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5.2.2.1 Bird Strike Damage

The common perception that birds are a relatively soft and yielding foreign object does not apply to the impact conditions in an engine due to the high typical impact speeds. A bird with a mass of a few hundred grams can not only seriously damage the compressor blading, but it may even overload solid components such as housing struts.

Statistical data for bird strikes:

The statistics that describe the frequency of bird strikes in engines are heavily scattered, and in some cases even contradict each other.
This is due partly the large differences in the definition of bird strikes; for example, whether this term includes mere bird impact, noticeable damage, damage in need of repair, or even aircraft accidents.

This is further complicated by the fact that various bases are used for comparisons:

Aircraft movement: Takeoff + landing = 2 aircraft movements

Aircraft flight: 1 takeoff + 1 landing, but no indication of the number of engines involved

Engine flight: Each flight of each engine is counted separately (1 takeoff + 1 landing);
Furthermore, in many countries there are either no available data or only rough estimates, especially for military aircraft.

Civilian flight:
There is comprehensive data available from various sources. The following conclusions can be made on basis of this:

  • Birds strike several hundred aircraft per 106 flights.
  • In roughly 3/4 of all cases there is no damage to the aircraft.
  • In about 20% of all bird strikes on aircraft than one or more engines are affected. However, it is very rare that multiple engines are struck simultaneously.
  • Only less than 10 % of all bird strikes on aircraft, i.e. 35% of bird strikes on engines result in engine damage that requires immediate repair.

Simultaneous damage to multiple engines making continuation of flight impossible is an extremely rare occurrence in civilian cargo aircraft. In one case in the mid-1970s, a large, three jet, passenger aircraft was destroyed due to consequential damages resulting from several birds weighing roughly 2.5 kg each being ingested into one of the large-bypass fan engines. In general, after this type of incident, the certification regulations are changed accordingly (made stricter) in order to decrease the probability of similar damage occurring in future aircraft types.

Figure "Influences on the risk of bird strikes" (Ref. 5.2.2.1-1): This statistical analysis is based on military aircraft in the early 1980s. The greatest bird strike risk was in fighter aircraft, followed by large cargo aircraft (top diagram). The relatively large percentage of missions that fighter aircraft fly at low altitudes should increase the probability of bird strikes considerably (see Fig. "Increased risk at low altitudes").
Large cargo aircraft have a large engine intake surface area, which should also be a reason for increased bird strikes.
Since most bird species are active during the daytime, it is natural that the greatest risk of bird strikes was during this time (middle diagram). However, it is interesting that there were also a considerable number of bird strike incidents registered at night.
By far the greatest bird strike risk is around airports (bottom diagram) which is partly due to the low flight altitudes in this area. However, this also means that a large number of birds are evidently present around airports. This is the reason for the extensive effort to remove birds from areas near airports, which continues today with limited success.
Bird strike is relatively rare in helicopters, even though these operate primarily at low altitudes (top diagram). One explanation for this is the relatively low flight speeds, combined with possible deflection of the birds through the angled or vertical rotor wash.

Figure "Increased risk at low altitudes" (Refs. 5.2.2.1-2 and 5.2.2.1-3): The greatest number of bird strike damages occurs at altitudes below 500 meters. This is correlated with the corresponding flight situations, i.e. takeoff, landing, and approach flight (Fig. "Influence of proximity to ground"). The documented cases involving fighter aircraft show a similar distribution. This is due to both the preferred low flight altitudes of birds, as well as the fact that flight missions of modern fighter aircraft are often at low altitudes below radar height, even during cruising flight.

Figure "Probability of significant damage" (Ref. 5.2.2.1-20): This diagram from a statistic (DOT/FAA/CT-91/17) of the U.S. authority FAA betrays a clear connection between bird weight and 'significant' damage at the aeroengine. The definition of the damages is not distinct. Concerned may be damages which make a repair necessary (Fig. "Small bird damage"). Interesting is the markedly increase of the damage risk up to about 2 kg bird mass. Even birds with around 100 g are obviously in the position to cause bigger damages. Thereby it can be supposed, that it's about several birds (flocks, Fig. "Turboprop I") and not about single exemplars.

Figure "Impact forces" (Ref. 5.2.2.1-12): This concerns theoretical estimated impact forces (Fig. "Endangered components"). Considered is a bird impact at the outer skin of an aircraft fuselage. Anyway, at least magnitude and tendencies may also apply for aeroengine components like fan blades, casing struts and nosecone as well as the intake duct.
The forces in the chart will heavy scatter in the real case. This has several reasons. The calculations where based on an impact angle of 90° . With this the real impact angle in the individual case was not considered. An angle of 30° may be more realistic, at least for the fuselage surface. This would besect the stated impact forces. With the introduction of an “bird diameter” (effective diameter) this inaccuracy shall be a little reduced. The forces can be also a multitude higher, depending from the supposed `bird consistence'. Contrary a flexibility of the impacted surface will act diminishing.

