Aeroengine Safety

Fig. "Design compromisses for maintainability" (Lit. 19.1.4-3): The maitainability of aeroengines must be adjusted in compromises with higher-ranking requirements. So, for military aeroengines the insensity for shelling from the ground is especially important. That requires the positioning of the assessories with the many different (pipe-) lines at the upper side of the engine. For fighter aircrafts, where the engines are inside the fuselage however, is the accessibility to the accessories from underneath easier (Fig. "Designed overhaul frendliness").

Here is the positioning of the accessories, especially for civil fan engines to mention. In this case it's just the point to minimise the risk of a simultaneous outage of both engines of a twin-jet in case of an uncontained failure. The accessory components which contain/pipe potential dangerous media (oil, fuel) are positioned at the so called „wet side“ (volume 2, Ill. 8.2-3.2).

Figure "Designed overhaul frendliness" (Lit. 19.1.4-2 and Lit. 19.1.4-3): A summary of important considerations and guide lines (without claiming completeness) shows measures, which are at the disposal of the designer to optimise the maintainability of aeroengines.

In the foreground stand the experiences, especially of the operator in practice (Fig. "User relevant development of a maintenance manual" and Fig. "Safety risk by not reported manual problems"). This is a basis for the definition of requirements from maintenance and overhaul. From this the concept for maintenance respective overhaul can be developed.

To recognise typespecific problems in time, today already 3-D computer programs are available in the design phase. With it problems of assembling and fitting can be identified. Anyway an early buildup of a hardware engine model is advantageous.

The consideration of some guidelines enable the designer already a configuration, suitable for maintenance. In the center of the considerations are the exchangeable units („line replaceable units” = LRU). Here acessories with their pipes and cable connections are of special interrest. It is essential to minimise the effort of often repeated maintenance tasks like oil control, magnet plug control and exchange of filters. A requirement is the best as possible accessibility which also inccludes especially the possibility of a visual control.

Is the exchange of aggregates necessary, the attachments which must be separated schould be located in one level. So, for example, a pipeline connection at a far spot necessitates a long, sumbersome exchanging part where complications during the fitting can be expected. That could enforce a displacement of other parts/components and as well increase the assembling effort as influence the safety potential (Fig. "Technicians unfamiliar with aircraft"). Rests of oil and fuel can deteriorate neighboring parts with problems like swelling or embrittling of elastomers or the formation of annoying vapours (Fig. "Smell and odour of cockpit") up to the development of cracks at metallic parts (Fig. "Contaminated Hydraulic fluid"). To avoid an uncontrolled exit of fluids (oil, fuel) during the loosening of connections, suitable drainage devices must be provided.

The demand for the solely use of standard tools for work, especially the exchange of LRUs, simplifies the logistics and avoids unauthorised variations. Precondition is of course a suitable design. This includes not at least enough space and accessibility to handle the tools.
Standard tooling requests also for standard connecting elements like flange screw joints or tube fittings.

A securing of the bolting with wires will be avoided on modern aeroengines with self-locking systems.

Tolerance attachments like washers, e.g., at connections for actuator cables or feedback cables or for the adjustment of a bearing clearances (e.g., on radial shafts) do not only mean a higher assembly effort. They are also a source of potential failures. Therefore tolerance attachments should not be planned by the designer.

The safety serves also a good visual recognisability of identification markings in the built-in condition.

Fig. "Measures for ergonomic maintainability" (Lit. 19.1.4-2 and Lit. 19.1.4-3): This example of a modern aeroengine type for use in big airliners shows besides the compliance with basic guidelines (Fig. "Designed overhaul frendliness" and Fig. "Considerations for installation of a hing" and Fig. "Designing optimal maintainability") many measures for good maintainability and overhaul. To this belongs the requirement, that an exchange of accessories must be possible within 130 minutes.
An important measure is the standardisation of connecting components like bolts, pipes fittings and clamps (chapter 23.5). Connection of pipes must be placed in the level of the module main flanges.Thereby the maintenance activity is confined to a limited space and the size of the exchangeable modul is minimised.

Therefore also a connection of a fuel pipeline at accessories like gears or fuelpump must be integrated.

