centrifuging from rotating parts like bearing cages such oil droplets develop.

C”: Similar deposits like from an oil mist (layer strukture see „II“ in Ill. 22.3.2-3.1) produce a periodic wetting by a not constant oil stream. Such a situatuion may be found for example in scavenge lines.
With a suitable preparation, the chance exists to draw from the layer structure (Fig. "Deposition with layer formation") conclusions at the frequency and time periods/intervals of the layer formation. With this, operation conditions and causative influences (e.g., temporary lekages) can be identified.

D”: For example such conditions can arise in the oil sump of a main bearing (layer structure see „I“ in Fig. "Formation mechanisms and oil coke features"). If oil puddles form, they are especially concerned by coke formation (puddle coking).

E”: In the area of burning oil, neighboring surfaces are very fast (fraction of minutes) heated at temperatures above 700°C. Thereby existing coke deposits will burn. Then these surfaces are free from deposits and show overheating features like plastic deformations and oxidation. In the direction to lower temperatures, often exist very rough and thick carbon deposits, developed from oil vapour (Fig. "Intershaft oil fire"). These are signs for frequent arising oil vapour. This is especially interresting in unexpected regions. They are a sign of at least temporarily leakages. There are often connections with labyrinths, which seal oil-conveying regions like bearing chambers. Situations with the exit of oil vapour occur, e.g., in military aeroengines during certain manoeuvers and high flying speed near the ground (volume 2, Ill. 9.2-8 and Ill. 9.2-9; Lit 22.3.2-2).

 Intershaft oil fire

Fig. "Intershaft oil fire" (Lit 22.3.2-2): Failure of a hollow turbine shaft after an oil fire. This shaft of a 3-spool aeroengine runs with a narrow gap in an outer shaft. The extremely high temperature in the region of the oil fire burnt the coke deposits and softened the shaft up to plastic deformations.
The thick coke coatings on both sides of the overheated zone may be produced by frequently oil vapour containing leakage gases from the main bearing chamber at the turbine side, before the fire occurred (volume 2, Ill. 9.2-8 and Ill. 9.2-9).

 Formation mechanisms and oil coke features

Fig. "Formation mechanisms and oil coke features" (Lit. 22.3.2-3 and Lit. 22.3.2-4): Depositions can accumulate from particles which already developed in the oil. These particles are caused by a sort of condensation around tiny foreign particles. Typical is metallic abrasion of bearing races, cages, gear tooth flanks and spline toothing. Also air from the intake transports particles as dust into the oil. At those salts from the sea atmosphere can agglomerate. For example these paticles can get into the oil with leakage air through labytinth seals of the bearing chambers. Such, around foreign particles formed `hybrid particles', can contain very different contents of binding resinified oil. This effect lets distinguish between pure oil deposits and particle deposits. This is easily possible with a chemical analysis.

I“: Smooth lacquer like layers/coatings are at the beginning sticky with a light brown colour which with the operation time gets darker. Thereby the layers/coatings get hard and glassy brittle, with rising temperature hazy and cracked. Influences at the growth of the coating thickness can be seen in diagrams „C” and „D“ of Fig. "Formation of depositions in hot oil systems". With the coating thickness also the chemical composition changes. To this belogs the water content. It is released during the polymerisation (resinifying/gumming).
Under static conditions from the oil film a resin like layer/coating develops. This hardenes similar to a clear resin lacquer. With rising substrate temperature and with the wetting, the amount of oil also increases the quantity of deposits. However if the temperature rises very high, the evaporation of the lighter componds occurs. With this the weight of the deposits diminishs (not necessarily the thickness!). The layer gets dull und cracked. A similar effect is created by an intense air flow which accelerates the evaporation of the oil components. Thinkable is, that the layer/coating because of the formation, step by step shows an assessable layered structure (Fig. "Deposition with layer formation"). This enables conclusions at the frequency of the wetting event.