Figure "Endangered components" (Ref. 5.2.2.1-18): In this picture will be tried to assign typical, for the bird impact outage endangered aeroengine components, sizes of birds.
Birds around 100g
can already represent a danger. This is especially to expect for flocks. They can lead to changes at/in the `booster' (3) (deformations of blades, cross section reduction) which trigger a surge (Fig. "Influence of proximity to ground"). This is especially risky because bird impacts occur during the start phase at full aeroengine power.
The rotorblades of the 1st stage high pressure compressor (6) after a `swan neck channel' (`S-channel') are from experience also endangered by overload from small birds. These blades are located behind the booster (Fig. "Axial stiffening rib") or at three shaft engines between intermediate pressure compressor and high pressure compressor (Fig. "Sensitivity caused by S-shaped ducts"). Here the danger exists, that bird rests accumulate at the struts in the channel and then, as a bigger mass, act at the filigree rotorblades as low speed impact (Fig. "Low flight speeds").
Even small single birds can destroy sondes/probes (intake temperature, pressure/flow velocity) which extend into the flow channel and `consternate' control units and the cockpit instruments.

Birds up to 1 kg are above the problems with smaller birds already in the position to produce alarming failures with a direct impact. This is also true for the nosecone (rotating or fixed, “1”). Such made of fiber reinforced plastic had to be suitable strengthened (Example "Spinner fragments"). Also big fan blades (2) from a high strength titanium alloy can be already so deformed, that a replacement is necessary (Fig. "Small bird damage"). At smaller aeroengines a blade fracture is absolutely possible.
Also indirect heavy aeroengine failures/damages can be triggered already from birds of this weight class. This is the case if cowls/shrouds of the fuselage or the nacelle respectively the intake channel to the aeroengine are ripped off (Fig. "Intake duct liner", Fig. "Damage at spinner" and Fig. "Indirect threats of bird damage"). The secondary failures are as well heavy mechanical damages of the blading as also vibration overload from compressor surge and/or flow disturbances.

Birds up to 2,5 kg can produce impact forces in the range of 10-tons. With this the danger exists, that already struts in the intake to the front bearing chamber break (Fig. "Canada goose I"). A direct hit at the spinner respectively the low pressure rotor shaft can destroy main bearings. In such a case high rotor unbalances and catastrophic secondary failures must be expected. Occurs an abrupt braking of the rotor by intense rubbing, also in the rear aeroengine area catastrophic failures by overloads like the fracture of casings are possible.
Heavy rubbing processes and/or the fracture of oil flown through casing struts can cause fire in the intake region of the aeroengine (Fig. "Canada goose I"). Thereby obviously also plastic components can be ignited and Al honeycomb structures of the lining from the nacelle can be molten.

Birds more than 2,5 kg let expect the already discussed damages/failures in a far disastrous dimension (Fig. "Canada goose I").

Example "Bird strike during take off" (Ref. 5.2.2.1-4):

Excerpt: “…During rotation on the takeoff roll the right engine of (a two engine transport aircraft), ingested a bird. Several fan blades broke, however there was no fire and the engine continued to run until shut down by the flight crew… Feathers removed from the engine belonged to a bird with …an average weight of 3.8 pounds. The engine performed within the limits of its certification criteria.”

Comments: The bird strike during takeoff is a “classic” bird strike situation (Fig. "Influence of proximity to ground"). The pilot heard a loud bang and felt heavy vibrations. Inspection of the feathers and remains of the bird that were found in the fan, compressor, and combustion chamber showed that it was a duck. The given value is the average weight of that species of duck. With this large bird, the contained failure of fan blades is allowable. The special stressing of the fan rotor blades is to be expected during low speed impacts such as this one (also see Fig. "Canada goose III" and chapter 5.2.2.2).

Example "Spinner fragments" (Ref. 5.2.2.1-5):

Excerpt: “A 28 ounce (about 900g) duck was ingested into a …turbofan engine of a (4 engine aircraft) on takeoff, impacting and breaking the 10 ply fiberglass fan spinner. Downstream engine destruction followed. Engine manufacturer and FAA was aware of a number of previous bird-strike failures with this spinner. New and stronger 20 ply spinners are being phased in; however thin spinners are still in use…“

Comments: This case involves a passenger aircraft with four large fan engines with high bypass ratios. The pilot was able to shut down the engine in a controlled manner and land the aircraft safely. The damage investigation showed that the engine damage had been caused by fragments of the spinner (rotating nose cone) and fan blades.
At the concerned aircraft type obviously spinners have shown in an early version during bird strike as not sufficient against the high impact forces (Fig. "Impact forces"). After this the number of glass fiber layers was doubled.