To ensure the loosing and tightening if only from one side accessible threaded, the connections, the nuts must be appropriate fixed.

Boltings/plugs for liquids or borescope holes are designed selfsecuring. This corresponds the proscription of lock wires. Of course is in case of self securing the reusability must be explicit checked and defined.

In the case of clamps for pipelines it depends on whether they are not opened or closed by ductile bending during assembly (Fig. "Problems of a fitting assistance can trigger").

To avoid problems during the exchange of an engine modul, the acessories may not be attached/fixed at the main flanges of the module.

Componets that mus be frequently exchanged, like acoustical inserts respectively casing rings with rub coatings in the area of the fan or booster (see sketch) must be exchangeable at the installed engine on the aircraft („on wing“).

„Full Authority Digital Engine Control” (FADEC) as accessory in modern aeroengines is for maintenance and overhaul suitable to incorporate. The considerably cable connections for in- and outcoming control/monitoring impulses demand suitable plug connections and with colour encoded cables. Those must be secured with cable clamps without the use of bolts.
Also the connections of fluid containing pipelines must be suitable labeled. This prevents the mechanic from „surprises“.

Figure "Problems of a fitting assistance can trigger": To assemble a so called P-clamp at least one side of the pipeline must be loosened to slide the the clamp on. Is the pipeline mounted on both ends the clamp must be bend open (lower sketch). Such a procedure during the maintenance lead obviously in several cases to the failing of the clamp during operation. With it the danger of a fatigue fracture of the affected pipeline existed. Two deadly flight accidents of fighters can be apparently related to the failing of an air tube. This shows also the danger of a fatigue fracture of a pipe affected by the failing of a clamp. So it is absolutely wise, that for new aeroengine types the solely use of clamps for pipelines which must not be bended open („lock bush clamp”) is prescribed (Fig. "Measures for ergonomic maintainability").

Figure "Failures by unfavourable assembly conditions" (Lit. 19.1.4-17): This is an impressive and hopefully instructive example, how an insufficient consideration of the human factors can provoke dangerous situations.

The oilfilter is mounted in a very narrow space. That makes its adjustment very difficult (left frame). During assembly the adapter (flange) is pulled with smooth tightening of the flange side nuts to the seat of the casing/housing (frame in the middle). A problem like the not sufficient adjustment of the flange can increase the tensening forces already dangerously and overload the flange. This is hardly to notice because of the high power gear ratio of the nuts (Fig. "Bad accessibility needs prwecaution"). Also the dismounting of the flange is hampered. This can motivate for the loosening use of a not suitable tool like a screw driver (left frame). A larger operator with an overhaul shop has reported several such incidnets. With a precautionary inspection ot the aeroengines in his aircrafts further two cracked parts were found. Now cracks should be found with a nondestructive test. Additional filter housings of a more sensitive elder models will be exchanged with a new version, developed by the OEM.

Figure "Considerations for installation of a hing" and 19.1.4-4.2 (Lit. 19.1.4-4 and Lit. 19.1.4-5): Military helicopters can count as very ambitious for optimal maintenance and overhaul work. Such work is already to be carried out in unfavorable surroundings. Thereby is the position of the engine at the fuselage in an area which can not be reached without auxilary means (high above the ground) especially demanding. In this connection also the weight limitation of the components which must be dismounted respectively exchanged gets understandable.

An esspecial emphasis is obvious pointed at clear, distinct and well-arranged documents. From this results, that the description incorporates the whole necessary field of activities. Thereby the distinct identification of the position relatively to a point of reference respectively to a reference axis as well as limits for placements plays an impotant role. Those must guarantee the compliance of the tolerances and positions. For example to avoid the tensioning of pipelines (danger of fatigue cracks!) or the contact and the fretting of lines (chapter 23.5).

Of course suficient clearances must be scheduled, to enable an acceptable assembly. To meet this demand for the field work, gauges and devices/jigs are necessary. The exchange of lines must be possible with simple, also in military situations available auxilary means.

All for the maintenance necessary markings have to be positioned good readable with a safe process. Not suitable are for example markings which can be damaged by fluid media from a leak. To those belong adhesive tapes which are sensitive for fuel and oil.

The military use has to be considered for pipelines with a suitable position and arrangement (Fig. "Design compromisses for maintainability").

