Aeroengine Safety

Fig. "Optimal use of human sensations": Frequently the multiple possibilities for monitoring and testing which sole sense organs without aids, enable are not aware (Ill. 25.2.2-1 and Fig. "Overload crack and fracture seen by an observer"). Naturally experience and expertise are crucial for the success. In the following, typical perceptions and its possible conclusion should be shown at examples (see also Fig. "Much can be seen during passing by an engine").

Visual: Damages can have component specific very different features. So scratches and notches, especially at pipe lines, are triggers of vibration/fatigue cracks of importance (Fig. "Risk of titanium pipe lines 1", -7.2). Fretting (Fig. "Dangerous rubbing contacts of components") is extreme dangerous especially at tubes of titanium alloys. Deformations like bulges can promote fatigue cracks (Fig. "Vibration of dinted pipe lines") with notch effect and tensioning.
Deformations, especially unusual annealing colours or locally oxidation at hot parts or at compressor casings are important features. So at casings from titanium alloys, as consequence of fierce inner rubbing processes, develop at the outside dull grey oxidized zones (volume 2, Ill. 7.1.3-20). These must be checked for material embrittlement.
Signs like soot streamers for escaping gas (e.g., at a combustiomn chamber casing), let suggest at crack formation (volume 3, Ill. 11.2.2.2-9). In this case, in short time the fracture of the component must be absolutely expected. During high inner pressure it can come to a bulging in the overheated region (Fig. "Much can be seen during passing by an engine"). Such a situation exists, if flames exit from the combustion chamber ( Ill. 21.2.1-9, volume 2, Ill. 9.3-3 and Ill. 9.3-5) or hot gas from the turbine impinges at the casing.
Control for metal splashes as cause of violent rubbing processes or fragment formation in the hot gas stream (volume 1, Ill. 4.1-6).
Unusual many sparks in the exhaust gas during ground runs can give a hint at deteriorating rubbing processes. Suspicious deviations in the outer appearance can give hints at unapproved parts (suspected unapproved parts = SUPs, Fig. "Visible features of SUPs" and Fig. "Suspicious packing indicating at SUP").

Touching: Roughness in an unusual extent and arrangement is alarming (volume 4, Ill. 16.2.2.6-13). Especially attention must be payed during the assembly at sticking slugs (melt beads), at disks as well as at guide vanes and blades. This can be weld splashes (e.g., electrom beam welding, volume 4, Ill. 16.2.2.6-6). They have a high deteriorating potential, which crucial decreases the cyclic lifetime. At accessibility the folliwing problems can be observed

• Foreign object damages/impacts.
• Burr formation (e,g., during erosion, volume 1, Ill. 5.3.2-2 or at bores, volume 4, Ill. 16.2.2.2-4).
• Touching of locally loosened coatings (e.g., damping foil, volume 1, Ill. 5.3.2-6).

Feature of escaping media (Fig. "Contaminated Hydraulic fluid".1) is humidity. This is a sign for a leakge, which e.g., during crack formation or failing of a flange connection, can lead to a catastrophic failure. Also temporary escaped or spilled media can be concerned.

• Hydraulic fluid/oil can trigger cracks (Fig. "Contaminated Hydraulic fluid") and/or affect thermal and electrical insulations in its effect.
• Deteriorating electrical contacts (Ill. 19.2.1-1.1, Fig. "Malfunction of electrical plug" and Fig. "Contamination in electric connectors").

A speciality are sensible gas jets (Fig. "Dangerous situations by air duct leaks"). Such appearances can be identified during a test run. These point at a leak like crack formation, a failing seal or a flange connection of an air duct.
Are vibrations unusually intense, this could hint at a failure (e.g., bearing, blade, pumps). These are also a danger for fatigue of components like pipe lines (Fig. "Resonance vibration caused by P-clamps").
Also a first temperature evaluation in the nonhazardous temperature region is possible (hear radiation, vaporisation). So the necessary cooling for the use of a borescope can be checked (Fig. "Borescope versions and applications").