Example "Dangerous takeoff abortion" (Ref. 5.2.2.1-6):

Excerpt: “During the takeoff roll after V1 was called by the officer, a bird was ingested in the left engine.
…according to the captain, he observed an object flash past the airplane. The first officer reported he saw a bird on the right of the nose of the airplane. Shortly afterwards he heard a loud `explosion'…The captain reported that the explosion, louder than any compressor stall he had ever experienced, created a shudder in the airplane….the airplane was subsequently stopped approximately 750 feet off the departure end of runway…“

Comments: This was a smaller twin jet passenger aircraft with two older fan engines with relatively low bypass ratio. The bird was a hawk that weighed roughly 120g. Evidently the damage to the engine was not as serious as the noise and vibrations of the triggered surge (Ill. 11.2.1.1-7) caused by the bird strike seemed to suggest.

Figure "Influence of proximity to ground" (Ref. 5.2.2.1-2): In the low flight altitudes at which bird strikes are most frequent, civilian aircraft (left diagram) will generally flying at relatively low speeds (takeoff, landing, approach flight; right diagram). Therefore most strikes will below speed impacts that especially stress the fan rotor blades.
Fighter aircraft are a different situation, since these usually travel at relatively high speeds at low altitudes. In this case, the resultant high speed impacts will especially stress static engine parts in the air flow such as housing struts or intake guide vanes (also see chapter 5.2.2.2).

Bird strikes in military aircraft

There are no accurate published statistics for military aircraft. In general, the situation should be similar to that of civilian aircraft, with the major difference being that there is an additional and probably dominating danger factor due to the low altitude flights over land and water at high cruising speeds (Example "Flock of birds"). Also, the frequency of bird strikes is probably considerably increased by the fact that the engines of pursuit and fighter aircraft are integrated into the fuselage. If one refers to the civilian aircraft damage statistics (Fig. "Impact area distribution"), the fuselage was the most frequently affected part of the aircraft. Assuming that due to the considerably smaller dimensions of military jet aircraft fuselages, roughly 50% of birds will strike the relatively large engine inlets (Fig. "High speed impact"), then the bird strike rate for fighter aircraft should be roughly twice the 20% engine strike rate in civilian aircraft. The risk of total thrust loss is further increased in all single-engine aircraft. It should only be decreased relative to large civilian aircraft due to the fact that the (absolute) engine inlet surface area is considerably smaller. Fig. "Impact area distribution" shows that the strike rate only decreases underproportionally with smaller engine face cross-sections.

Example "Flock of birds" (Ref. 5.2.2.1-8):

Excerpt: ”…during low altitude flight at 50 meters over the ocean, flew into flock of birds, lost cockpit roof, engine failed, and crashed.”

Example "Low altitude" (Ref. 5.2.2.1-7)

Excerpt: ”…during low altitude flight at about 60 meters in three-aircraft formation, aircraft struck a bird and crashed into the Baltic Sea…“

Example "Approach flight" (Ref. 5.2.2.1-9)

Excerpt: ”…during approach flight…crashed due to engine damage caused by flying into a flock of ducks…“

Comments: These are only three examples of many similar incidents, involving this single-engine fighter aircraft type. It is also typical that many of the incidents occurred at low altitude flight missions over the ocean. The (single cycle) engine was an older model with adjustable intake guide vanes. The blading of the rotor and stator was made from Cr steels.

Figure "Species and living habits" (Ref. 5.2.2.1-2): The fact that in this year 50% of all bird strikes were caused by gulls and a further 12% were caused by lapwings indicates that airports and low-altitude flight paths in coastal areas and expansive wetlands are especially dangerous. Neither of these bird species is migratory. This statistic is taken from the European region.
The number of bird strike damages in the Nordic countries seems considerably smaller than that in central Europe. This is probably due not only to the shorter stay of migratory birds in the north, but also the relatively small number of flights.
The danger is especially great in the migratory bird routes (Israel, etc.) and their land bridges (Denmark, Gibraltar, etc.) in the spring (May) and autumn (October), since expansive, dense flocks of birds during these times increase the probability of several birds striking one or more engines simultaneously (Fig. "Multi bird impact").
Additionally, flocks of birds often fly at greater altitudes during migration than they would otherwise, increasing the danger zone for aircraft.
However, by far the largest influencing factor is the area surrounding an airport (feeding areas, garbage dumps, fields, nesting areas, wetlands). In altitudes above 4500 meters there is virtually no risk of bird strikes since even large birds of prey such as eagles, condors, and vultures are rarely found at these heights.

