Aeroengine Safety

Fig. "Approved lubrication media" (Lit. 22.4-19): The persued operation properties of anti friction media can be found from spezifications and brochures of suppliers. In contrast, potential problemes and dangers are mostly discussed unsufficiently. Therefore in the following, especially specific dangers during the use are deepened. Besides lubrication greases, dry lubricants stay in the foreground.
The experience shows, that metals and metal compounds (e.g., sulphides ) are used for the anti friction effect. However, just these have a high failure/damage potential. Thereby operation influences like a high temperature, water, corrosive media and tension stresses are important. Containing molybdenum disulphide (MoS₂ ): The particle size of the MoS₂ is, depending from the thickness of the lubricant film and the roughness of the sliding surface, of high importance of the anti friction effect. For smooth/polished surfaces like these of bearing, markedly finer particles are necessary. Too coarse particles leads to extreme abrasion/wear. So it is understandable, why the change to other products, alone with the argument of the same composition, is dangerous.
The temperature limitation of use for MoS₂ containing producs lays under 400 °C. It depends from potential deteriorating mechanisms of the materials, which will be lubricated. In this range already markedly decomposition by oxidation into MoO₃ and SO₂ occurs. Is MoS₃ existing, which acts intense pic (volume 2, Ill. 6.2-15.1), serious friction problems appear. With the absorption of humidity, the coefficient of fricttion markedly rises from 0,02 up to more than 1. If humidity effects this compound it is the contrary of a anti-friction agent! This is the reason for many negative experiences with this anti friction medium in the normally humid atmosphere (sketches in the middle and below, Ill. 22.4.1-4).
Is MoS₂ containing lubrication media used at temperatures like those, typical for hot parts, sulphur can be released. Especially at Ni-alloys catastrophic sulfidation damages (Fig. "Significance of using approved lubricants" and Ill. 23.3.1.2-9) with the failure of the component can be triggered (volume 1, <U>page</U> 5.4.3-3 and Ill. 5.4.5.1-4). Are cross sections in contact with MoS₂ under sufficient high tensile stresses, spontaneus crack formation and fracture must be expected (Fig. "Unsuitable lubricant leading to fracture"). Also during an application by spraying (e.g., of an anti-friction agent) attention must be payed, that no hot parts are contaminated with MoS₂ .

Polytetrafluorethylene (PTFE, „Teflon ®“): If at high temperatures fluorine or its compounds are released, for metallic materials the danger of corrosion, similar to chlorine, exists. Especially dangerous is stress corrosion (stress corrosion cracking = SSC) at components under sufficient hich tensile stresses. To the potential endangered materials belong titanium alloys and high strength steels. Terefore attention must be payed, that also no unintended depositions, e.g., during the use of sprays like release agents or anti seize agents, get on components with high operation temperatures.

Graphite is used in lubricants and protects metallic surfaces against seizing/galling. However it oxidises already markedly at relative low temperatures (from about 300°C). Graphite is resistant up to about 1300°C without the access of oxygen. So it can stay over long operation periods in tight threads, also at typical hot part temperatures in aeroengines. With this it would be also helpful, loosening connections without damage. Its lubricity needs the access of humidity (vapour, air moisture). Gets graphite unintended or by abrasion/wear into electric isolating areas, it can trigger shorts respectively creepage currents.

Hexagonal boron nitride is an inert dry lubrication that contains its lubricity in an oxidising atmosphere up to about 600°C operation temperatures.

Metal oxide forming high temperature lubricants („Neverseez ®”) contain metals like nickel, copper and zinc in a compound with petroleum as carrier oil. This evaporates at operation temperature and leaves oxides of nickel, copper and zinc. These prevent a seizing/galling. This allows the loosening of a connection/bolting without damages, also after longer operation periods. However here attention is necessary. If even so, high break loose torques arise, a not with non destructive testing detectable deterioration is possible. This can prevent a further use of the bolts.
Further the effects of the metallic contents from the lubrication grease must be tested during an approval.
The danger of the formation of a corrosion cell prohibits the use in contact with corrosion susceptible alloys. This applies for low operation temperatures, during which no protecting oxides can develop and at least temporary humidity exists (e.g., condensate during stand still).