Acoustically: Unusual noises can give hints at deteriorations like crack formation (Fig. "Hearing cracks and failures") or seizing/galling processes during the assembly (Fig. "Hearing cracks and failures"). Appear during the turning of the rotor by hand unusual noises, these ca be in connection with rubbing, indicating a problem (e.g., blading, labyrinths). Noises striking during operation, in frequency (sound) and/or intensity can also announce problems. To these belong gear failures.

Smell can announce failures and even suggest at its causes. So the typical smell of overheated electrical insulations is known - „it smells of ampere”. Also the use of aeroengine specific prohibited auxilary materials can reveal with smell. To these belong clorine containing degreasing fluids (perchloroethylene, trichloroethylene) or rust remover. Also burned aeroengine oil or hydraulic fluid/oil can aggressive attract attention. An unusual intense smell of fuel should also be alarming.

Fig. "Support of sensations": The practitioner knows absolutely aids, which can support him with his sensations. Here two typical examples are presented:

Screwdriver (sketch left): Applicable for the identification of noises during turning of a rotor by hand. Especially useful is a model with end-to-end blade. This can transfer the structure-borne sound of a vibrating wall (e.g., pipe, casing) directly to the touching ear. With this, the interresting sound will be more intense and clearer, at expense from sound disturbances from the outside (airborne sound).

A coin for the evaluation of a vibration can only be used on flat/horizantal faces, like at a test rig. The coin will be carefully vertical placed and its behaviour at the vibrating plane evaluated (falling, movements). Naturally, this observation can be only evaluated in comparison with the normal behaviour. Natually it must be guaranteed that the coin will not remain as a foreign object.

Fig. "Overload crack and fracture seen by an observer": Features at components caused by overload in many cases can be identified from an experienced expert without aids or testing techniques (see to this also Fig. "Optimal use of human sensations").

Plastically deformation: Concerned is a lasting deformation. These can form in different ways, depending from material, component geometry, load type and load direction. Possible are lenthening, or compression e.g., of a bolt. Yieldings and internal stresses are shown with warpage and buckling. Contraction/necking: Plastical elongation is connected with neckingist. Compression leads to a locally thickening. Frequently the part also buckles/collapses. Examples are overloaded actuator rods and actuators of variable compressor guide vanes.. Plastical deformations occur at notches and scratches, e.g., by impact of foreign objects or improperu handling.
Sufficient high unbalances, caused by a plastic deformation of rotor components, get noticeable by vibrations. To this belongs the plastic lenthening/yielding by the creep of bolts at rotor flanges in the hot part. Also many other hot parts like blades, underlie creep deformations (volume 3, Ill. 12.5-10). Do the bolts at pipe line flanges lengthen, a markedly leak can develop.

Cracks are mostly difficult to identify without aids. However, they change component properties markedly, if the size is sufficient. To this belons damping (dull tone, volume 4, Ill. 16.2.1.7-10) and elasticity, respectively matural frequency (unusual tone during plucking). Also indirect features like leaks can reveal cracks. However, the probability of such a large crack is rather low (besides at thermal fatigue). Probably a fracture will occure before.

Fractures become noticeable frequently by influencing the operation. Do rotor components like disks and rings fracture; the exit of the fragment (uncontained) must be expected (volume 2, Ill. 8.1-1.2). This leaves corresponding features like puncture holes.

Fig. "Borescope versions and applications" (Lit. 25.2.2.1-2 up to -7): A borescope consists of objektive lens, light guiding arrangement and ocular. Primarily two borescope types are used. Its specific advantages determine the application. Besides the observation through an ocular, frequently the additional possibility exists, to view the picture at a television screen. This enables further viewers a discussion and evaluation. Additionally a conventional (photographic) or digital documentation is possible.