Figure "Impact area distribution" (Ref. 5.2.2.1-2): The left diagram shows that with engines on the wings of large civilian cargo aircraft, the frequency of bird strikes increases considerably more slowly with inlet cross-sections larger than a certain size. There is no similarly clear limit in engines on the fuselage. It is not clear whether the underproportional bird strike probability at larger inlet cross-sections is due to the introduction of modern fan engines with large bypass ratios. It is possible that the pinching of the inlet flow may have an effect on bird strikes (Fig. "State of operation").
The right diagram shows the distribution of bird strike damages across the entire aircraft during the early 1970s. If one adds up the fuselage components (nose, cockpit windows, hull), over half of bird strikes are on the fuselage.

Figure "Multi bird impact": It would be estimated rather unlikely, that several aeroengines of a single aircraft suffer a bird strike simultaneously. Especially, that both aeroengines of a twin-engined aircraft are concerned. The statistics of the US aviation authority however teaches us something different (Ref. 5.2.2.1-18).
A spectacular case shows this picture. Both aeroengines of this airliner dropped out by bird impact shortly after the start. The pilot had to make an emergency landing, which he could manage in a large city at a river nearby. The airplane floated so long on the water, until the passengers could be rescued.
This is the more astonishing because, during the `splashdown' of a modern airliner with big fan engines at the wings ,a lateral touch at the water let expect rather catastrophic damages (Ill. 10-12).

Fig. "Turboprop I": The displayed 4 engined military transport aircraft in the upper sketch crashed after a flock of pewits brought three of four turboprop engines to an outage (Ref. 5.2.2.1-18). The aircraft was in the landing approach and tried to avoid the flock of birds. It is not clear as far the evasive manoeuvre or the loss of power from the aeroengines was cause of the accident.
The documents on hand let not identify if the compressors have been damaged or if the opening of the oilcoolers intake, also is orientated to the front, are causative concerned.
So turboprop engines are not fully protected against bird impact by the centrifuge effect of the propellers.
It may be noted, that the turboprop engines are relatively small. The high-speed filigree compressor is as well FOD sensitive for the clogging by bird remains as also for mechanical damages. Additional aggravating is the mostly S- shaped bended intake channel, which brakes the bird and so promotes a low-speed-impact (Fig. "Influence of flight speed") in the compressor.

Example "Twin engine failure" (Fig. "Flocks of small birds", Ref. 5.2.2.1-2):

Excerpt: ”….(the aircraft) which ingested lapwings in both engines on take-off. One crew member, of the eight persons on board, was injured, but the six occupants of a passing car were killed when the disabled aircraft slid across a road which passes the end of the runway”

Comments: The top diagram in Fig. "Typical bird strike damage" shows the aircraft after the accident. It is a relatively small civilian aircraft with two correspondingly small (and therefore sensitive to bird strikes) engines located on the rear fuselage. There have also been comparable damages in aircraft from other manufacturers. It is not clear whether or not this indicates an especially high sensitivity of this configuration. However, it is plausible that the small distance between the engines due to the relatively small fuselage diameter may increase the probability of both engines failing even due to small flocks of birds.

Figure "Flocks of small birds" (Ref. 5.2.2.1-2): The probability of multiple engines failing and possible total thrust loss depends largely on whether or not birds are encountered in flocks. There have been several reported cases in which a twin-jet aircraft crashed (top diagram) because both engines failed due to a flock of birds (Example "Twin engine failure"). It is also important whether or not birds favor certain flight formations. These can cause several birds to strike the same engine. There is a documented case in which several wild geese caused catastrophic damage to a large fan engine (Example "Engines with large bypass ratios").

Example "Engines with large bypass ratios" (Ref. 5.2.2.1-11):
Excerpt 1
(Ref. 5.2.2.1-10): “…after ingestion during takeoff of a number of turkey-sized seagulls…The No. 3 engine on the right wing separated, possibly as a result of unbalanced rotational forces, and the entire right wing was engulfed in flames by the time the pilot … was able to brake to stop near the end of the runway….All 139 persons on board evacuated safely…
About 30-40 dead seagulls weighting up to an estimated 15 lb. were found in the area of the runway were the strikes occurred….
Parts of the engine were strewn over half the length of the runway (see sketch below).

Excerpt 2 (Ref. 5.2.2.1-11): ”…engine damage…was caused by a massive multiple bird strike of heavy birds in excess of any FAA certification test requirements….more than two birds and perhaps as many as seven in the 5-6 lb. weight class could have been ingested in the No. 3 engine…these large birds severely damaged the fan blades…“

Comments: This damage occurred in the mid-1970s and caused the certification and safety authorities (FAA, NTSB) to critically consider the problem of the ingestion of large birds into large engines with large bypass ratios (see also Ill. 9.4-7).. The results of these considerations are included in current certification and verification regulations. The birds involved in this case were probably wild geese rather than gulls (Fig. "Canada goose III").

