Aeroengine Safety

Figure "Parts effected by erosion": Many different engine components can be directly or indirectly damaged through erosive particles:

“E1” fan and LPC blading: Roughening through material removal and/or deposits (fouling), changes in blade profiles, increased tip clearance, blade fractures caused by dynamic fatigue fractures originating in erosion notches.

“E2” housing/casing coatings in the fan and LPC: The erosion material removal causes increased tip clearances to the rotor as well as steps and roughness in the path of the gas flow.

“E3” HPC blading (Fig. "Typical erosion on compressor blades"): see “E1”.

“E4” HPC housing coatings ( Fig. "Typical erosion on compressor blades"): see “E2”.

“E5” spacer coatings (Fig. "Singularities in the gas flow"): Erosion of the relatively hard spacer coatings increases the tip clearances between the rotor and compressor stator blades with no inner shroud. Additionally, the roughness of this area of the hub contour can be increased.

“E6” injection system: Erosion of the injection nozzles through coke formation in overheated fuel. This can result in overheating in the combustion chamber area, even including flames escaping from the combustion chamber housing/casing.

“E7” turbine blading (Fig. "Erosion of soft coatings"): Erosion and increased oxidation especially at the leading blade edges caused by dust, coke from the combustion chamber, and spalled ceramic thermal barriers.
Blocking of cooling air ducts and cooling air film bores, which can cause local overheating.
Chemical reactions and erosion material removal in ceramic thermal barriers.
If dust collects in a fir-tree joint, it can unallowably alter the dynamic behavior of the blades (Ref. 5.3.2-1).

“E8” bearing damage (Fig. "Axial face seals"): Erosion in bearing casings and gas ducts caused by circumferentially-traveling trapped particles of ingested dust and labyrinth material wear products. Weakening of wall cross-sections, even causing the walls to fail.

“E9” labyrinths (Fig. "Axial face seals"): Erosion in labyrinths, especially on softer abradable coatings.

“E10” air extraction ducts and air ducts (Fig. "Volcanic ash"): Erosion of walls, especially in the outer area of pipe bends.

“E11” Variable vane adjustment: Dust in the guide vane bearings can cause jamming and wear.

“E12” blade root contact surfaces: In rotor blades and guide vanes, an abrasive process can increase fretting wear due to dust that has penetrated into these areas.
Changes in the friction relationships can alter the dynamic behavior of the blading (Ref. 5.3.2-1).

“E13” Elastomeres: Concerned are rub in coatings from filled silicone rubber. This is used in the front region of the compressor (fan, booster, LPC) at interstage seals. This coating usually is located on the inner rings of the stators opposite the labyrinth tips on the rotor. There is the suspicion, that traces of wear on these coatings are in connection with ice formation. The exact mechanism seams not yet clear. However it is known from overhaul processes, that particles from water ice can be used as blasting abrasive to remove soft contaminations (Ref. 5.3.2-25). In this case however the provider calls it a non abrasive process. An abrasion effect however may merely depend from the process parameters. This is also true if the ice contains abrasive particles (e.g., dust) as contamination.

Figure "Erosion damage on suction an pressure side": Changes to the blade profile at the micro-level, especially the leading edge, can affect the aerodynamic behavior of the blade considerably. The impact of sand grains causes notches and burrs, as well as increased roughness primarily on the pressure side of the blade. On the suction side, especially, burrs promote local flow stalls (bottom diagram).The consequences include a decrease in compressor efficiency and also increased dynamic loads on the individual blades. Combined with the notch effect of the impact notches, this increases the risk of fatigue fractures. Clear signs of high dynamic loads (extrusions, see Fig. "Coating damage I") in erosion notches were verified through use of scanning electron microscopes (SEM).

Figure "Behavior of shaft-power engines" (Ref. 5.3.2-2): The erosion-induced changes to the compressor blades, such as increased roughness and/or changes to the blade profile at the micro- and macro-levels, result in a considerable worsening of the compressor efficiency. This, in turn, affects the operating behavior of the engine and the damage frequency and life spans of other components, especially the hot parts. In order to maintain the same engine performance, the efficiency decrease must be compensated by increasing the gas temperatures. This raises the temperatures of the hot parts (combustion chamber and turbine). In addition, the decreased play to the surge limit can cause it to be crossed, leading to extreme short-term overheating in the turbine with extensive consequential damages.