Rigid borescope (upper frame): It consists of a mantle which contains the lens system. Is the objective lens directed forward, we speak about a `direct arrangement'. Is a narrow angle of view (Fig. "Borescope check of turbine blades") of 10° choosen,we get a strong magnification with only a low depth of sharpness. Larger angles permit at a larger depth of sharpness only a lower magnification. As normal count angles between 50° up to 80°. Wide angle objectives have up to 90° angle of view. With additional supplements and optional equipment, special demands like a large distance to the objective (`work distance') can be considered.
Angular positioned objective lenses enable a viewing angle of 90°-180°. Naturally the selection of the borescope must consider the suitable viewing angle the proper orientation of the objective.
There are borescopes with jacket tubes of different diameter. In the diameter range of 1,7 mm up to 14 mm, 5, 8 and 10 mm are the standard. The length can be, depending from the requirement, some centimeters up to several meters. For the selection attention must be payed at spatial restrictions. They must not hinder the manipulation and the accessibility to the ocular.

At repaired borescopes an exact orientation of the objective can be problematic, if the angle or view deviates from the condition as new. This affects e.g. in an instruction, the specified orientation of the borescope.
The exact knowledge of the magnification is requirement to determine dimensions (e.g., gap width or crack positions) sufficient exact. For this, data (diagrams/charts), should be attached to the borescope in the application description.

Flexible borescope (fiberscope, middle frame): These devices have a flexible optical fibre from the objective to the ocular. This enables the sight at a otherwise hidden part. The diameter of the light guide lies between 4 and 12 mm, with length between several centimeters and some meters. In flexible borescopes the `image guide' and this of the illumination light, which both consist of glass fibres, are separate. However they differ in properties and functions. Tereby exist producer specific differences.
A light guide, which only is used for illumination or in pyrometers (Fig. "Pyrometer"), consists with 5 mm diameter of about 1000 end-to-end glass fibres. Every with 1/3 the thickness of a human hair.

Much more complex is an optical `image guide'. It consists of much thinner fibres. Its length must coincide to each other at objective and ocular. Such a guide consists, depending from the image resolution, out of 20 000 up to 100 000 fibres. The quality of the image/quality depends crucial from the exact fibre position and the failures (fractures and damages) of the fibres.
The objektiv can be adjusted from the region of the ocular with setting screws by fibres. Additionally at the ocular, optical properties (focal distance, eye dioptres) can be adjusted. Anyway, the user should let check his eyes periodically.
The flexibility of the image guide/light guide has also disadvantages. At a large free length, a support may be necessary. In such cases, special application specific guiding tubes are used (sketch below right). These offer simultaneously a protection against damages of the sensitive guide. This will be slided through the first inserted tube.

Problems: The function efficiency of every borescope is a requirement for the full use of its potential. To guarantee this basic rules must be kept.

• Before them first use the instructions of the directions for use must be understood.
• Checking of the condition from light guide and image guide. Suitable cleaning of the lenses from ocular and objective from dust and depositions. For this, recommended media/cleaning agents must be used. For example, grease can change unnoticed the optical properties (focal distance).
• Don't store the borescope at extremel temperatures (hot, cold). So aging and cracking can be prevented.
  The usual upper temperature limit lays below 100°C. A borescope must not be inserted before the temperature in the operating area does not certainly fall below this value. This suitable temperature is usually reached, if the direct wall can be touched with the bare hand for about 30 seconds.
• No continued use of a borescope, whose front lense is damaged. Otherwise the failure can enlarge rapidly.
• Borescopes must be at once after use put back in the original storage and transported with this. Otherwise, from experience, the increased danger by dropping or stepping on it exists.
• Don't expose borescopes X-rays or gamma radiation.Glass fibre bundles of flexible borescopes are especially susceptible.
• The flexible side of the objective may be only bended in the specified limits.
• During the extraction, the tip must be supported/held. So a damage through drop down can be prevented.
• No forced insertion. No too narrow bending radius of the guide.
• You must always know, where the tip of the objective resides. For example, rotor blades can shear off a too far inserted borescope.
• Checking of the guide for damages. A damaged tip deflection/adjustment can lead to get caught, with high damage costs.
• A borescope must not be pulled out with a bended tip.
• Don't try to remove an, at a borescope fix connected illumination guide. This is only possible at special versions.