II”: Rough coatings form from sprayed on droplets (dynamic conditions). Thereby on the hot substrate surface resin particles develop, which include extreme tiny oil droplets. They can melt together to a smooth layer. Evaporate these in the resin captured rests of oil, a rough and porous structure forms.
Drops with the coating thickness the temperature, because of its insulation effect, the single resin particles of the following layer can no more fuse. Due to the surface tension the soft droplets form beadlets. In fact these adhere to one another, however don't form a smooth layer. It is not clear how far this mechanism prevails. Droples can also already harden with the shape of beads in the hot air stream and so generate a powder. Increases the amount of spray (dynamic conditions) this shortens the time for a hardening process and so prevents deposits (diagram „E“in Fig. "Formation of depositions in hot oil systems").
With the amount of oil the quantity of deposits also rises, depending from the polymerisation rate (diagramm „F“in Fig. "Formation of depositions in hot oil systems"). The cause is, that the particles are no more sweeped off.

III”: The deposits out of the vapour phase are determined from temperature and time. Macroscopic they are dull and can form a crust over a longer period of time. Usually these are rather hard and thin. Mikroskopical a characteristic “globular” structure exists.

 Deposition with layer formation

Fig. "Deposition with layer formation" (Lit. 22.3.2-3): This illustration shows the microscopic picture of the metallographic section through an oil coke deposit. Clearly a layer structure can be identified. Besides the assessable number of layers and its thicknesses, further important features can be seen. From the arched cracks can be concluded at a plastically deformable, soft primary condition. This may be hardened and embrittled during operation time (Fig. "Formation of depositions in hot oil systems"). At this suggestion point the, perpendicular to the arched cracks, orientated and starting brittle cracks. The envelope from a porous soft structure of the whole particle (grey sheathing) permits conclusions at the conditions of the deposits which formed at the end of the process (Fig. "Formation conditions shown by coke deposits" and Fig. "Formation mechanisms and oil coke features").

Fig. "Deteriorating effects by oil coking" (Lit. 22.3.2-4 and Lit. 22.3.2-5): Oil coking can affect deteriorating the components of aeroengines in a very different way. This applies not only for the oil system itself, but also for the outlying components. Therefore a direct connection must not be at once identifiable.

A”: The perhaps most frequent problem is the complete or partly clogging of oil lines/tubes. Usually these are scavenge pipe lines (return oil) which transport the hot oil, mixed with air (foam). Those are not like a pipe line for fresh oil flown through by cooling pressure oil. In the fresh oil supply oil jets/nozzles are endangered. Its small discharge cross section is clogged by the particles transported from the oil. In a fresh oil pipeline and the oil jets itself, also coke formation can occur (volume 3, Ill. 11.2.2.2-5). This is the case if during stand still, from the neighbouring hot parts heat the empty or only partly filled pipe line, by conduction or radiation to more than 250°C (heat soaking). This danger especially exists in the zones of pipes (Fig. "Problems of bearing chambers near hot parts" and Fig. "Oil coaking endangers main bearings") which lead through struts in bearing supporting structures (hot struts) in the hot gas stream on the turbine side. A reduced oil flow is especially dangerous for bearings and gears (overheating, wear/abrasion). Endangered are also vent lines which only transprt a mixture of oil vapour and air, which tends to coke formation (Fig. "Formation mechanisms and oil coke features" „III“).

B”: Oil coke which peels off will be carried by the oil and can deteriorate the through flown components. To this belongs the clogging of filters, screens/sieves and heat exchangers (sketch „G“). Like a reduced oilflow there is also a change and/or deflection of the spray cone dangerous for bearings and gears.

 Deteriorating effects by oil coking

C”: Will be sliding ring seals or labyrinth seals abraded/worn by particles in the leakage flow the amount of escaping leakage oil will rise. So this process reinforces itself. This can lead to the formation oi oil coke outside the oil system. With this the danger of damaging rub processes arises (Ill. 21.3.3-5 respectively Fig. "Overheating by rub causef of coke formation") and in an extreme case of an oil fire (volume 2, chapter 9.2). Does oil coke in the gap of a labyrinth during stand still, this can cause stiffness of the rotor up to its jamming. The potential consequences are start problems up to the danger of an overheated turbine. Similar problems can also not ruled out for brush seals (volume 2, chapter 7.3). Here a clogging of the brush with an elastic stiffening and its malfunction must be expected.

