Airworthiness Compliance Verification of Bird Strike on Civil Aircraft Windshield Frame

With the frequent occurrence of bird strikes, minimizing the impact damage effect of birdstrike on flight always is the goal of airworthiness certification. As a large windward component in front of the aircraft, the windshield, and frame have more probability to be suffered from birdstrike. This paper introduces the common connection forms of the windshield frame structure of civil aircraft, and through the interpretation of the airworthiness requirements related to windshield in CCAR-25, puts forward the process and method of compliance verification of windshield frame birdstrike. Based on the SPH method, the dynamic analysis of bird impact on windshield window frame is carried out, the test is carried out to verify that the analysis method is reasonable and conservative, and the results show that the bird impact performance of the windshield frame structure meets the airworthiness requirements.


Introduction
Bird strike refers to a flight accident caused by a collision between an aircraft and birds flying in the air.According to the operation experience of civil aviation, bird strike is a major threat source of aircraft external structural damage.Although the external structure of the aircraft is exposed to various threats of foreign object damage, such as hail, lightning, runway debris, etc., about 90% of these incidents are reported to be caused by bird strikes [1].In the accident of bird striking aircraft, because the windshield, canopy, radome, and other components are located in the nose of the aircraft and have a large windward area, the probability of their collision with birds is correspondingly high.According to the statistics of FAA from 1990 to 2006, the probability of a bird striking the nose structure of an airliner is 44% of that of the whole aircraft.Therefore, the verification of the bird impact compliance of the nose windshield is particularly important for transport category aircraft.

Structural connection form of windshield and frame
As a part of the airframe structure, the windshield of the aircraft participates in bearing the airtight load in the cabin and plays a role in providing the pilot's vision and protecting the pilot.According to the connection and stress between the windshield and the frame, the installation forms are mainly divided into three categories: the first category is the form that the windshield is not directly connected with the surrounding window frame structure, which is called "pressure plate"; The second type is that the windshield is connected to the window frame structure by bolts, but does not transmit in-plane loads, which is called "non-load-bearing bolted type".The third type is that the windshield is directly connected to the window frame structure and transmits the in-plane load, which is called the "loadbearing bolted type" [2].The installation form of the Class III windshield is shown in Figure 1.a) Pressure plate type: the windshield installed with the pressure plate type only bears the airtight load acting on it.The stress form of the windshield is simple.It is easy to install and disassemble.When the windshield is damaged, it will not affect the overall structure of the cabin cover.However, because the pressure plate windshield does not bear the load, it is equivalent to a large opening in the structure that needs to be strengthened, and the weight of the structure is relatively heavy.At the same time, the windshield is not directly connected with the window frame structure, and when all structural layers are damaged, the windshield may be separated from the structure under a pressurization load.
b) Non-load-bearing bolted type: the windshield is directly connected to the window frame structure through bolts, which increases reliability.If the pressure plate and the windshield are connected together, the pressure plate can be designed to be relatively weak, and certain weight can be reduced.Its disadvantage is that the windshield does not bear the load, the structure has a large opening that needs to be strengthened, and there are many bolt mounting holes around the windshield, which has a certain impact on the life of the windshield itself.c) Load-bearing bolted type: The load-bearing bolted type windshield can transfer the in-plane load, and can be strengthened without following the design principle of a large opening in the structural design of the engine body, so as to effectively reduce the structural weight.At that same time, the rigidity of the window frame can be reduced, the section height of the window frame structure can be reduced, and the space in the cockpit can be increased.Its disadvantage is that the stress form of the windshield and the structure is complex, and when the windshield is damaged, it will have a certain impact on the structural force transmission of the canopy.In the process of type certification, airworthiness compliance verification is also complex.
In regional aircraft, there are many applications for load-bearing bolt-on windshields, such as ERJ and CRJ aircraft, due to the large windshield opening relative to the overall aircraft nose size.Airbus series aircraft adopt the design and installation method of pressure plate, including the A380 and the newly developed A350 aircraft.The Boeing family of aircraft, from the B707 to the B787, uses a nonload-bearing bolt-on windshield.
3 Airworthiness requirements for bird strikes on the windshield frame Bird strike, as a discrete source and specific risk, must be verified in type certification to ensure that the aircraft can continue to fly and land safely after a bird strike.Current airworthiness standards for transport category aircraft, CCAR25, 14CFR25, and EASA CS25, all provide airworthiness requirements for bird strikes, and the airworthiness requirements related to windshield frame bird strikes are primarily § 25.571 (e) (1) and § 25.775 (b).It clearly defines the area of bird impact, the speed of bird impact, and the weight of the bird body [3].§ 25.571 (e) (1) requires that the aircraft must be able to complete the flight if it is subjected to structural damage from a 1.8 kg bird impact.§ 25.775(b) requires that the windshield and its supporting structure must be capable of withstanding the impact of a flying bird of 1.8 kilograms without being punctured when struck by a bird at a velocity of Vc at sea level.
Section 25.571 (e) (1) requires a damage tolerance (discrete source) assessment to ensure that the aircraft structure can complete the mission in the event of damage caused by a bird strike from a discrete source.The damaged structure must be capable of withstanding the static loads reasonably expected to occur in flight (considered as the ultimate load case).If the damage to the structure causes a significant change in the stiffness or geometry of the structure, its effect on damage tolerance must be studied; Section 25.775(b) requires the ability of the windshield and its supporting structure to withstand bird strikes.The scope of consideration (verification object): the transparent part of the windshield and its supporting structure located directly in front of the driver who normally performs his duties, excluding the transparent part and its supporting structure of the sliding window, fixed window, and cabin window on the side of the cockpit.The birdstrike condition considered is the impact of an aircraft with a 1.8 kilogram bird at the sea level Vc velocity selected in § 25.335 (a) relative to the bird along the aircraft track.
The requirement that the windshield and window frame not be penetrated after a bird strike is intended to avoid the following consequences: a) After impact, structural damage affecting flight operation occurs, such as damage to the pilot and other passengers due to windshield breakage, structural element rupture and bird penetrating the windshield glass into the cockpit, or damage to the normal work of the pilot due to tearing of the cabin liner; b) The bird penetrated the windshield and the surrounding structure, causing a loss of pressure in the cockpit.