It is interresting, that from the point of view of the job safety, the source of light can be problematic.

• The light source can ignite flamable vapours, fluids (fuel) or dusts. Therefore it must be postioned safe above the ground.
• Insulation failures/danages at the electrical power supply can have dangerous consequences. Also during the exchange of the lamp, the power supply mus be switched off, to preclude an electrical shock. Usually the lamps (e.g., xenon lamps) are very hot, therefore a sufficient cool down time must be kept.
• No contact with water (splash water).

Fig. "Borescope check of turbine blades" (Lit. 25.2.2.1-9): This example gives an impression of the necessary production engineering for the borescopy of a turbine blading. Thereby the certainty of the indication and the effectivity (expenditure of time). Crucial is the size and the orientation of the „angle of view“. It corresponds the opening angle of the objective. It can be seen, that for the optimization some planning, prework and adjusting work is required. Different adjusted objective configurations are to be applied. These comply with the viewing angle (direction of the objective to the investigated surface), the distance to the investigated surface and the necessary magnification. This influences the selection of the objectives (wide angle, narrow or normal angle, Fig. "Borescope versions and applications").
Naturally the knowledge of the appearance and the position of the failures to be expected is a requirement.

Fig. "Borescope inspection of turbine blading" (Lit. 25.2.2.1-8): Application example of a flexible borescope (Fig. "Borescope versions and applications"). Concerned is the condition monitoring of single components from a fighter engine. In the shown case, the turbine guide vanes (sketch above) in front of the last turbine stage, are checked from both sides. This happens from the exit and by means of a borescope opening in the combustion chamber, i.e. in front of the turbine (sketch in the middle). For this the tip of the fiber optic is equipped with a retractable hook from aluminium (probably that the blades are not damaged, sketch below right). This hook could be hinged at the suitable location of the blade edge, here the trailing edge, during entering from the combustion chamber (sketch below left). With this, 180° of the blade could be controlled. This procedure was carried out at all blades at the circumfrence. It was repeated at every blade. For this the borescope had to be unhinged and pulled back. The rotor was turned corresponding further with the compressor.

In this manner it was possible to identify deteriorations and to document. These data could be compared of the next inspection and so the chronological failure progression could be evaluated. With this the requirement for optimal logistics with an acceptable failure risk was created.

Ill. 25.2.2.2-7 (Lit. 25.2.2.1-14): This illustration shows typical pictures from inside of a compressor, like the inspector can see it in the borescope.

“A”: Radial fatigue crack in guide vanes and rotor blades without shroud.Cause are high frequency vibrations of a higher mode (“lyramode”, volume 2, Ill. 7.1.3-4; and volume 3, Ill. 12.6.3.1-6).

“B”: Break outs at soft run in coatings in the compressor casing (violume 2, Ill. 7.1.3-3), face to face with the tips of the compressor rotor blades. Probably some of these failures through high frequency vibrations of the casing in connection with the “blade passing frequency”.

„C”: Erosion of the softer rub in coatings at the casing side (e.g., Ni-grafite-spray coating) above the tips of the rotor blades (volume 1, Ill. 5.3.2-10 and volume 2, Ill. 7.1.3-3)

“D”: Foreign object damage (FOD) in the leading edge of a compressor blade (volume 1, Ill. 5.2.1.1-6 and Ill. 5.2.1.1-11).

“E”: Vibration fatigue crack “E1” in the guide vane, parallel to the edges as a result of torsion vibrations (volume 3, Ill. 12.6.3.1-6).