Silver containing pastes let expect a lubrication effect (e.g., at threads). This may apply similar pastes, like are also used for the heat dissipation at electronic components. At hot parts from nickel alloys, silver can act dangerously deteriorating in different manners (volume 3, Ill. 12.4-14). This goes from the promotion of a sulfidation, deterioration by diffusion (loss of strength) up to a spontaneous crack formation during tensile stresses and the influence of molten silver (Fig. "Brittle failure modes of bolts and nuts", LME volume 4, Ill. 16.2.2.3-10.1). Similar dangerous are pastes with metallic lead to estimate. Also here it must be reckoned during high temperatures with initial fusings and LME at steels, nickel alloys and titaniujm alloys (volume 4, Ill. 16.2.2.3-10.2).

At Titanium alloys already above 200 °C, the direct contact with solid silver acts embritteling (SMIE, volume 4, Ill. 16.2.2.3-11). Basically the use of silver should be omitted for alloys of Al and Mg because of watery corrosion due to formation of a corrosion cell.

Fig. "Properties and application range of lubricants" (Lit. 22.4-2, Lit. 22.4-9 and Lit. 22.4-10): This chart contains a selection with remarks about the properties and the application range of frequently used lubricants. In this chapter also at other locations it is pointed at several of these auxilary media.

Fig. "Environmental influences of lubricants" (Lit. 22.4-16): The behavior of lubricants can change totally under environmental influences. This must be recognised during testing and approval/certification. So the following factors play a role:

• Access of environmental/surrounding media (Fig. "Approved lubrication media" and Fig. "Beware unsuitable lubricants").
• Contact with auxilary materials.
• Component specific operation parameters.
• Materials/tribological system.

Also during change, respectively the introduction of new auxilary materials/media or new products (also the change of the producer/product) like washing fluids and solvents, anti icing agents or contakt sprays, pay attention at the effects at already used auxilary media. If necessary a proof of the harmlessness is needed from the OEM. In case of doubt, basically the agreement of the responsibles must be obtained. Typical, potential deteriorating mechanisms are:

Intrusion of foreign particles into the lubrication gap: A leakage flow through the bush bearings of variable guide vanes (sketch in the middle) can transport corrosive dust particles and dispose these between the sliding faces. So with the use of MoS₂ containing lubricants (also as infiltration and in sliding bushings), the danger of corrosion and jamming of the system exists (Fig. "Approved lubrication media").

The ingression of liquid media: These can act corrosive, intensified by the lubricant. Larger amounts and with a solving effect can elutrate and cause the metallic contact between the sliding faces (wear/abrasion).

Entering of vapours: This already applies for air humidity if it changes the lubricating properties of the auxilary medium at operation conditions. Also vapours of solvents or oil vapour can alarming influence not sufficient tested auxilary materials.

Fig. "Beware unsuitable lubricants": During the testing of the thrust reverser, the function of the pivoting mechanism was proved in serveral test cycles. Thereby, after a short engine run, the pivot bearings have been treated with salt water spray, that afterwards acted overnight. Already after frew test cycles the bush bearings jammed with catastropic damages by orverloading the actuating mechanism. Investigations showed, that obviously the used MoS₂ containing dry lubricant film at the sliding faces, together with corrosive water, caused corrosion products with a volume increase (Fig. "Approved lubrication media"). Thereby certainly the deteriorated anti-friction properties of the MoS₂ during contact with water, have been from especially significance. After a conversion to the synthetic aeroengine oil with which a periodic lubrication was carried out, no more failures occurred. This proved itself during the following long years series operation.