Figure "Turboprop II": The sketch shows a double engined civil aircraft at which the intakes of the turboprop engines are located at the upper side (compare intake below in Fig. "Turboprop I"). During start the dropout of one aeroengine occurred after the entrance of a 120 gram bird (Ref. 5.2.2.1-17). The compressor didn't show mechanical damages, although everywhere rests of the bird where found. Seemingly the remains have markedly affected the quality of the compressor.

Illustrations 5.2.2.1-6.4.1 and 5.2.2.1-6.4.2 (Ref. 5.2.2.1-18): A canada goose caused the displayed aeroengine failure. At the entrance of such a large bird extensive damages in the aeroengine can not be avoided. However the casing wall was not penetrated by fragments. With this, also such a failure meets the specifications. The failure pictures are obviously caused from two different incidents what, for example, can be seen in the region of the (fixed) nose cone. Concerned is in both cases an elder aeroengine type with low bypass ratio. The fan blades are, compared with modern fan engines (Fig. "Canada goose III") relatively small and light. They have in the upper half of the blade supports (clapper). These can influence the bird impact behavior favourable (Fig. "Low speed impact II" and Fig. "Influence of clappers"). The compressor rotor is supported in front. The bearing is supported by an entrance guide vane apparatus (struts to the bearing chamber “1”). This design has a markedly influence at the behaviour during a bird impact (Fig. "Influence of inlet guide vanes") and the secondary failures to be expected.
In both cases obviously the struts in front have been destroyed. With this the low pressure rotor looses its support and it comes to an extreme rubbing process. This probably promotes the extensive damages of the fan blades (“2”). Additionally a fire of the escaping oil from the damaged bearing chamber must be expected.
In the right picture obviously many fan blades fractured above the clapper. This is probably caused by an intense rubbing process after the drop out of the front bearing. Obviously the rubbing process and/or burning oil from the destroyed bearing chamber triggered a fire in the cowl lining. It came to a melting of light metal. This may derive from the lining of the intake cowl, made from aluminium-honeycomb.

Illustrations 5.2.2.1-6.5.1 and 5.2.2.1-6.5.2 (Ref. 5.2.2.1-18): Those destructions in a modern fan engine are caused by a canada goose. During a direct hit, forces up to 40 tons must be expected (Fig. "Impact forces"). With this serious aeroengine failures like the fracture of the clapper stiffened fan blading, can not be avoided. Important is, that the fragments specification conform do not exit the aeroengine (contained).

Figure "Small bird damage" (Ref. 5.2.2.1-18): This fanblade damage is caused by a relatively small bird of about 1/2 kg. Remarkably are the plastic deformations of the blades at the impact position. This adumbrates the high pressures of the bird mass in the contact area, which behaves as a sort of liquid (Fig. "Laboratory tests on bird strikes" and Fig. "Phases of deformation").

Figure "Typical bird strike damage": Bird strikes are especially dangerous for the following typical parts of multiple-shaft fan engines with small bypass ratios (frequently used in Fighter aircraft):

Engines without inlet guide stars/struts (top) and no front bearings, but with rotating nose cones (spinners):

  • Deformation or fracture of the fan rotor blades (1a; Fig. "Vertical take-off and landing aircraft")
  • Fracture of the fan rotor blades above the clapper (1b; Figs. "Low speed impact I" and "Low speed impact II")
  • Deformation and fracture of guide vanes/stator at the outer zones, contact with the rotor blades (2a)
  • Fracture of guide vanes at the inner shroud (2b)
  • Fracture of guide vanes at the connection to the housing/casing (2c)
  • Stressing and deformation of the housing/casing after a rotor blade fracture (3a)
  • Fracture of threaded connections on the housing/casing due to imbalances (3b)
  • Fracture (especially with fiber-reinforced synthetics; Example "Spinner fragments") or deformation (especially with metals) of the rotating nose cone (spinner, 3c). In some cases overloading of the rotor`s fixed bearing.
  • Overstressing of the axial blade connections/fixing/roots (4a)
  • Fracture of the spinner connection/fixing (4b)

Engines with front bearings (below), housing/casing struts, adjustable inlet flaps or inlet guide vanes (Ref. 5.2.2.1-14), and fixed inlet cones:

  • Deformation and fracture of the fan rotor blades (1c)
  • Fracture or deformation of the fixed/stationary (non rotating) nose cone (3d)
  • Damage to the housing/casing struts (5) and possibly also the rotor bearings
  • Damage (deformation, fracture) to the adjustable inlet flaps/guide vanes (Fig. "Variable vanes") or changes to the blade angle (6a)
  • Damage to the adjustment mechanism for the flaps or guide vanes (6b)
  • Damage to the inner flap bearings (6c)