D“: (Anti friction) bearings are primarily endangered indirectly by washing in of coke particles with the lubrication oil. Connected with this, is the wear/abrasion of the races and the cage (Fig. "Wear loaded sliding surfaces in bearings"), as well as a markedly shortening of the fatigue life of the running surfaces. Especially during stand still the danger of heat soaking exists. The result is a coking of the bearing and its surrounding area with a considerable damaging effect (Fig. "Problema by heat soaking", Fig. "Importance of resting time at idle" and Fig. "Oil coaking endangers main bearings").

E”: Gear pumps (Fig. "Fuel influencing fuel pumps" and Fig. "Gear pump failures") are used in the oil system as fresh oil (pressure oil) pumps and scavenge pumps. Washed in hard oil coke particles can abrade/wear the tooth flanks and promote fatigue (Fig. "Failures of gear wheels"). This metallic abrasives can cause further secondary failures, for example in bearings. It is also dangerous, if abrasive particles get with the squeeze oil into the sidewise sealing gaps and cause wear. With this the efficiency, e.g., the delivery rate drops.

F“: Oil coke, built up to thick layers can trigger dangerous self reinforcing failures by rub processes. An example is the formation of oil coke between concentric shafts (Fig. "Overheating by rub causef of coke formation", description Fig. "Disimprovements by remedies"). The rubbing process leads with heating and wear to a shaft fracture. In such a case the now occuring overspeed of the turbine, leads to the burst of a turbine wheel.

 Overheating by rub causef of coke formation

G”: Oil coke acts heat insulating. This disturbs the cooling of the oil in the return oil (scavenge oil) pipe lines. So the temperature level of the oil can rise, what further accelerates the aging of the oil, respectively shortens its life. This promotes the formation of coke in hot regions of the oil system.
Heat exchangers serve for preheating of the fuel to avoid icing, if there is suitable a water content (Fig. "Fuel supply problem by low temperature"). At the same time, the oil temperature is reduced. Coke particles which are washed in, can settle. This deteriorates the heat transfer and with this the function of the heat exchanger.

 Problems of bearing chambers near hot parts

Fig. "Problems of bearing chambers near hot parts" (Lit. 22.3.2-2, Lit. 22.3.2-8 and Lit. 22.3.2-9): The helicoper engine shown in the upper sketch, had after its introduction in the 80 ies big problems. These could be led back at the coking of pipe lines in the oil system (example 22.3.2-1). Also from many other big (Fig. "Oil coaking endangers main bearings") and small aeroengine types (Fig. "Problema by heat soaking") problems and failures in connection with oil coking emerged. Mostly concerned was the region of the bearings, respectively bearing chambers at the turbine side. Therefore the example of a typical bearing chamber at the turbine side was chosen (frame below) with its associated peripherals. So it is possible to present the problems which are connected with coking.

Fresh oil (pressure oil) line ond oil jet/nozzle (Fig. "Oil coaking endangers main bearings"): Obviously an adherent is coking inside the pipe line in the oil stream not so probable. The conditions, that a coke layer forms, seems after shut down of the aeroengine very much more likely (Fig. "Formation mechanisms and oil coke features" „I“). These layers can produce failures corresponding to Fig. "Deteriorating effects by oil coking" „A” and „B“.

Bearing failures can be triggered from lack of oil or washed in coke particles from the oil jet/nozzele or the cage (Fig. "Deteriorating effects by oil coking" „D”).

Seals of the bearing chamber guarantee a pressure inside the bearingchamber, which prevents an oil leak. Mostly these are labyrinths. Also sliding ring seals (face seals, Ill. 23.4.2.2-1) and recently brush seals (volume 2, chapter 7.3) are used. Failing of seals can, depending from the pressure gradient, enable hot gas break-in or oil exit. In an serious case the ignition of an oil fire (volume 2, chapter 9.2) outside or inside the bearing chamber is possible. Further failures like dangerous rubbing processes describes Fig. "Deteriorating effects by oil coking" „C“.

Bearing chamber: To this also belong the supporting struts. Does crack formation occur, e.g., through vibrations or thermal fatigue, the danger of a hot gas break-in with an oil fire exists (volume 2, Ill. 9.2-11).