Fig. "Maintenance of lubricated systems" (Lit. 22.4-16): Understandably the maintenance has an essential influence at the safety of a lubricated system. Thereby the re-lubrication plays a crucial role. Do here failures occur, respectively will be deviated from specifications/instructions, especially in the manual, catastrophic failures can occur (Fig. "Ill maintained lubrication leading to crash 1"). Also media of cleaning activities in the surrounding of dry or grease lubricated components, their function can be affected (Fig. "Environmental influences of lubricants"). This applies also for abrasive cleaning particles (e.g., from cleaning `felts') and dust (e.g., compressor wash, chapter19.2.3).

Of course only the auxilary media and lubricants, which are explicite approved/specified for the particular application by the responsible, may be used. An auxilary medium, which is sheduled for an application, can fail at an other component with other operation conditions. Therefore an unauthorised deviation is potential dangerous and must be omitted.

Fig. "Warning signals" (Lit. 22.4-3): Several components send before a catastrpophic failing `warning signals'. They only must be registered, indentifyed and know, how to interpret. Understands the experienced technician the „language“ of the part, he can intercept dangerous consequences of function deviations in time. To this belongs the remedy respectively the introduction of the adequate measures. Typical „warn signals” are:

High moving/actuating forces and breakaway forces can indicate by relating, high power consumption, respectively the rise of forces and hydraulic pressure in the actuation mechanism. Examples are actuators of thrust reversers or the thrust nozzle adjustment at aeroengines of fighters. Also an asymmetric movement, standby position or overload features at systems of power transmissions (e.g., jackscrews, levers), are symptoms. Do those arise, naturally not only the deteriorated area must be repaired respectively exchanged. It is essential to identify the cause, i.e. if necessary the sluggish joint/bushing and to clear the failure mechanism.

Unusual pronounced „stick-slip-effects“ (Fig. "Influences of the stick slip effect" and Ill. 20.1.14) are caused by deteriorated sliding properties. They often draw attention by noise emission. Typical is squeaking (high-frequency, an example are brakes in a car) or creaking (low-frequency, an example is an old door), chattering/rattle and „wobbling”. Also vibrations can be the result of a movement. They can create instabilities in control systems and admeasurement systems/valves, which show in the aeroengine behaviour or at displays/instruments.

Features of the lubricated area and appearance of the lubricant: Already abnormal amounts of escaping lubricant can apply as a hint at deviations in the tribological system. The cause are an unsuitable lubricant and/or unusual operation influences/conditions (e.g., high temperatur). This applies contrary also for unexpected gumming or brittle deposits (Fig. "Ill maintained lubrication leading to crash 1").
Especially alarming is the so called „bleeding“ of the lubrication gap. An indication are red brown up to almost black run-down signs (Fig. "Lubrication caused fretting wear"). Usually those occur during fretting (fretting corrosion at steels, volume 2, Ill. 6.1-3 and Ill. 6.1-4). In such a tribological system, fine wear/abrasion products oxidise. The flushing of these wear paricles let expect an unacceptable increase of the clearance, if it not yet happened. Unusual discolourated remains of lubricant can also point at the reaction with other media like solvents or wash fluids.

Deposits at sliding surfaces/contact surfaces announce with colour, structure and consistency problems and allow conclusions at the causes (volume 2, Ill. 6.1-3). Also here red brown discolourations hint at fretting corrosion. Naturally the inherent colour of the lubricant (frequently black for MoS₂ or graphite) must be considered.

It is especially alarming, if there are no remains of the lubricant (Fig. "Ill maintained lubrication leading to crash 1"). This applies just, if during correct maintenance respectively assembly those must be expected. Then the conclusion at deficits, during the prescribed lubrication processes and relubricatios, is obvious.

Features of wear, respectively fatigue at sliding faces and pitch surfaces (races,flanks): At anti friction bearings such a deterioration can attract external attention with vibrations. In grease lubricated bearings, often particles caused by wear and fatigue can be found. At bush bearings, the advanced wear arises usually as a markedly increased clearance.

