Aviation Safety Paper on Operations Under Meteorological Hazard with Early Detection

Reviewofcurrentresearchtrendsinbirdstrikeandhailimpactsimulationsonwingleadingedge.pdf

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Review of current research trends in bird strike and hail impact simulations on wing leading edge

Zdobyslaw Jan Goraj and Kamila Kustron Department of Aircraft Design, Warsaw University of Technology, Warsaw, Poland

Abstract Purpose – Bird strike and hail impact resistances are considered in relation to the fulfilment of airworthiness/crashworthiness regulations as specified by appropriate aviation authorities. Before aircraft are allowed to go into service, these regulations must be fulfilled. This includes the adaption of the wing leading edge (LE) structure to smart diagnostics and an easy repair. This paper aims to focus on the wing LE, although all forward-facing aircraft components are exposed to the impact of foreign object during the flight. The best practices based on credible simulations which may be appropriate means of establishing compliance with European Aviation safety Agency and Federal Aviation Administration regulations regarding bird strikes, together with the problem of collisions with hailstones, are overviewed in aspect of accuracy and computing cost. Design/methodology/approach – The best means of evaluating worldwide certification standards so as to be more efficient for all stakeholders by reducing risk and costs (time and money consuming) of certification process are recommended. The very expensive physical tests may be replaced by adequate and credible computer simulations. The adequate credible simulation must be verified and validated. The statistical approaches for modelling the uncertainty are presented in aspect of computing cost. Findings – The simulation models have simplifications and assumptions that generate an uncertainty. The uncertainty must be identified in benchmarking tests. Instead of using “in house” physical tests, there are scientific papers available in open literature thanks to the new trend in worldwide publication of the research results. These large databases can be efficiently transform into useful benchmark thanks to data mining and knowledge discovery methods and big data analyses. The physical test data are obtained from tests on the ground-based demonstrator by using high-speed cameras and a structural health monitoring system, and therefore, they should be applied at an early stage of the design process. Originality/value – The sources of uncertainty in simulation models are expressed, and the way to their assessment is presented based on statistical approaches. A brief review of the current research shows that it widely uses efficient numerical analysis and computer simulations and is based on finite element methods, mesh structure as well as mesh free particle models. These methods and models are useful to analyse airworthiness requirements for damage tolerance regarding bird-strike and hail impact and haves been subjected to critical review in this paper. Many original papers were considered in this analysis, and some of them have been critically reviewed and commented upon.

Keywords Benchmark, Bird and hail impact, Civil aviation system, Simulation modelling, Wing leading edge

Paper type General review

Symbols

PH = Hugoniot pressure; PS = steady-flow pressure, tD = total duration of the impact; L = length of impactor; u0 = relative impact velocity; X, mX = output from the RBD system and its mean value; Y, mY = corresponding output from the model and its

mean value; and VC = cruise velocity.

Acronyms and Abbreviations ALE = arbitrary Lagrangian Eulerian; AMC = acceptable means of compliance; AOG = aircraft on ground;

ASTM = American Society for Testing and Materials; CS = certification specifications; DAS = design assurance system; DEF STAN = defence standard; DP = damage prognosis; EASA = European Aviation safety Agency; FAA = Federal Aviation Administration; FAR = Federal Aviation Requirements; FEM = finite element method; EOS = equations of state; FOI = foreign object impacts; GASP = Global Aviation Safety Plan; GBD = Ground-based demonstrator; GUI = graphical user interface; ICAO = International Civil Aviation Organization; IBIS = ICAO Bird Strike Information System; JAR = joint aviation requirements;

The current issue and full text archive of this journal is available on Emerald Insight at: www.emeraldinsight.com/1748-8842.htm

Aircraft Engineering and Aerospace Technology 90/4 (2018) 602–612 © Emerald Publishing Limited [ISSN 1748-8842] [DOI 10.1108/AEAT-02-2017-0053]

The work described in this paper and the research leading to these results have received funding from the European Community’s Seventh Framework Programme FP7/2007-2013, under grant agreement no 604013, AFLONEXT project, 2013.

Received 9 February 2017 Accepted 3 March 2017

602

http://dx.doi.org/10.1108/AEAT-02-2017-0053

LE = leading edge; NASA = National Aeronautics and Space

Administration; QS = quality system; RASP = Regional Aviation Safety Group; SAE ARP = Society of Automotive Engineers, Aerospace

Recommended Practice; SHM = structural health monitoring; SMM = safety management manual; SMS = safety management system; SPH = smoothed particle hydrodynamics; SSP = state safety programme; WBA = World Birdstrike Association; and V&V = verification and validation.