Bird impact process and damage mechanism
According to the regulatory requirements, the aircraft is required to withstand the impact energy of a 1.8 kg bird relative to the aircraft at a flight speed of Vc.Taking Vc = 180 m/s as an example, the energy of the bird is: E=1/2 m×Vc2=29.2kJ.
The angle ș between the windshield bird strike range structure and the flying bird trajectory is usually between 15 and 40°, as shown in Figure 2.
Because of the large relative speed between the aircraft and the bird, the bird impact process is completed in an instant.It can be seen from the simulation of finite element software and the high-speed video recording of the bird impact test that the time T of bird impact has a great relationship with the stiffness of the impact point area.When the stiffness of the impact point area is very large, the structural deformation is very small, the bird body is quickly dismembered by the great reaction force, and the bird impact process is over.The whole process is about 1 ms. Figure 3 shows the bird body energy-time curve at the high-stiffness impact point.Figure 4 shows the energy-time curve of the bird body when the stiffness of the impact point area is very small.At this time, the body deformation is large, and the bird will slide along the deformed structural skin for a certain distance.During the sliding process, the energy of the bird body gradually decreases, and the whole process is about 3 ms.The load F applied by the bird on the airframe is related to the velocity Vc and mass m of the bird, the angle of attack ș at the impact point, the stiffness of the impact point and the impact contact time T, and the load F can be approximately calculated by the following formula: The peak load of the bird on the airframe may reach 208 kN.

Compliance design
According to the damage mechanism of bird strike, the impact factors of bird strike on windshield and window frame structures mainly include impact angle, sea level Vc, stiffness of surrounding structures, and other factors.Therefore, the bird strike compliance design of windshield and window frame structure is mainly considered from the following three aspects.a) Under the condition of meeting the functional requirements such as windshield vision, the inclination angle of the windshield and window frame structure shall be designed to minimize the bird impact assessment area; b) The bird impact velocity is taken as the sea level Vc value.When the design speed envelope of an aircraft is initially determined, the value of Vc should be reduced as much as possible under the condition that the overall design requirements of the aircraft are met, which is of great significance for improving the passing rate of the bird strike assessment of the aircraft; c) The supporting structure around the windshield is designed to minimize the design features of large corners, so as to reduce the stiffness mutation of the connection between the windshield and the window frame and reduce its adverse impact on the absorption and transmission of bird impact energy.