Exterior discolouration of bush bearings/anti friction bearings respectively casings/boxes: Are tempering colours concerned, this is a clear sign of a too high operation temperature. The cause may be the increase of the friction forces and with this the frictional heat. This again points at an already dangerously advanced bearing failure. Such an indication on a grease lubricated bearings is also supported by leaking grease, that fluidised too much. Simultaneously the seal of the bearing can be deteriorated by over temperatures.

Fig. "Significance of using approved lubricants": Concerned is an aeroengine of a fighter (sketch above). The assembly of the turbine module should be carried out `especially fine'. To increase the torque efficiency of the bolts (Fig. "Influence of pretension at fatigue" and Fig. "friction forces on bolt connections"), e.g. to guarantee the necessary pretension with the specified fastening torque, the bolts of the coupling (curvic coupling, detail in the middle) have been treated with MoS₂ containing lubrication grease. Prescribed was synthetic aeroengine oil. Unintended, the toothing was contaminated with remains of lubricant. Part temperatures in the range of 500 °C decomposed the MoS₂ and a sulfidation attack (detail below left, Lit. 22.4-19) occurred. These, also if small deteriorations/notches (in the range of a tenth millimeter), unfortunately have been located where they are relevant for the lifetime of the part because of the very high loads. A fracture mechanics evaluation could not certain enough rule out a dangerous cyclic crack growth. Therefore several suspect turbine modules had been disassembled. These expensive parts (turbine disk, sketch right) had to be scrapped.

Fig. "Unsuitable lubricant leading to fracture" : In this case it was tried to improve an assembly process. Like in Fig. "Significance of using approved lubricants" the required initial tension should be guaranteed with a `better' lubricant of the central bolt thread. This thread is in the middle sketch located at the right end of the central tension bolt. This tensions over toothed couplings (curvic couplings) the turbine wheels of the gas generator.
The thread, which is treated with a MoS₂ containing lubricant, is moved through the hub bore of the turbine wheels. These consist of a Ni alloy casting. The hub bore of the 1st turbine stage wheel is so tight that lubricant, which laps the thread, is wiped off into the hub bore. This is during service extremely high loaded.The position of the centering lappped-type flange of the central bolt was just exactly positioned in this zone. So it was possible, that at a materials temperature of more than 500 °C the decomposed grease released sulfur. Because of the access of air hindrance, no burning occurred. In the micropores which are for these casting materials in certain limits inevitable and so specified/accepted. Into such, which have been opened by the machining of the bore, sulfur intruded. This caused a sort of stress corrosion cracking with very fast, accelerated crack propagation. Already with an axial length of few millimeters, the crack gets instable (volume 3, Ill. 12.2-1). The fracture of the wheel with fragment exit (uncontained failure) occurred. An extensive and immediate readjustment at about 100 already delivered aeroengines, which have been assembled according to the suspect procedure, was necessary.

Fig. "Ill maintained lubrication leading to crash 1" (Lit. 22.4-3): This crash of an airliner into the sea is caused by the failing of the elevon actuator. Parts of the airplan could be salvaged. The investigation arose as cause of the crash a failure at the adjustment spindle of the elevon. So this was no aeroengine failure. However the occurrences are in causative connection with maintenance deficts. They are also interresting for the field of aeroengines.
The sequence of the flight accident and findings at the wreckage pointed at the failing of the elevon (left side, frame above reight). This is actuated by a screw nut on a threaded spindle with a length in the range of meters, with about 10 cm in diameter (left side, frame below right). It is driven by electric motors.
Parts of the elevon actuating system with spindle and actuating nut could be recovered. The threaded sleeve inside the actuating nut (lleft side, below left), was broken and showed pronounced wear damages. Thread pitches down to the root radius had disappeared and so the nut got out of engagement. The forced fracture at the end comes from a circumferential LCF crack (low cycle fatigue) with accordant crack propagation features (striations). Inside the nut no residues of a lubricant have been found. However in the neighboring outside section there were red brown and black residues of the lubrication grease (Fig. "Warning signals"). The bore for a local lubrication was blocked by a hard black deposit (Fig. "Warning signals"). This deposit contained wear particles from the nut.
About 20 cm long copper coloured wear/abrasion chips out of the nut had warped aroud the spindle (right side above). It was noticeable that the spindle showed in the operating distance, except tiny rests, no traces of old or fresh lubricant. Outside the operating distance, remains of lubricant have been found in the thread roots.