“E2”: Fatiguem crack, triggered by bending vibrations of the edge of the blade.

„F“: Erosion in the region of the blade leding edge (volume 1, Ill. 5.3.2-4).

Fig. "Borescope findings at turbine blades and vanes" (Lit. 25.2.2.1-10): Shown are some typical pictures from high pressure turbine blades, as the inspector sees them in the borescope.

“A”: Local, rather superficial oxidation failure. Also occurs in the region of a component specific hot spot in connection with hot gas corrosion. Such a “hot spot” develops e.g., there, where a protcting cooling air veil does not act sufficiently. The oxidation inhibiting diffusion layer is already consumed, the base material passed through to the surface (volume 3, Ill. 11.2.3.1-9 and Ill. 11.2.3.2-1).

“B”: Typical thermal fatigue crack with decelerated crack growth (volume 3, Ill. 11.2.3.2-7 and Ill. 12.6.2-4) in the transition to the outer shroud of a turbine blade.

“C”: Impact at a rotorblade through an internal foreign object (own object damage = OOD). The position at the suction side of the leading edge is typical for the turbine. Such foreign objects in the high pressure turbine are for example coke particles from the combustion chamber (“carbon impact”) or broken out ceramic particles from thermal barrier coatings (volume 1, Ill. 5.2.1.1-12).

“D”: `Burned' leading edge (volume 3, Ill. 11.2.3.1-10) in the region of the tip from a shroudless turbine rotorblade. A blocking of the cooling air channels/holes can cause the overtemperature. This will do a blocked dust removal opening/bore (volume 3, Ill. 11.2.3.3-2 and Ill. 11.2.3.2-2) or a restriction caused by a deformation (OOD, e.g., `carbon impact', , volume 1, Ill. 5.2.1.1-11).

“E”: Heavy oxidation (“burning”) and little thermal fatigue cracks at the leading edge of a turbine blade. This appearance is typical for local overtemperatures. It is also called “orange peel eneffekt” (volume 3, Ill. 11.2.3.1-10 and Ill. 11.2.3.2-7).

“F”: Turbine blade, at which foreign material escapes from the bores of the cooling air veil and melts (volume 1, Ill. 5.3.2-13 and volume 3, Ill. 11.2.3.2-2). Concerned may be labyrinth abrasion of rub in coatings in the compressor.

„G”: This, at the pressure side of a cooled turbine rotor blade in axial direction proceeing dark line, can have different causes. This means also, corresponding different risks.
In a harmless case, coked oil is concerned. This should be easy to remove and so identifiable.
However it is extremely dangerous, when a crack is on hand. Just at cooled turbine blades the crack (thermal fatigue) can develop at the colder side, i.e. inside (!) around the cooling air channels/bores and grow to the outside (volume 3, Ill. 12.6.2-9). If this is the case, the identification is a lucky `chance'. The blade (probably the whole set) must be exchanged at once, because an immediate failure/fracture must be expected.

„H“: Concerned is oxidation in the tip region of a turbine rotorblade. This locally material loss occurs at high pressure turbine blades through the hot gas leakage stream (volume 3, Ill. 11.2.3.2-4).

Fig. "Borescope inspection for failure control" (Lit. 25.2.2.1-11,see also Fig. "Risk potential of brazing repairs"): After the start, during climb a loud bang occurred at the right aeroengine (sketch) of the two engined airliner. The high pressure rotation speed of the aeroengine dropped, at the same time the exgaust gastemperature rose. After this, the airplane landed at a the depart airport.

The aeroengine was removed and inspected at the operator. Thereby the region of the high pressure turbine nozzles (= HPTN) down to the low pressure turbine, showed heavy damages. The high pressure turbine (= HPT) is single staged. The failure started obviously in the HPTN.