Vent lines: Vapours and gases which flow through only guarantee little effective cooling. With this the pipes can heat markedly, especially in the region of hot struts (Fig. "Oil coaking endangers main bearings"). Oil vapour and a high air supply promote the formation of coke (Fig. "Formation mechanisms and oil coke features" „III”). A further danger exists in fatigue cracks, caused by thermal stresses between the pipe and the turbine casing (volume 2, Ill. 9.2-10). Breaking-in hot gas can trigger a fire with the help of easy igniting vapours. This is in the position to expand also through longer lines. A breather separates the oil from the gas phase and returns it to the oil circuit. It can be clogged by spalling oil coke.

Scavenge lines (reflux lines): Inside these, the oil flow is so little, that already during operation, oil coke can settle (Fig. "Formation mechanisms and oil coke features" „I“ and „II”). This is promoted by high scavenge oil temperature. The heating during operation and after the shut down can rise the temperatures of the line walls dangerously. The formation of coke can be promoted if additionally washed in particles from the bearing chamber settle (Fig. "Formation of depositions in hot oil systems").

Components of the oil system, like gear pumps (Fig. "Deteriorating effects by oil coking" „D“) and heat exchangers/oil coolers (Fig. "Deteriorating effects by oil coking" „G”), are endangered by particles, washed in from the bearing chamber.

 Main bearing failure by coaking

Example 22.3.2-1 (Fig. "Problems of bearing chambers near hot parts", Lit. 22.3.2-8 and Lit. 22.3.2-9):
For a helicopter engine several ADs (airworthiness directives) have been edited, which deal with a coking problem in the power turbine. Coke formed inside the vent lines through the casing struts to the bearing chamber of the turbine. Leaking hot gas from the seals heated the pipes heavily. There was oil deficiency because of the coking which caused main bearing failures. The consequence was a failure of rotor components from the engine with the exit of fractured fragments.
The concerned aeroengine type had a very high temperature level because of production problems and leakages.
Obviously the problem of the bearing failures was promoted by the behaviour of the maintenance personnel, which deviated from the specifications. So in one case the oil analysis showed heavy metal wear debris. However this was not indicated from the warning light in the cockpit. The cables to the chip detektor have been separated during maintenance. In an other case the warning light flashed indeed several times, however the pilot didn't take notice of this.
Aggravating may be, that it was very difficult for the OEM to get satisfying informations from the operators. These have been often limited to asking phone calls.
The OEM developed new seals and introduced a 0,003mm oil filter which withholds coke particles. With this an oil analysis of the filter residues with ferrography was possible. It is markedly more informative as the spectrometric analysis (SOAP, Fig. "Informations of oil analysis" and Fig. "Suitable test methods") of the acid content of oil samples (TAN), used untill now. Additionally the display of the chip-detektors is monitored exactly.
At the same time a less coking sensitive lubrication oil was introduced. It correlates MIL-PRF-23699 HTS specification (Fig. "Lubrication oils") and is suitable for about 40 °C higher oil temperatures.
Above this a device was attached, which limits the axial displacement of the shaft in case of a bearing failure. This device shuts down the aeroengine, if the fracture of the shaft occurred. So by means of the fuel supply an overspeed of the engine and the danger of a wheel fracture with catastrophic results is prevented (volume 1, Ill. 4.5-8).
To demonstrate the funktion at the OEM in a test, the oil was cut off 20 minutes. In fact the shaft moved back after the forced bearing failure, but there have been no catastrophic secondary failures.

Comment: These failures occurred in the 80ies. This is a reconstruction from a multitude of citations. Therefore differences in details are possible. For example it is not clear, how far shaft fractures occurred. With this the danger of overspeed and the fracture of turbine wheels exists. But obviously also bearing failures triggered unbalances and dangerous secondary failures.