The investigation of further according actuating nuts from other airplanes of the operator showed wear damages similar in type in different stages. Investigations of the lubricant remains arose, that these didn't fit to one of the both specified products. This pointed at a mixture of the greases. During tests in connection with one of the the greases (Aeroshell Grease 33, Ill. 22.4-2) a dark redbrown discolouration was observed. This obviously concerned a desired, i.e. harmless reaction of the additives. Extensive wear tests showed, that a mixture of the specified/approved greases (Aeroshell 33 and Mobilgrease 28) not noteworthy differ from the single greases. Also 5 % water or deicing fluid did not change this favourable behaviour. Only during deficit of grease, a wear was observed, similar to the wear of the accident parts. Therefore the mixture of the greases could be ruled out as cause of the extreme wear.
The failure sequence arose, according to the investigations, as follows (right side below):

At the beginning the wear in the moving nut caused the blockage of the actuation.
Then during the frequent actuating trials, a shear of the remaining nut thread occurred. The nut was torn over the spindle up to the stop. It came to an extreme, aerodynamically caused swelling load ot the elevon. This lead to a cyclic overload of the threaded bushing, with crack formation and subsequent forced fracture. With this the elevon got free over the stop and triggered the final crash process.

The cause of the etremely wear of the actuating nut must be seen in an unsufficient mantenance caused lubrication.

Thereby possibly additional effects acted:

• Lacking lubrication, respectively re-lubrication.
• The technician was not informed about the procedure during the re-lubrication. For example he did not know, that during lubrication grease must be visibly extruded out of the engaged threads.
• Extension of the lubrication period by the operator from 1600 flight hours to 8 months (ca. 2250 h). Thereby, without sufficient experience and coverage/proof, it was acted from the assumption, that this is justified with a change of the lubrication grease product.

A later inspection of the responsible authority (FAA) arose in thre maintenance field of the operator a multitude of deficits:

Management:

• The position of the maintenance director was since two years not occupied and is since a short time administrated by two persons.
• The position of the plant manager is vacant.
• The director for the safety is simultaneously responsible for the quality control and the training. He has no direct report authorisation at the highest management level.
  

Maintenence training:

• The manual of the operator does not discribe training handouts or the practical trainings.
• The training program is only informal in the discretion of the instructors. There is no structured plan, the determination of the subject matter and criteria of success.
  

Manitenance program:

• The maintenance manual (general maintenance manual) does not render the actual, at the operator used processes.
• The maintenance manual does not contain action instructions for extensive planning and controlling (heavy check).
• The maintenance manual does not contain a complete approach for the problems of an aircraft after a heavy check (airworthiness release).
• In single cases the documentation of the maintenance was incomplete.
• Deficits in the labelling and the trace back of spare parts.
• Shift transfer did not always take place, according to the maintenance manual. Deviations of the documentation (deviating forms, lacking signings).
• Work sheets for special tasks are only provided with a stamp, but not with a signature of the issuer.
• Though notations of a not finshed work process are existing, they lack for the completion.
• Changes on the work sheets without coumter-signature of the engineering or the quality control.
• The maintenance manual contains no instructions like work sheets, which must be distributed and monitored during the maintenance.
  

Continuous problem analysis and monitoring.

• Manuals show no copies of examples of check lists for such a program.
• The monitoring is not continuous. Check intervals in periods of one or two years with an overdrawing of half a year.
• Check/inspection methods and techniques with the, at the operator used program, do not correlate with the prescribed safety standards.