Figure "Vertical take-off and landing aircraft" (Ref. 5.2.2.1-12): This VTOL crashed after a bird strike (Ref. 5.2.2.1-12); this type of aircraft should be especially sensitive to bird strikes due to the relatively large engine inlets and extended operation at low altitudes. Parts (1) of the nacelle or engine inlet also appear to have entered the engine. Several fan blades have fractured above the clapper in typical fashion (2). This is due to the stiffening effect the clapper has on the entire blade ring (Fig. "Influence of clappers"). The fan housing/casing (3) evidently contained the fragments.

Damage types and damage development

The typical damage symptoms of bird strikes are deformation, crack initiation, and fractures of the engine components. This is caused either directly by foreign object impact or indirectly through developments such as bent compressor rotor blades damaging the neighboring stator vanes (Fig. "Damage mechanisms in the compressor") or becoming jammed in the latter (Fig. "Contact of rotor with stator vanes"), causing one or both to break off. Catastrophic blade failure can be expected if the flexure causes the entire cross-section of the blade to be plastically deformed near the root (Fig. "Resulting deformation of impact-bending"). The corresponding maximum loads are several times greater than the loads at which the outer fibers of the profile (usually the edges and the blade spine) begin to be plastically deformed due to bending stress greater than the yield strength.
Mechanical impact stress on parts made from brittle materials (Fig. "Unsteady forces during bird strike") results in visible effects from reflexion and overlaying of impact waves. This causes typical multiple fractures in locations where they would not occur under static loads. The extent of the damage caused by the impact itself may be limited at first, but continued engine operation can cause catastrophic development after a few seconds or minutes. Typical examples of this type of damage include dynamic fatigue fractures on compressor bearings after rotor imbalances and vibrations, and also dynamic fatigue fractures in deformed or damaged blading. These dynamic fatigue fractures frequently occur in plastically bent rotor blades, since the centrifugal force creates high mean stresses by unwinding the blade and causes additional flow disruption (flow stall at the deformed blades; Fig. "Consequential damage") and notch effects (impact notches, cracks) which result in high dynamic stress. Bending of the blade along the circumference is less disruptive than a permanent twist. Permanent twist has a strong influence on the flow and can cause a stall at the damage blades. In subsonic compressors the torsion angle change must be less than about +- 10° of the set value, in transsonic compressors the maximum deviation is +- 2° of the desired angle.
If the above damage causes compressor surges, then the blading is subjected to additional dynamic loads. This is accompanied by the danger of the hot parts overheating due to a lack of cooling air. It has also been observed that before birds enter the compressor, they can break off parts of the cowling or inlet duct, and these fragments can also cause extensive FOD in the compressor.
Aside from the compressor parts described in Fig. "Typical bird strike damage", the following engine parts are especially sensitive to bird strikes:

Special damage due to bird strikes in the compressor area

  • In compressors with titanium blading, if the engine continues to run after a blade fracture a titanium fire may ignite. This creates the danger of fire burning through the housing and escaping from the engine, ultimately causing the aircraft to crash.
  • Bearings, especially fixed bearings, are stressed by powerful axial and radial forces caused by imbalances after a bird strike. This can result in bearing track fatigue, plastic deformation of the bearing tracks, or even spontaneous force fractures. Typical points of origin for a force fracture in an outer bearing ring are the corners of grooves or studs designed to keep the ring from rotating.
  • Connections (bolts, nuts, serration) between the compressor disks can fracture due to large deceleration moments (deceleration of the rotor when the bird passes through), in extreme cases causing the compressor rotor to separate.
  • Blockage of air intake vents, ducts, and valves (for cabin air, de-icing air, hot part cooling air, etc.)

Damage to the combustion chamber area due to bird strikes:

  • Blockage of the combustion chamber vents and bores for the cooling air film and combustion air, resulting in danger of uneven temperature distribution and local overheating in the combustion chamber (can burn through the combustion chamber wall and combustion chamber housing, resulting in an engine fire), and unallowably poor temperature distribution in the gas flow, which can cause the downwind turbine blading to fail.

Damage in the turbine area caused by bird strikes:

  • Overheating of turbine guide vanes (nozzles) due to poor temperature distribution in the gas flow
  • Blockage of cooling air bores in turbine rotor blades and guide vanes, resulting in overheating with burning-through of walls and crack initiation
  • Overheating of the turbine due to a lack of cooling air and reduced excess air during compressor surges; additional overheating caused by the regulator increasing the fuel flow in order to balance out the decrease in engine performance/efficiency
  • Mechanical damage through fragments or metallic melts (for example, after a titanium fire).