 Problema by heat soaking

Fig. "Problema by heat soaking" and Fig. "Importance of resting time at idle" (Lit. 22.3.2-10 and Lit. 22.3.2-11): Aeroengines have relatively to their power, respectively conversion of thermal energy, a small oil volume, related to piston engines. Depending from the size/type of the aeroengine, this is about between 2 and 40 liters (Fig. "Flight time dictated by oil shortage"). From this amount of oil, as well the lubrication as also the cooling of bearings and gears, is carried out. On the other side do aeroengine components store (especially turbine disks) with high operation temperature and big masses considerable heat quantities. These flow off during stand still and can heat neighbored regions so intense, that coking of oil occurres (`heat soaking').
In airliners the aeroengines run during landing approach and also some time after landing (roll) with low power respectively in idle. This enables a cooling period of the aeroengine. In contrast to this, the usual operation profile of the helicopters does not offer this. There is no landing approach with low engine power and a following rolling to the passenger exit. Quite the contrary is the hovering during landing of the helicopter, which demands a high engine power.

 Importance of resting time at idle

The shown small aeroengine has the problem of „heat soaking“ (Fig. "Deteriorating effects by oil coking" „A”). Thereby the bearing of the turbine and its surrounding region of the bearing chamber are heated after shut down through the shaft from the hot turbine wheel. The introduced heat is unsufficient discharged, because of the lacking of cooling air and flow of fresh oil. This promotes especially in elder aeroengine types the coking of oil in the bearing chamber on the turbine side.
For this reason the aeroengine will not be simply shut down from operation. Instead the OEM specifies the timely procedure during the shut down process (volume 3, Ill. 11.1-17).
In the sketched case the aeroengine runs about 2 minutes in idle. Thereby the exgaust gas temperature drops from about 700°C to about 400°C. After the shut down, the air in the exhaust gas region still has about 150°C. But it heats again by heat soaking to about 320°C. Those temperatures displays the instrument for the exhaust gas temperature in the cockpit.
Newer aeroengine types with digital electronic control (full authority digital engine control = FADEC), usually must not keep such shut down procedures. A reason is, that by coking endangered oil is pumped down and valves prevent a following of the oil.

Note: The OEMs as responsible for the design of the aeroengine must exploit all design possibilities, to avoid potential damaging coke formation.

Note: The process of an oil conversion and/or mixing of different oil types must be established in detail and it must be arranged, that the maintenance personnel keeps exactly with it.

 Oil coaking endangers main bearings

Fig. "Oil coaking endangers main bearings" (Lit. 22.3.2-1): This example shows, how longsome and difficult it is to get coking problems under control. The situation is especially demanding, if many aeroengines of a fleet of airliners are concerned. Here the availability for operation must be kept. ETOPS operation (volume 1, Ill. 3-8) intensifies the problem.
The bearing in the turbine exit casing showed in several cases failures, which could be traced back to coking problems in the fresh oil (pressure oil) supply(!). But also scavenge oil lines showed heavy coking. The problems could assigned to a special oil type. Obviously oil accordant the U.S.-Spec DOD-L-85734 AS (Ill. 22.3.1.1-1) was used.
An investigation of the OEM showed, that the oil nozzle/oil jet of the bearing was blocked by a flaking coke layer from the fresh oil pipe line.
Unfortunately from the available literature can not be seen, if the oil coke developed during stand still, shortly after the shut down (heat soaking) or during operation. The oil flow reduced by the coke deposits decreased the cooling. This accelerated the coking in the fresh oil line and the scavenge line, up to a complete blocking.
Obviouly the fresh oil supply through the casing struts was already susceptible for coking, caused by the design (frame below). At this finding the OEM reacted with a cleaning instruction for the shop. Concerned was a thermal processing with the burning (?) of the deposits inside the oil pipe lines (baking method). So seemingly the failures could be limited.
However after further heavy cases of coking occurred, the oil type was changed. The whole fleet of the concerned operator changed over to the oil according U.S. Spec MIL-L-23699 Type II (Ill. 22.3.1-1). This oil has proved itself at other operators. It was decided for an oil change at the whole fleet, to avoid an uncontrolled mixing with oils of aeroengine types from other airlplanes of the operator. Form the oil change the OEM prescribed a dumping and re-filling (drain and flush) of the oil system. With warning labels, the manitenance personnel was warned not to mix the oils.
Unfortunately the coking problem got really serious after the oil conversion. Already a quarter of a year later, a bearing failure with heavy vibrations occurred. The same symptoms arose shortly later at an other aeroengine. An other operator made similar experiences. Further cases with coking of bearings followed. The aeroengines had between 500 and 1000 start-shut down cycles after the last cleaning of the oil sump.
An unsuitable influence obviously came from the cleaning of the pipes by „burning out“. With this the adherence strength of the ramained coke layers dropped. Now these could easier flake and hinder the oil flow. This brought the OEM to a change to an alkaline rinsing (flushing method). Because every method has its specific advantages, the operator, to be on the safe side used both methods successive.
It showed, that the type of oil change is essential for the flaking already existing coke deposits in the fresh oil pipe line. Responsible was the high concentration of additives in the fresh oil. These support a flaking of the coke layer. However an intensification of the coke formation, caused by mixing the old with the new oil type, could not be ruled out.
A measuring of the flow rate at the aeroengines on wing was the only precautionary check for the blocking of the oil jets/nozzles. This demands a special device, provided by the OEM. Already the first aeroengines did not pass this test. Now obviously the situation got hectic. With a simplified blow through test developed by the operator of the potential concerned pipe lines, all aeroengines have been checked within few days. Thus probably sevreal cases of bearing failures could be prevented.
The fhrough flow tests are carried out periodically in a monthly period (during the A-check of the airplane).
A conversion of the oil change procedure took place from drain and flush to the top-up method. Additionally the concerned oil pipe lines have been designed for disassembly. This facilitated the maintenance work in critical areas. Investigations of the OEM let eypect additional safety with a monitoring of the oil pressure.