Compliance verification methods
Acceptable methods of compliance for bird impact verification primarily include MOC0, MOC1, MOC2, and MOC4.
(1) If there is an approved similar model, the MOC0 method can be used to make a declaration of compliance; (2) For the newly designed aircraft, the MOC2 method is used to carry out the dynamic analysis, which is used to formulate the bird impact test plan and screen the severe points under various bird impact conditions; (3) In order to determine whether the corresponding parts of the airframe meet the requirements of bird impact, the bird impact test is carried out on the selected severe points by using the MOC4 test method, and the accuracy of the MOC2 analysis method is verified.
Based on the SPH method, this paper establishes the bird impact dynamic model of the nose and windshield structure and carries out the dynamic analysis of MOC2.According to the dynamic analysis results, the relative critical points of bird impact are screened out, which provide input for the selection of compliance verification test points.Then the bird impact verification test is carried out by using the air gun method, and the dynamic analysis of the windshield frame structure in the test state is carried out, which is compared with the test results, indicating that the bird impact dynamics analysis method is reasonable and conservative, and the bird impact dynamics analysis results of other positions of the windshield frame structure are valid based on the above model.It can be verified that the bird impact performance of the windshield frame structure meets the requirements of the airworthiness regulations.

SPH method
The Smoothed Particle Hydrodynamics (SPH) method is applied to the explicit finite element method for the analysis of large deformation problems.Lucy [4] presented the first hydrodynamic method for smooth particles, which is essentially a gridless algorithm based on numerical interpolation.The SPH method assumes that the material is a set of fluid particles with mass, velocity, and energy, and each particle is equivalent to an interpolation point with physical characteristics.The basic idea of this algorithm is to discretize a continuous solution domain into particles, regard each particle as an interpolation base point with known physical characteristics, and calculate its field function value with interpolation function, so as to approximately describe the field of the whole solution domain [5].
Considering the rheological behavior of bird bodies during high-speed impact, the Murnaghan equation [6,7] is usually used: where ‫‬ and ‫‬ are the current pressure and initial pressure, ߩ and ߩ are the current and initial densities, B is the bulk modulus of elasticity, and ߛ is the exponent.

Selection of test points for bird impact dynamics analysis
Due to the lack of a reliable simulation calculation method for windshield glass, the supporting structure area around the windshield glass is selected for evaluation in the simulation calculation comparison.For the peripheral area of the main windshield, due to the rigid structure of the window frame, the bearing form of each side of the window frame is a simply supported beam, and the deformation of the middle position is relatively large when it is hit by birds, so the middle position is more severe than other positions.The relatively dangerous point A in the middle of the left window frame and point B in the middle of the upper window frame is selected, and at the same time, point C in the center of the lower window frame is selected.For the side windshield area, point D in the middle of the upper window frame is selected.

Bird Impact Dynamics Analysis
The finite element method is used to model and analyze the nose structure.The pre-processing software is hypermesh and the post-processing software is PAM-CRASH.The windshield frame, canopy framework, ring frame, and skin panel are modeled by refined mesh, the element type is Shell, the element size is 5 mm ~ 7 mm, and the fastener is simulated by Plink element.Components in other areas are modeled by the coarsening grid, and the unit size is 15 mm ~ 40 mm.This area is used as the support or transition section of the assessment area.
The geometric model of the bird body is shown in Figure 6.It is a cylinder with hemispherical ends and a length-diameter ratio of 2:1.The mass is 1.8 kg and the density is 950 kg/m 3 .The diameter of the sphere at both ends is 113 mm and the length is 226 mm.

Figure 6 Bird body model
The displacement deformation of the four impact points is shown in Figure 7 to Figure 10.The windscreen window frame was deformed elastically or plastically, the maximum displacement was 7.9 mm, and no damage occurred, so the bird body would not penetrate the window frame.b) The speed of the bird striking the windshield shall be greater than or equal to the value of Vc at sea level.If the speed value has a lower deviation, it cannot be accepted by the approving party.If the upper deviation value is too large, the bird strike verification of the windshield is too harsh.The bird impact velocity tolerance zone is typically 0-2%.
The structural damage at impact point A is shown in Table 1, and the test results and the calculation and analysis results are shown in Table 2. From the comparison, it can be seen that the test results and the analysis results have the same structural damage mode and equivalent damage area.In the simulation analysis, the upper skin of the window frame has a warped flanging, but there is no warping in the test.The analysis is slightly more severe than the test.It is proved that the finite element model and the finite element analysis method of the dynamic analysis are reasonable and conservative, and the dynamic analysis results of bird impact at other positions of the windshield frame are valid based on the model.