Figure "Typical erosion on compressor blades": The life span of a helicopter engine depends largely on the presence of a dust filter in front of the engine inlet and the separation effectiveness of the filter (bottom right diagram, Ref. 5.3.2-18). It can be seen that only filter systems with high separation rates permit desirable engine life spans.
The top left diagram is a schematic depiction of the typical erosion symptoms in a rear compressor stage. The centrifugal effect of the blading in the air flow concentrates the particles near the casing wall. This makes the erosion effects more pronounced in radially farther outward areas. This especially weakens the root areas of stator blades (“1”), promoting dynamic fatigue fractures in case of flexure. However, the clearance between the rotor and blades without inner shrouds is not affected, minimizing the efficiency decrease in this part of the compressor.
The rotor blades, on the other hand, are worn primarily at the tips (“2”), where the clearance increases are spread over a larger circumference, affecting the compressor efficiency more than the stator blade erosion.
In addition, increased erosion of the relatively soft abradable coatings on the inside of the casing (“3”) further increases the clearance at the rotor blade tips.
With stator blades with inner shrouds and soft abradable labyrinth coatings facing the rotor (“4”), there is a danger of clearance increases due to erosion, although the erosion stress is lower than near the rotor blade tips (lower dust concentration near the hub).
The erosion of the blade profile, with roughening of the leading edge and burr formation on the suction side (“5”), promotes local flow stalls and therefore also compressor stalls.
The erosion behavior (erosion sensitivity) varies considerably between different compressor designs (see diagram bottom left, Ref. 5.3.2-2). Axial compressors (“A”) show the greatest sensitivity. Combined axial-radial compressors (“B”), such as are common in helicopter engines (Fig. "Erosion sensitivity of different compressor designs"), perform better. Side-mounted inlets can result in the blades being damaged especially at the highly dynamically stressed transition to the root platform (sketch above right, Example "Resonance"). Machines with pure radial compressors show the most robust behavior (“C”). This positive performance can be further improved through erosion protection on disks and diffusers (“D”). The horizontal line “E” shows the operating limits of the engine according to regulation AS/AV E-85 930.

Figure "Profile change due to erosion" (Ref. 5.3.2-4): “Rollover” as an extreme profile change. The leading edge of a helicopter engine compressor rotor blade has been rolled up by sand erosion.

Figure "Delamination of coatings": Coatings on engine parts, especially blades, can locally erode or be damaged by foreign objects so that the flow, combined with blade vibrations, cause the coating to delaminate. This is especially the case with ductile coatings such as organic erosion protection coatings (top diagram) or metallic foils such as damping foils (bottom diagrams), in which the bond with the base material is considerably weaker than the coating strength. This kind of delamination or rolling-up of the coating causes considerably more flow interference than superficial material removal or flaking. The delamination of larger coating areas also poses a threat of damage or blockages in the rear engine area (for example, the cooling ducts of hot parts).

Figure "Blade tip hooking" (Refs. 5.3.2-3, 5.3.2-2, and 5.3.2-17): In blade tips, especially those located in turbulent flow zones such as around areas where air is extracted through bleed valves(Example ""), a typical erosion symptom known as “hooking” can appear in the rotor blade tip area. This causes increased wear of the trailing edge below the corner of the blade.

Figure "Erosion patterns on radial compressor disk" (Ref. 5.3.1-5): Radial compressors are usually particularly erosion resistant (Fig. "Typical erosion on compressor blades"). Depending on the upstream configurations, such as radial or axial inlets and axial or radial compressors, the erosion symptoms on the blading can vary greatly (Fig. "Particle size"). In this case a multiple-stage axial compressor is located upstream. During an erosion test with coarse dust a sudden power decrease of 15% occurred. This was followed by the mechanical failure of several blades.