Connectors/couplings for actuating respectively feedback line cables and rods must be easy to identify, adjustable and exchangeable. The requirement for a safe and easy adjustability is especially obvious. Preconditions are good visibility and certain identification to avoid confusions (e.g., adverse to the documentation) with catastrophic consequences. Often the adjacent connecting cables are turned over rolls. This function has to be considered during maintenance. The derailing out of such a guidance or a blocking must be avoided.

Are shear connections used as couplings (shanks with cross bolts), the compunents must be secured against loosening (e.g., sliding out of the bolt, Fig. "Cotter pins and fixings of splints"). Doors and covers for the necessary accessibility of the aeroengines must be designed to easy dismantle or exchange. Thereby hinge-joints must be only dismantled as ads a whole. The separation of a hinge-joint itself (e.g., by pulling out the bolt) must not be possible (Fig. "Risk of shift changeover with not accomplished maintenance"). Is the aerodynamic affected by the cover/door those must be sufficient exact to position, to avoid vortex forming contours.

Illustrations 19.1.4-5.1 and 19.1-5.2 (Lit. 19.1.4-9): During a training flight in the late sixties of the last century in the mediterranean area, the pilot noticed on the cockpit instruments a serious oilloss of the engine.This triggered the nozzle to open and so for this single engine fighter type to a dangerous loss of thrust. After that an emergency landing followed on a, for this purpuse in fact too short civil airport of an island. The airplane overrun the landing strip and plunged head first into the sea (upper sketch). The pilot catapulted himself with the ejection seat from 3 meters water depth out of the aircraft. He survived badly injured.

After recovery the engine was removed and investigatet for the reason of the problem. At the causative failing of the scavenge pump could be suggested by the observations of the pilot and the displays of the aircraft instruments.The investigations were hindered by the fact, that the underside of the aircraft front, especially the scavenge pump was heavy damaged by the impact on the ground.

It was found that the drive pinion gear („A“ in the scetch bottom right) has failed because of a crack by bending fatigue from one side (details bottom left). Additional it could be definitely seen when the fracture surfaces are merged, that the pinion gear shaft was plastically deformed. This could explain the bending fatigue load. But how could the bending of the shaft happen?

A microscopic investigation of the pinion gear showed at the front face of a single tooth a plastic deformation (volume 1 Ill. 4.3-6). Its form was explainable with the contact of an axial moved part, the corresponding drive gear.

A request at the military operator unfolded, that only a few hours before the flight accident at a multitude of aircrafts in a non-standard action work was performed at the scavenge pump on the installed engine. The reconstruction showed, that thereby the pump had to be mounted under bad accessibility. Obviously it came during sliding on of the pump casing on the stud bolts („B”) to a face contact of the teeth. Even though there is an explicit warning in the manual about this situation, perhaps triggered from similar corresponding experiences (Fig. "Warning for bad maintenance accessibility"), it is obvious that with the help of the nuts „C“ the resistence against the assembly can be overcome and so the shaft of the pinion gear was plastically bent.

An at once inspection of the engines from other suspect airplanes showed further five parallel cases, which however did not lead to a breakdown of the pumps.

Figure "Advantage and danger of module design" (Lit. 19.1.4-10): A modul design has many advantages like reduced logistic as well as time-saving and cost saving. But it there are also risks. A special problem are damages by pushing/sliding together for the connection of modules.Usually the separation of the modules takes place in the area of a main bearing. For this normally the inner race (ring) will be pulled off, that means separated with the cage and the rollers from the outer race (ring). The sliding on of the exchange modul often is severe disturbed by bad access/visibility or narrow gaps to the joining area. The heavier the modules which must joined are, the higher is the danger of an unnoticed forced damage of the joining area. From experience theseare races/surfaces, cages and rolling elements/rollers of roller bearings (Fig. "Critical joining of big engine modules 2" and volume1, Ill. 4.3-6), labyrinths (tips, soft or/and brittle/hard rub coatings). Also filigree fitting diameters (bores, hubs) as found on thin walles sheat metal casings (Ill, 19.1.4-6.2, lower sketch respectively example 191.4-1), are included. This is especially true for the fan module of large aeroengines (sketch above left).