Concerned was crack formation by thermal fatigue (volume 3, Ill. 11.2.3.2-7 and Ill. 12.6.2-4). The cracks are positioned at the convex side of the blade (suction side, detail). When a crack separated a blade a big cooling air leak occurred. This lead to an overheating, which `burned' the trailing edge. With this, the HPT rotor blade failed, as result of a HCF vibration fracture. Obviously it was excited in the resonance by the flow disturbance behind the damaged nozzle.

In this aeroengine type, in the past frequently crackes formed at the convex side of the HPTN vane. It came also there to break outs. In such a case, a dangerous flow disturbance must be expected. It can excite vibrations of the turbine rotor blades up to a fracture (Fig. "Borescope inspection interval"). Without such an excitation even parts with faults of 2,5 mm seem not to be endangered. Therefore rotorblades of the HPT, which merely show during the borescope inspection cracks below this limit, will stay in the engine (!).

To minimize the risk, the OEM demanded a repeated borescope inspection of the crack endangeed blade surface. Thereby especially attention for cracks had to be payed. The inspections began, corresponding to a service bulletin (SB), at 3 200 start-stop cycles since new. They had to be repeated corresponding the crack finding after 800, 400 or 100 cycles. This check was omitted with the exchange of the blades against an improved version.

The removed blades underwent a repair process. This closed the cracks with high temperature braze (Fig. "Diffusion brazing" and Fig. "Risk potential of brazing repairs"). Such blades have been proved at three operators. In the current failure case, an correspondent part was concerned.

It came to ambiguities about the intervals of a borescope inspection and the necessity to check the convex vane side. At the operator it was decided without sufficient consultation with the OEM for an interval of 1600 cycles. This was once more repeated, then the failure occurred. This time interval was obviously markedly too long for this repair version. The OEM had rather planned a control interval of merely 800 cycles.

Fig. "Borescope inspection interval" (Lit. 25.2.2.1-12): The investigations of the failed aeroengine showed:

• First the turbine rotor blades of the 1st high presure stage failed (sketch middle right). These fractures proceeded about 25 mm above the root platform cross the blade. Further 32 blades showed cracks at the corresponding location. The laboratory investigation showed in the region of the fracture typical features of overheating. Fractures and cracks could be identified as caused by creep (volume 3, Ill. 12.5-3). They can be traced back at high component temperatures. These have been markedly above the design data and stood in connection with problems of the adjustment of the power lever. High temperatures are especially dangerous, because the turbine blades of this early fan engine (frame middle left) have already during normal operation a very high temperatrure level. Contributory material faults or pecularities have not be found.
• The failure at the rotorblades overloaded the following turbine stator.
• This was excited to extreme vibrations (several times higher than normal). This could be concluded from component tests. So it came ti fatigue fractures in 32 guide vanes at the transition to the inner shroud.
• As result the loose inner part of the stator moved under the gas loads backwards. As consequence the separating inner part of the stator moved backwards under the gas loads.
• It came in contact with the front side of the disk from the 2nd stage of the high pressure turbine. The abrasion process during rubbing (volume 2, Ill. 8.2-21.1, Ill. 8.2-21.2 and Ill. 8.2-22) separated the bladed rim (sketch below right).

This aeroengine type had as one of the first a computer supported monitoring. To identify critical sudden deviations the recorded parameter data have been printed and evaluated one time in weekly. Actually already more as one week before the failure important parameters (exhaust gas temperature, fuel consumption and high pressure rotation speed) have been markedly drifted. This showed an intensifying failure. Unfortunately the evaluation in the acute case took place not before several days after the failure.

The aeroengine also possessed a vibration monitoring. It did not show the supposed markedly vibrations.
Already days before adjustion problems and a rise of the gas temperature obviously hinted at the fracture of HPT rotor blades. However the flight engineer obviously did not succeed, enforce at the management a problem analysis. This showed the subsequent analysis of the operation documents.

Measures: To minimite the risk with help of an early failure identification the following actions have been introduced (see informations in picture below left):

X-ray check of the aeroengines HPT to find stator vanes failuresin time (see also Ill. 25.2.2.2-5).

