Aeroengine Safety

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 "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").

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).

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.

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").

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.

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.