Recommended measures to prevent potentially dangerous oil coking:

Design features:

  • Oil flow as strong as possible.
  • Oil temperature as low as possible.
  • Temperature of oil wetted walls as low as possible (insulation, cooling).
  • No sharp pipe bends.
  • No flow disturbing cross sections.


Maintenance:

  • Suitable cleaning of the oil system components.
  • Monitoring of the oil parameters during operation.
  • Measuring the flow rate during operation.
  • Determined methods of the changing and mixing of lubrication oil.


Oil:

  • Selection of optimal operation specific suitable oiltype with thermal stability as high as possible.

 Risk of oil coaking in vent and scavenge lines

Fig. "Risk of oil coaking in vent and scavenge lines" (Lit. 22.3.2-13 and Lit. 22.3.2-14): The concerned airplane type is certified ETOPS 180. The investigation of the failed aeroengine by the authorities showed :
The turbine casing of the intermediate pressure turbine (IPT) has been ripped at the whole circumfreence. The shaft of the IPT („2”) in the reginon of the rear bolting with the disk (`drive arm') was separated in the area of the cooling holes. A material investigation let suggest at component temperatures of more than 1000°C. Obviously the following overspeed of the free turbine disk („1“) led to the centrifuging of all blades. The associated main bearing chamber showed two burned holes with diameters in the region of centimeters.
From the upper part of the vent line (vent tube, „4”) only a small piece, mounted at the turbine casing, with signs of extremely overheating remained. The part of the pipe line inside the bearing chamber missed, except a small piece. The heat insulation („5“) was missing.
An investigation of the parallel aeroengine showed:
Soft, grainy coke deposits inside the vent line in the region of the bearing chamber wall. The pipe line was not fully blocked. These deposits show an unusual structure, compared with the usual coke inside the vent lines. It promotes the autoignition (Lit. 22.3.2-13) and acts as flame holder. This accelerates the coke buildup. The ventline and the scavenge line show heavy, but accordnat the OEM not unusual fretting wear. The heat insulation („5”) around the pipes lacked.

Additionally findings:
The concerned operator was the only one, who used for this aeroengine type a certain oil (HJO 291). However a coking test showed no deviations from the instructions/specifications. During the investigations it was discovered, that obviously the oil approval did not sufficient consider the demands of modern fan engines.
Former incidents: Already about 10 years before this failure, coking was found at two aeroengines at the same place. The OEM responded with a service bulletin. It planned periodic borescope inspections (all 1500 operation hours) on wing. If necessary a cleaning of the pipe must be carried out. Alarming coke deposits, obviously have been observed only after 3000 hours and with a certain oil (ASTO 560). This the OEM removed from the list of approved oils. Since even after more than 10 000 operation hours, no noteworthy coke deposits have been found. After this the inspection was canceled. Therefore the later used failed aeroengine and its parallel engine with ca. 15 000 operation hours at ca. 2300 cycles since new, have never been been inspected.