Figure "Intake duct liner" (Ref. 5.2.2.1-18): In this case seems the dimension of the damage are also connected with fragments of the liner from the air intake duct. It can be supposed, that even the disturbance of the airflow by the rests of the liner prevented a further operation because of compressor surging.

Figure "Damage at spinner": Damage at the rotating nose cone (spinner) of a fighter engine by bird impact. The component is made from high strength titanium alloy sheet. A bird with the size of a duck hit the spinner at high flight speed near the ground and deformed it heavily. In spite of extreme unbalances and crack formation obviously the centrifugation of fragments did not occur.

Figure "Low speed impact I" (Ref. 5.2.2.1-13): This relatively small fan engine of a aircraft used ground attack and training has wide chord fan blades. The chord length of these blades gives them enough stiffness to make flow-disrupting clappers unnecessary. The lower diagram shows the fan rotor (Fig. "Influence of inlet guide vanes") after a bird strike test with a 1.8 kg bird traveling at 210 m/s. This arrangement is sensitive for a “low speed impact” like in this case (Fig. "Influence of flight speed"). Such a low speed impact can be also expected at higher flight velocities if the bird hits the wall of the inlet duct and is there braked. With a bird of this size it is practically impossible to prevent fractures even in solid blades of this size. The relatively brittle fracture of several neighboring blades immediately above the foot platform is interesting. The fact that the preceding blades in the direction of rotation are heavily bent indicates a damage mechanism which started by one blade fracturing due to the bird impact. The fractured blade became jammed in the housing, and the following blades “ran over” it, either fracturing or bending in the process (Ill. 8.1-15.1). Another interesting characteristic is the serious deformation of the metallic spinner, which indicates that at least some parts of the bird struck here first.

Figure "Damage symptoms on rotor blades": This diagram is a schematic depiction of a compressor rotor stage that has damage symptoms typical of those caused by bird strikes (compare with Fig. "Resulting deformation of impact-bending"):

  • Plastic deformation of a blade that was struck at the tip “1”
  • Blade that was struck slightly farther down and plastically bent in a typical S-shape “2”
  • Blade crack caused by local overstress “3”
  • Blade that has been torqued by high loads on the leading edge “4”
  • Blade that as fractured at the root “5”

Figure "Variable vanes": This older single-cycle military engine was found to be prone to damage to the adjustable guide vanes of the forward stages through ingestion of relatively small birds such as gulls (Example "Flock of birds", Ref. 5.2.2.1-14). These blades are often hollow and designed to carry the de-icing air. As shown in the bottom diagram, overstress such as that caused by bird strikes causes the blades to crumple and/or burst open. The damage to the adjustable guide assembly can disrupt the compressor flow seriously, resulting in stalls and engine failure.

Figure "Low speed impact II": Damage from the low speed impact of a duck during landing approach flight; the fan rotor blades of the first stage fractured above the clapper. All fragments were contained. This example shows the stiffening effect the clapper has on the blade ring (also see Fig. "Influence of clappers"). The narrow fan blades of this older engine type created relatively low-energy fragments that did not threaten the housing. However, the heavy, clapperless, wide-chord fan blades (Fig. "Low speed impact I") found in modern engines require special blade design to ensure acceptable behavior during bird strikes. The large fragments resulting from a blade failure must be contained by a sufficiently elaborate, i.e. heavy, containment.

Figure "High speed impact": This diagram shows a guide vane behind the first fan rotor stage that sheared off at the hub. At high flight speed a bird traveled through the rotor blade largely without striking anything, due to the aerodynamic design of the blading; in extreme cases birds can pass whole between two blades. This stresses the stator with maximum kinetic energy. This will destroy part of the relatively filigreed stator and can lead to extensive consequential damages to the rotor.

Figure "Indirect threats of bird damage" (Ref. 5.2.2.1-15): In fighter planes, the danger that a bird will strike the engine directly is compounded by the possibility that a bird will strike the fuselage and then enter the engine. In this case, a bird will “spray” in a manner similar to a jet of water. The pieces of the bird are far less threatening to the engine than a whole bird. This effect is frequently taken into account by military certification regulations with regard to verification of engine behavior in case of large bird strikes. In the tests a bird is guided into the engine by deflector plates. This procedure is recommended especially with aircraft with engine inlet ducts that are shaped in a way that makes direct bird strikes on the engine unlikely.
However, there is an increased risk if a bird strike creates fragments that can also enter the engine (parts of the radome, air inlet, covering, etc.). In the depicted case a single-jet fighter aircraft was struck by a seagull, causing parts fragments of the fuselage cover to break off and enter the engine.