Figure "Extreme erosion stress during sand storms": This diagram illustrates the technical relationships in the extreme erosion stress mentioned in Example "Sand storms", which can evidently occur during a sand storm. The engines involved were shaft power engines of the 3000 kW class. The blading of the axial compressor in this engine type is made from a high-strength titanium alloy and is relatively filigreed. The involved helicopter is not known to have been equipped with any particle separating devices such as filters. Therefore it can be assumed that the compressor blading experienced considerable profile changes of the blades (similar diagram at bottom left) due to the sand erosion. In this case an unallowable influencing of the engine`s operating behavior and power output was concluded to have occurred.

Figure "Singularities in the gas flow": Engine-specific singularities in the gas flow direction can affect the erosion symptoms and the especially erosion-stressed components in characteristic ways. In engines that have a S-shaped housing/casing ducts (swan neck duct, upper figure - left sketch) between the high-pressure and low-pressure compressors, particles are evidently deflected towards the hub. This can be understood by considering the particle deflection in a radial compressor (see Fig. "Erosion sensitivity of different compressor designs"). The first compressor spacer (upper figure - left sketch) after the swan neck duct is especially erosion stressed. This stress should decrease for the following spacers due to the repeated outward centrifuging of the particles as they travel farther through the compressor. Experience seems to confirm this. The erosion symptoms of the spacers usually show the spacing of the upstream rotor blades in a more or less pronounced manner. The blades maintain their position to the spacer. Experience shows that the particles collect on the pressure sides of the blades.
The sketches on the right side are a schematic depiction of typical erosion symptoms on soft housing coatings (such as Ni/graphite spray coatings). The darker zones are the more heavily eroded areas. Similar to the case with rotor spacers, the erosion pattern corresponds to the upstream blades right side upper sketch left), although in this case the blades are the stator vanes, which do not move relative to the housing coating. Evidently the particles can also travel along the stator vane profiles and become concentrated. This typical damage symptom is usually not very pronounced when the rotor blades run-in heavily (bottom - bottom figure).

Figure "Erosion of soft coatings" (Example "Extreme reduction of lifespan", Ref. 5.3.2-7): The erosion of the usually soft abradable coatings in compressor housings/casings is a common problem in modern engines. The high power concentration at the best efficiency levels and good operating behavior demands, among other characteristics, minimization of the compressor rotor blade tip clearance. This can only be achieved by having the blades run-in when the clearance gap is breached. In this case, the blade tips must not be unallowably damaged. This requires soft coatings. Evidently even a multi-stage fan in a bypass engine is not sufficient for centrifuging all the ingested particles and preventing the core engine from experiencing any erosion stress. It can be assumed that, along with the particle concentration which increases near the housing, housings that become conically smaller along with the gas flow promote the erosion of housing coatings as in the depicted case (middle and bottom diagrams).

Figure "Erosion of turbine blades": Particles from the combustion chamber and other engine zones, located upstream in the engine, can cause serious erosion on the pressure side of turbine stator blades and the suction side of turbine rotor blades (also see Figs. "Influence of impact direction", Erosion of turbine blades" and "Thermal barrier coating I"). Typical particles include soot from the combustion chamber (Example "Improved combustion liner") and particles from crumbling ceramic thermal barriers, rub coatings of the seal segments, or hard armor coatings on blade tips and labyrinths.
Even damage to thin diffusion coatings (typical thickness less than 0.1 mm) can result in oxidation of the base material, promoting damaging erosion which can considerably reduce the life spans of the parts.

Figure "Low coke combustor" (Example "Improved combustion liner"): To minimize the life shortening erosion by coke from the combustion chamber of this helicopter aeroengine, a version of the combustion chamber with less coke formation (low coke combustor) was introduced. This combustion chamber has a new designed effusion cooled inner wall. So there was the hope to double or triple the life time of the blades. At the same time also the visible smoke production should be minimized. The effusion cooling enables a more even temperature distribution without hot spots. With this, the lifetime of the inner wall may be markedly extended.