Improvement of the vibration monitoring.

Visual check by inspection of the eyhaust pipe for splashed metall particles. Obviously thes must be expected, when intense rubbing leads to the fracture of blades or to abrasion.

Fig. "Thermal fatigue cracks in turbine vanes" (Lit. 25.2.2.1-15): The failure investigation of the concerned aeroengine showed:
The turbine stator consists of 11 segments, every with 2 vanes. Many vanes show up to 10 mm long cracks at the leading edge (middle scetch, above left, volume 3, Ill. 12.6.2-4). In the most cases, the cracks proceeded into the inner cooling channels. The cracks gaped to the surface due to oxidation. This is typical for thermal fatigue (volume 3, Ill. 12.2.1-12 and Ill. 12.6.2-10).
A vane was extensive burned through (middle sketch below left). The edges have been heavy oxidized. The position corresponded with the crack formation of the other vanes. Such a failure can be expected, if the crack influences the cooling air flow unacceptable (volume 3, Ill. 11.2.3.2-7). The consequence is an overheating with the existent typical features.
As a result of the flow disturbance (Fig. "Borescope findings at turbine blades and vanes") at the broken out vane a turbine blade of the 1st stage fractured. It failed through bending vibrations in the HCF range (volume 3, Ill. 12.6.1-6). The fracture lays near below the root platform, i.e. above the fir tree root (detail middle left). The fracture surface shows typical features of a fatigue crack (sketch below).

History: The concerned 1st stage turbine wheel had since the assembly about 10 000 operation hours and 13 600 start-stop cycles. During this time, all associated turbine blades have been exchanged. The in front positioned turbine guide vanes/nozzles are not lifetime limited by the OEM. Therefore for these no operation documentation was conducted. As „on condition” parts they merely undergo a periodical inspection. An exchange takes place, when the crack formation or the metal loss lay above the permissibility.

Comment: From the available documents the type of periodical inspection does not emerge. Probably borescope inspections are concerned. This procedure offers itself for the failure mechanism of thermal fatigue. Usually here delays, at least in the initial phase, the crack growth at turbine guide vanes. This is the case if the crack adds to a relief (volume 3, Ill. 12.2-23 and Ill. 16.6.2-12). Requirement is, that additional loads like gas bending forces don't already early overload the cross section, weakened by a crack. In the case on hand, the wedge shaped widening by oxidation of the crack shows, that a sufficiently slow growing crack is concerned. This can be monitored with a borescope inspection in suitable time periods.

Fig. "Abusing a borescope" (Lit. 25.2.2.1-13): In the existing case, in an airworthiness directive (= AD), a borescope inspection of the vent tubes is demanded. Concerned are bearing chambers in the region intermediate pressure and high pressure turbine region (three shaft aeroengine, sketch above). The pipe lines can clog with coke and so trigger an oil fire (volume 2, Ill. 9.2-2). With this, the danger of a shaft fracture with the exit of fragments exists (uncontained, volume 2, Ill. 9.2-9). To identify this in time, a borescope inspection is suitable. For this the borescope is as followig inserted into the pipe line.

• Insertion of a 8 mm diameter flexible boroscope (Fig. "Borescope versions and applications", sketch below). It is pushed through the whole tubelength up to the bearing chamber. Is this possible, the inspection must be repeated at the latest after 6400 operation hours or within 1600 start-stop cycles.
• If this is not possible with a 8 mm borescope because of the coke deposits, a 6 mm borescope should be used. Does this also fail, repeated inspections in scheduled intervals (all 1600 operation hours or 400 start-stop cycles) must be carried out during service.
• Can the 6 mm borescope not be pushed through, the pipe line must be suitable cleaned or exchanged. This procedure is described in a separate AD.

Comment: Obviously here the possibility of an optical evaluation is less used. Rather concerned is a mechanical inspection of the continuity.