Even an apparently slight damage of a main bearing which can be seen as a dent from the rollers on the inner race, can lead in a very short operation time after the assembly to a catastrophic bearingfailure.

The damage of the sensitive labyrinth tips and/or break-outs in the rub coating (sketch low, right) can trigger a catastrophic failure. Even with a melting through neighbouring schafts and the following overspeed of the concerned turbine must be reckoned.

Figure "Parting planes of modules" (Lit. 19.1.4-10): In the parting plane of the modules are potential damage endangered components/part zones located. The shown examples are based on experiences and incidents. The sketch obove left shows a military three shaft aeroengine with pronounced module design. In the area of the marked parting plane resides the bearing chamber with several concentric and/or axial shifted labyrinth seals, which must be simultaneously inserted. The new, that means with labyrinth tips not worn by rub cause significant smaller gaps/clearances than they were at the preceded disassembly. That may increase the problem under not optimal assembly conditions in military use (volume 2, Ill. 7.2.2-6).

The upper right sketch shows the case of a serious bearing failure with the breakdown of the aeroengine from a big airliner during strar (Fig. "Advantage and danger of module design", Fig. "Critical joining of big engine modules 2" and volume 1, Ill. 4.3-6).

The lower sketch refers to the example 19.1.4-1. In this case it came during the assembly of the low pressure turbine (LPT) module to the damge of turbine guide vanes with extensive secondary damages.

Figure "Borescope inspection" (Lit. 19.1.4-2 and Lit. 19.1.4-12): The visual inspection and monitoring of the component/part condition (low left detail ) when they are assembled, especially on wing, is of high importance for the safety (chapter 25.2.2.1). Those controls are performed during maintenance by means of fibre-optics (boreskope) through suitable positioned holes in the casings (upper sketch). The designer defines on basis of relevant experience and the available borescope types number and position of the borescope holes (upper sketch).

Figure "Health risky materials" (Lit. 19.1.4-1, Lit. 19.1.4-14 and Lit. 19.1.4-16): This examples from practice for engine components respectively its materials with a risky health hazard (Fig. "Processes with poisonous materials") can still be expected today only in elder aeroengine types. A particular danger exists during repair or maintenance work when dust/particles are procuced (e.g., grinding, Fig. "Risks of health hazardous materials"). Alternative materials for asbestos based interlayers of clamps for pipes (sketch above left) or isolating (thermal, electric) fixing/mounting elements proved in some cases as problematic. So the tribology/sliding features and the formability (brittleness) of the alternative did not suitable correlate with the original.

Besides bolts/screws (sketch above right) also many other parts like compressor casings and rotor discs are fabricated of cadmium-plated corrosion sensitive steels. At raised operation temperatures a Ni-intermediate layer (Ni-Cd-plating) must avoid a deterioration during operation (Fig. "Casing behaviour like apressure vessel" and Fig. "Danger of LME and LMIE by galvanic platings") of the base material by diffusion of Cd (LMW, SMIE). Also here substitute platings (e.g., zinc coating) can not always offer corrosion protection with equal good tribilogic/sliding features.

The use of dispersion hardened cast magnesium alloys with radioactive thorium oxide was commen in casings of accessory gears (sketch down left) and in the front compressor area of former especially military aeroengine types. Used was the low specific weight of a light alloy with high creep strength.

Compressor inlet guide vanes (sketch below right) must be hollow to guide inside warm anti-icing air. For this offer themselves good heat transfering copper-beryllium-alloys (CuBe) with high strength. Those materials are also used for springy components/parts in control systems/governors (e.g., pressure cells).

It can also be, that the poisonousness of a material is traced back to contaminants. For examle this is referred to Ni-grafite rub in coatings.

Figure "Risks of health hazardous materials" (Lit. 19.1.4-1 and Lit. 19.1.4-13, -14, -15, -16): This picture shows a summary of potential health hazards. In aeroengines can be many materials which are a risky to health (Fig. "Health risky materials"). Beyond, this is also true for processes/work of maintenance and overhaul (Fig. "Processes with poisonous materials").