Comparison of results
Point-A a) The window frame is not damaged by visual inspection, and the structure is in good condition; B) The fasteners connected to the window frame are not pulled out or sheared; C) The upper skin of the window frame is intact.
A) There is no damage to the window frame and the structure is intact; B) The fasteners connected to the window frame are not pulled out or sheared; C) The upper skin of the window frame is slightly turned up.
It is basically the same.The upper skin of the window frame has local warping and flanging during the analysis, but there is no warping during the test.The analysis is slightly more severe than the test.
The bird did not enter the cockpit.
The bird did not enter the cockpit.

Conclusion
Through the introduction of windshield and window frame installation structure and the analysis of CCAR25.571(e) (1) and 25. 775 (b), this paper discusses the compliance ideas, verification processes, and methods of airworthiness verification of windshield frame bird impact for transport aircraft.Based on the SPH method, the dynamic simulation analysis of bird impact is carried out, and the bird impact test of windshield through frame structure is carried out.The comparison between the analysis and the test results shows that the bird impact dynamics analysis method is reasonable and conservative.Based on the model established in this paper, the bird impact dynamics analysis results of other positions of the windshield frame are valid, which can verify that the bird impact performance of the windshield frame structure meets the airworthiness regulations, and provide a reference for the airworthiness compliance verification of bird impact on the windshield frame of transport aircraft.

Figure 1
Figure 1 Windshield and structure installation form Through the analysis of the installation and stress forms of the three types of windshields, their advantages and disadvantages are as follows:a) Pressure plate type: the windshield installed with the pressure plate type only bears the airtight load acting on it.The stress form of the windshield is simple.It is easy to install and disassemble.When the windshield is damaged, it will not affect the overall structure of the cabin cover.However, because the pressure plate windshield does not bear the load, it is equivalent to a large opening in the structure that needs to be strengthened, and the weight of the structure is relatively heavy.At the same time, the windshield is not directly connected with the window frame structure, and when all structural layers are damaged, the windshield may be separated from the structure under a pressurization load.b)Non-load-bearing bolted type: the windshield is directly connected to the window frame structure through bolts, which increases reliability.If the pressure plate and the windshield are connected together, the pressure plate can be designed to be relatively weak, and certain weight can be reduced.Its disadvantage is that the windshield does not bear the load, the structure has a large opening that needs to be strengthened, and there are many bolt mounting holes around the windshield, which has a certain impact on the life of the windshield itself.c) Load-bearing bolted type: The load-bearing bolted type windshield can transfer the in-plane load, and can be strengthened without following the design principle of a large opening in the structural design of the engine body, so as to effectively reduce the structural weight.At that same time, the rigidity of the window frame can be reduced, the section height of the window frame structure can be reduced, and the space in the cockpit can be increased.Its disadvantage is that the stress form of the windshield and the structure is complex, and when the windshield is damaged, it will have a certain impact on the structural force transmission of the canopy.In the process of type certification, airworthiness compliance verification is also complex.In regional aircraft, there are many applications for load-bearing bolt-on windshields, such as ERJ and CRJ aircraft, due to the large windshield opening relative to the overall aircraft nose size.Airbus series aircraft adopt the design and installation method of pressure plate, including the A380 and the newly developed A350 aircraft.The Boeing family of aircraft, from the B707 to the B787, uses a nonload-bearing bolt-on windshield.

Figure 2 Figure 3 Figure 4
Figure 2 Schematic diagram of aircraft nose angle of attack

Figure 5
Figure 5 Impact point selection for bird impact test of the windshield frame

Figure 7 Figure 8 Figure 9 Figure 10
Figure 7 Cloud Chart of Maximum Displacement of Windshield Window Frame after Bird Impact at Point A Figure 8 Cloud Chart of Maximum Displacement of Windshield Window Frame after Bird Impact at Point B

Table 1
Comparison of structural damage between test results and analysis results at impact point A

Table 2
Comparison of test results and calculation analysis results at impact point A