Figure "Consequential damage" (Ref. 5.2.2.1-16): In most cases in large fan engines, two rotor blades will sustain damage. The first blade against the direction of rotation, or “leading blade”, and the one following it, or “trailing blade” (diagram). Depending on how serious the damage to these blades is, there is a risk of dynamic overstress through flutter or a rotating stall. The influence of the rotating speed is not shown in this diagram.

References

5.2.2.1-1 R.P. Paxson, J.D. Vance, “A Bird Strike Handbook for Base Level Managers Thesis”, Report AD-A147 928 Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio, Nov. 1984.

5.2.2.1-2 J.Thrope, “Analysis of Bird Strikes reported by European Airlines 1972-1975”, CAA Paper 77008, Civil Aviation Authority, London October 1977, ISBN 0 86039 055 1.

5.2.2.1-3 T.L. Alge, J.T. Moehring,”Modern Transport Engine Experience with Environmental Ingestion Effects“, AGARD Conference Proceedings 558, “Propulsion and Energetics Panel Symposium”, Rotterdam, The Netherlands, 25-28 April 1994, Chapter 9, pages 3 and 6.

5.2.2.1-4 NTSB Identification NYC961IA022, Index for Nov 1995.

5.2.2.1-5 NTSB Identification ANC931A188, Index for Sept 1993.

5.2.2.1-6 NTSB Identification ATL96FA101, Index for July 1996.

5.2.2.1-7 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 344.

5.2.2.1-8 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 189.

5.2.2.1-9 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 346.

5.2.2.1-10.1 “DC-10 Strikes Birds, Burns At JFK Airport”, periodical “Aviation Week & Space Technology”, November 15, 1975, page 24

5.2.2.1-10.2 M.L. Jaffee, “GE Pushing Effort on CF6 Engine Fix”, periodical “Aviation Week & Space Technology”, March 15, 1976, pages 197 and 199.

5.2.2.1-11 M.L. Jaffee, “NTSB, GE in CF6 Engine Conflict”, periodical “Aviation Week & Space Technology”, April 5, 1976, page 22.

5.2.2.1-12 H. Blokpoel, “Bird Hazard to Aircraft” Problems and Prevention of Bird/Aircraft Collisions, Verlag Clarke, Irwin& Company Limited, Canada., Diagram 2-9

5.2.2.1-13 J.M. Foueillassar, A.R. von der Muhl, “Petites Turbomachines: Experiences sur la rupture des Disques”, AGARD Conference Proceedings No. 248, “Stresses, Vibrations, Structural Integration and Engine Integrity (Including Aeroelasticity and Flutter)”, Chapter 16, Diagram 7.

5.2.2.1-14 E. Heckmann, “F404 - das Jägertriebwerk unserer Generation”, periodical “Wehrtechnik”, 7/1986, pages 58 and 60.

5.2.2.1-15 J. Hild, “Vogelschlag und Flugsicherheit”, periodical “Aerokurier”, 10/1989, page 286.

5.2.2.1-16 M.Imregun, M.Vahdi, “Aeroelasticity analysis of a bird-damaged fan assembly using a large numerical moden”, “Imperial College of Science, Technology and Medicine”, London UK, The Aeronautical Journal, Paper No. 2487, December 1999, pages 569-577.

5.2.2.1-16 M.Imregun, M.Vahdi, “Aeroelasticity analysis of a bird-damaged fan assembly using a large numerical moden”, “Imperial College of Science, Technology and Medicine”, London UK, The Aeronautical Journal, Paper No. 2487, December 1999, page 569 up to 577.

5.2.2.1-17 NTSB Flugunfallbericht (NTSB/AAR-80-15),Flugunfall June 12, 1980.

5.2.2.1-18 A.Moffat, “Bird Strikes - Is the risk increasing?”, Mount Cook Airlines, Paper der “Australian Society of Air Safety” 2006.

5.2.2.1-19 C.A. Huertas-Ortecho, “Robust Bird-Strike Modelling Using LS-DYNA”, University of Puerto Rico, 2006, page 1-236.

5.2.2.1-20 J.Frischbier, “Vogelschlag in Flugtriebwerken - eine impulsartige Fluid-Struktur-Wechselwirkung in der Triebwerksauslegung”, www.mtu.de/de/technologies/engineering_news/entwicklung. page 1-23.

5.2.2.1-21 J.Frischbier, “Multiple Stage Turbofan Bird Ingestion Analysis with ALE and SPH Methods”, Herausgegeben von “American Institute of Aeronautics and Astronautics”, 2005, page 1-9.

5.2.2.1-22 NTSB Flugunfallbericht, NTSB-AAR-76-19, Flugunfall vom 12. November 1975.

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