Figure "Processes with poisonous materials" (Lit. 19.1.4-13, -14, -15, -16): Materials can come in different manners health dangerous into the human organism. Mostly by breathing, but also contact with the skin or the mouth are possibilities. The endangerment can, according to the formation, come from vapours, dust, droplets respectively aerosols, liquids and solid matter. Not always the practitioner on-site recognises the potential danger of the work conditions. Selected examples schall sesibilise against the risks.

The consequences for the health can, according to the medium, become immediately respectively short-term noticeable (e.g., inhaled gases of disssolvers) by indisposition in different steps depending from the medium. Medium term degradations of inner organs like liver and kidneys can arise. From radioactivity rather damages over a long time are to be expected. In an extreme case it comes to disablemnt up to death.

Generally apply at the workplace specified notes as well as preventive measures like exhaustions and ventilation or separated rooms/chambers. For personnel protection serve equipments like face mask respectively respiration filter or gloves (Lit 19.1.4-1).

Welding or brazing heats metals so high, that it evaporates. Those vapours can be inhaled when there is unsufficient protection (sketch above left). That is especially serious, when the pars additionally have easy evaporating platings/coatings like cadmium, lead or zinc, which should have been before removed. In brazing material also can be extreme poisonous components like arsenic. Radioaktive thorium oxide can be released as well during welding of cast dispersion hardened magnesium alloys (Fig. "Health risky materials") as also as a component from the tungsten electrodes of the TIG welding.

Cutting processes, especially grinding, sawing and milling with high speed hand operated tools (sketch obove right), can bring dust into the inhaled air. Especially alarming is the development of radioactive dust from Mg-Th-alloys (Fig. "Health risky materials") and tungsten welding electrodes. Those deposits can over a long period of time deteriorate the lung. Are the relatively brittle dispersion hardened cast Mg-alloys broken, e.g., at the materials testing of samples during goods receipt, dust clouds form (visible under favourable light incidence) around the fracture, which can be inhaled by the testing personnel. Also at the formation of poisonous dust like from beryllium alloys and cadmium platings especial attention is to devote.

During thermal spraying develop because of the extremely high process temperatures of the heating (e.g., electric arc) respectively of the carrying gas, metal vapours from the spraying powder. Recochetting particles/solidified drops form a health critical cloud of particles around the process. For example, to those belong also seemingly harmless nickel oxides, which are suspected to trigger long timd deteriorations.

Abrasive blasting and shot peening can form dusts by metal removal. Not only the materials of the parts itself are to consider. Also possible auxilary material like masking tapes from lead are problematic. Are heavy oxidised hot parts (e.g., from long operation in the repair shop) of Ni-alloys before a crack inspection (e.g., penetrant inspection) abrasive blasted, it must be reckoned with extra much Ni oxid dust. This demands a suitable arragement of the workplace. During the feeding of such a facility respectively withdrawal of the parts must be attended at dispersed dust. In this case a suitable protection mut be used.

To position complex shaped parts safe and exact for a chipping process, they are cast in a low melting alloy (sketch middle right). Those alloys can contain problematic metals like lead and bismuth. During melting vapours are to expect, inhaled they can trigger medium term organic damages. Removing the parts after chipping by breaking the brittle low melting alloy it must kept in mind, that forming dust is not inhaled. Such a dust development often is only to see in suitable light incidence.

The contact of poisonous metals with the skin (e.g., cadmium) and/or its chemical compounds (e.g., watersoluble salts) can through the bloodstream act as a health hazard.

Danger can also come from spray processes. For example the carrier gases/liquids represent a risk. But also the actiove agent itself may be not harmless. For example a spray may not be toxic to act deteriorating. Also the blocking of the lung over a long time with media like PTFE (separating agent) must be considered. Basicly the cans should bear labels with all necessary warnings and safety notes, which must be absolutely obeyed.

Health imperilling gases and mists are also to expect during many galvanic and chemical processes and baths. This must be accomodated with suitable facilities, corresponding with the relevant instructions. Especially risky processes like cadmium plating are as far as possible forbidden.
This counts also for degreasing stations on basis of chlorocarbons („Tri”, „Per“), which besides the deterioration of organs, can also create an addiction (dependence). Other processes like the galvanic chromium plating, where critical chromium oxides are to expect, will be as far as possible replaced by less risky processes (e.g., HCOF).