Further approach:
After the incident all aeroengines of the concerned type have, independent from the used oil, have been checked on wing. Thereby it was only possible to identify signs of a blocking in the vent lines. With this damages of the lines or the insulation could not be found. About 180 aeroengines with long operation time showed coke deposits, but less as in the acute case. However in 3 cases 75% of the pipe cross section have been clogged.
Then during overhauls at 70 aeroengines, the pipe lines with the heat protection have been inspected from the outside. In every second case the heat insulation at the critical zone (near the bearing chamber) and also in the outer region (near the turbine casing) was cracked, burst open or lacked.

Probable failure sequence:

  • Damaged heat insulation causes a higher walltemperature.
  • Build up of coke in the vent line.
  • Autoignition of the oil.
  • Overheating of the pipe line and the bearing chamber.
  • Oil lekage with oil fire.
  • Overheating of the IPT shaft till fracture.
  • Overspeed of the IPT.
  • Weakening of the turbine casing (rubbing?) and centrifuging of the blading with the exit of fragments (uncontained).

References

22.3.2-1 D.Santamaria, „Engine oil coking and its effect on performance“, Zeitschrift „Aircraft Technology & Maintenance-Engine Yearbook”, 1999, Page 48-53.

22.3.2-2 J.Schmidt, W.K.Hank, A.Klein, K.Maier, „The Oil/Air System of a Modern Fighter Aircraft Engine“, MTU-München GmbH, Page 1-20.

22.3.2-3 D.G.Dowse, E.Jantzen, K.Maier, „Deposition in Gas Turbine Oil Systems-Part 1: Analysis and Classification”, SAE-Publication SP-633 „Aviation Gas Turbine Lubricants-Military and Civil Aspects“, „Aviation Fuel and Lubricants-Performance Testing”, October 1985, Page 115-131.

22.3.2-4 E.Jantzen, „Tendency of Deposit Formation of Aircraft Engine Oils“, Symposium „Aviation Turbine Oils”, an der Technische Akademie Esslingen, December 14th-16th, ISBN 3-924813-11-6, 1988, Page 9.1-9.20.

22.3.2-5 K.Meier, „In-service Engine Oil Problems“, MTU-München, Page 4.1-4.10.

22.3.2-6 NTSB Identification FTW84FA216, Microfiche number 26590, „Turbine Assembly Shaft Fatigue”, Page 1.

22.3.2-7 Interavia AirLetter No. 10,675 - January 23, 1985 - 5, „Allison Fix for S.76 Engines“,Page 1.

22.3.2-8 E.J.Bulban, „Can Textron Lycoming Fix the LTS 101 - This Time?”, Zeitschrift „Rotor & Wing International“, November 1987, Page 34-36.

22.3.2-9 „Lycoming spends $30 million on LTS101 turbine”, Zeitschrift „Flight International“, 14 October 1989, Page 10.

22.3.2-10 R.Strecker, „Der richtige Dreh beim Abstellen”, Zeitschrift “aerokurier”, 2/2002, Page 60,61.

22.3.2-11 „Lubricating Systems“, www.globalsecurity.org/military/, 2005, Page 1-17.

22.3.2-12 E.Jantzen, „Behaviour of Aircraft Engine Oils at High Temperature”, DFVLR, Page 1-12.

22.3.2-13 V.Rosenecker, „NTSB, Safety Recommendation - In reply refer to A-06-85 through -87“, December 14, 2006, Seite 1-5.

22.3.2-14 „NTSB, Accident Report DCA04IA002”, April 25, 2006, Seite 1-5.

© 2022 ITTM & Axel Rossmann
22/223/2232/2232.txt · Last modified: 2022/02/20 17:00 (external edit)

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