Figure "Dust particle melting": The material removal from a hot part surface can be highly accelerated by a combination of oxidation and erosion. Brittle oxidation layers are especially erosion sensitive at steep impact angles (Fig. "Influence of angle of attack"), such as those expected on leading edges of blades. The erosion material removal from the protective oxide coatings exposes reactive fresh metal surfaces which oxidize correspondingly heavily.
A similar scenario (Ref. 5.3.2-17) occurs when dust particles are melted in the combustion chamber and then strike and stick (glassing, Ref. 5.3.2-4) to the relatively cold cooled surfaces of the hot parts, especially the blades.
The melted dust can cramp and/or react with the relatively coarse oxide layers, creating a powerful bond. Thus especially the flow through cross section at stators (nozzle) of the high pressure turbine can be clogged, so that during a power increase the aeroengine can easy get into a surge (Ref. 5.3.2-4).
When the engine is shut down, these layers harden into brittle coatings with crack initiation and break off partly or completely along with the protective oxide layers (Ref. 5.3.2-20)..
When engine operation is resumed, the fresh metallic surfaces oxide especially heavily, and the process is repeated when dust again enters the engine.

Figure "Desert environments" (Ref. 5.3.2-8): The described three-shaft engine type (top diagram) of a twin-engine fighter aircraft was used in a desert environment. The ingestion of sand and dust into the engine is a serious problem. Interestingly, the greatest problems do not occur due to erosion in the compressor, but in the hot parts. Sand particles are carried onto the turbine blading by the main gas flow (bottom left diagram). Here the bores for the cooling air film are blocked up by the sand, which has been made doughy by the combustion chamber. Further damage to the cooling air configuration of the hot parts is caused by blocking of the cooling air ducts (bottom right diagram) inside the blading. The sand layer stuck to the outside was removed with a blasting device. A probe with compressed air was inserted through the boroscope opening in front of the blades and blasted the leading edges of the blades with abrasive particles (Al oxide).
This kind of cleaning method is naturally only recommended in extreme cases, since there is a risk of the bores for the cooling air film becoming blocked by the blasting particles.
Newer developed blasting processes (chapter 19.2.6) can avoid this disadvantage.

Figure "Volcanic ash": This diagram is based on a press photo (Ref. 5.3.2-9, Example "Glassy, sticky ash layers"). The light discolorations in the nose area can be seen clearly.
In the photos it seems that it was not so much an erosion-induced roughening or delamination, but rather thick deposits of volcanic ash. This is supported by the windows which, as far as can be seen, are partly covered, and not only blind due to the erosion roughening.

Figure "Danger of volcanoes" (Ref. 5.3.2-15): In some regions in the world (in this example, Alaska) there are many active volcanoes. The dangerous ash clouds can threaten aircraft even at great distances (hundreds of kilometers) and altitudes up to 20,000 meters. In the lower sketch, the cross-section of the engine type from Example "Sulphur-laden ash" shows the areas especially threatened by the ingested ash. In this case glassy depositions of molten dust ('glassing') accumulated at the leading edges of the 1st stage high pressure turbine stator (nozzle, sketch in the middle). This detail shows a nozzle vane with pronounced signs of overheating due to blocking of the cooling air ducts. However these did not occur in the case of the example 5.3.5-5.

Figure "Erosion damage on air duct" (Ref. 5.3.2-15): The walls of this air extraction duct in the compressor eroded at the bend behind the extraction hole during erosion tests on an engine of a middle performance class. The increased erosion attack at the outer radius of the pipe bend is characteristic, since centrifugal effects and the redirection of the flow concentrate the erosion particles in this area.

Figure "Indications of volcano ash damage" (Ref. 5.3.2-26): Shown are changes in civil fan engines of a 4-engined airliner. It flew about 7 minutes through a `veil cloud' (cirrus cloud) contaminated with vulcano ash and gases. Subsequently concerned hot parts of the 4 aeroengines had to be repaired for a cost equivalent of the price from about 150 medium-class cars.

The assessment of the influence requires a comparison standard. This enable previous, documented routine investigations at the same aeroengine and/or sufficient experience.

As measures for the evaluation of the consequences offer itself:

• Borescope inspections of the blading from turbine (“S”) and compressors.
• Microscopic investigations of air filters like used in the air conditioning (“F1” and “F2”) or bleed air.
• Health - monitoring (chapter 25.1) of the aeroengine, e.g. observation of a change of the EGT (see above).
• Oil analysis (“O”) spectrometric type (chapter 22.3.4).