1. Introduction

Modern airframes are subject to intensive research and testing to ensure they are both airworthy and crashworthy before being certified for operation. Laminated composites have an important application in the modern airframes because of their unique properties. The composite mass fraction for the newest Boeing-787 and Airbus 350 is more than 50 per cent (Gardiner, 2014). The integrity of forward-facing laminated composite

elements, including wing leading edges (LEs), is compromised by the impact of foreign objects (FOI). The wing LE must be protected in the aircraft design process against critical damage caused by FOI, in accordance with damage tolerance certification specifications, e.g. CS-25 under European Aviation safety Agency (EASA) (EASA, 2015) or FAR-25 under Federal Aviation Administration (FAA, 2017). Although exterior aircraft structures are exposed to various threats of the FOI, the statistics show that 90 per cent of all events are reported to be caused by bird strikes (Heimbs, 2011; Khan et al., 2010; Hedayati and Sadighi, 2016). Bird strike is the significant problem for worldwide aviation. The worldwide cost of bird strikes exceeds billions of US dollars per year, and only about 20 per cent of bird strikes are reported by the staff. Even after the penetration of the LE, there should be no

damage to the front spar, thus ensuring safe flight and landing after impact. Additionally, the high lift devices of the wing which are mounted on the LE must be bird- and hail-resistant to the ensure that the take-off and landing phases of flight are safe while these devices are extended. Although bird strikes may occur during any phase of flight, they are most likely during take-off, initial climb, approach and landing phases because of a greater number of birds in flight at lower levels. As most birds fly mainly during the day, most bird strikes occur in daylight hours (Kulbi, 2014). Although bird strikes can affect the safety, the majority of

them cause damage that does not affect the safety but generate high costs as the result of the downtime for inspection and repair. Comparable problems exist for hail impact. Although hail impacts are rare, they can cause significant losses to the operator because of non-planned maintenance (aircraft on ground) for technical reasons. The wing LE and its high lift devices should be optimized with respect to the impact resistance to improve the effectiveness of maintenance.

Effective maintenance includes damage detection by using smart diagnostics, as well as easy repair. Bird and hail strikes are complex, strongly nonlinear and

impact at high velocity. The impact parameters are such that the projectile is considered as soft body and the wing LE as the target being a rigid body.

2. Bird strike and hail impact in the worldwide system

Worldwide efforts on reduction of environmental threats to airframes in service are a part of the complex global system of safety management in civil aviation. The initiatives to prevent bird and hail strikes are assisted by the International Civil Aviation Organization (ICAO). ICAO has been collecting bird strike data since 1965. The ICAO Bird Strike Information System (IBIS) has been in operations since 1980 and provides statistical analyses of bird/wildlife strike reports received from different States (Pinos, 1996). The analysis of this data reveals that approximately 90 per cent of bird strikes occur on or in the immediate vicinity of airports during the take-off, initial climb, approach and landing phases of flights, and one bird strike occurs in 2,000 flights. The need to solve this worldwide common problem has directed all efforts into setting up unified norms on the bird strike (wildlife) risk reduction to aviation as the result of experiences in all regions around the world. Although bird strike has been considered since the earliest

years of aviation history as a very important problem from the safety point of view, an international bird strike association was established in 2012 as the World Birdstrike Association (WBA). WBA is the international forum and association of bird strike committees and wildlife experts in aviation safety around the globe. This problem is part of the WBA Global Action Plan. The WBA mission is to be the global voice of the national bird (wildlife) strike committees and all other entities, civil and military. Additionally the WBA provides the platform for pursuing a constructive and cooperative relationship with all stakeholders (WBA, 2016). Currently a white paper, how to set up a certification system, is in preparation to assure the quality of personnel and products used for wildlife strike prevention and for the evaluation of training standards (WBA, 2016). Regarding safety problem in the widest sense, the ICAO is

promoting a new philosophy of comprehensive safety management, namely, the safety management system (SMS) provided by Annex 19 of the Chicago Convention that contains overarching provisions applicable to safety management functions related to, or in direct support of, the safe operation of aircraft. It highlights the importance of safety management at the State level across multiple aviation domains. The new Annex complements existing ICAO safety-related programmes and activities, including the revised Global Aviation Safety Plan, the Regional Aviation Safety Groups and ICAO’s safety management training offerings. With air traffic projected to double by 2025, this Annex supports the continued evolution of a proactive strategy to improve safety performance; the foundation of this proactive safety strategy is based on the implementation of a state safety programme that systematically addresses safety risks (ICAO, Safety Management Manual SMM, 2013).

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SMS means the complete established and clearly implemented security risk management and security infrastructure operating systems that emphasize the importance of safety management in a proactive way, and focusses on accident prevention and safety planning, undertaking risk information collection, analysis, monitoring and troubleshooting. At present, the civil aircraft design and manufacture

organization with type certification, production certification and airworthiness certification for its aircraft are the fundamental and mandatory requirements of national civil aviation authorities to ensure the safety and airworthiness of civil aircraft and to allow it to enter the aviation market. According to these regulations, civil aircraft design and manufacture organization must implement a design assurance system (DAS) for aircraft design and quality system (QS) for aircraft manufacture, fault/defect information report system and unsafe conditions correction and continuous improvement programmes, such as airworthiness directives and service bulletins for continuous operation for these aircraft. It is important to consolidate all requirements regarding the impact issues and assure impact-resistant structure in accordance with the damage tolerance strategy. There are no essential differences between the current

airworthiness management systems and SMS. It is feasible for aircraft type design and manufacturing organization with established current DAS and QS to establish SMS (Li et al., 2015). SMS has become a powerful means to effectively improve the overall safety level of complex aerospace systems in which the design of impact-resistant (including bird and hail strike) structures play an important role. All activities recommended for safety management in a global sense have been stated (ICAO, Safety Management Manual SMM, 2013).

2.1 Need to improve bird and hail reporting system Statistics of the bird strike threats are very selective because there is no objective and complete worldwide method of reporting or analysis. It is because there is no obligatory reporting to ICAO’s IBIS. WBA has been in existence for too short a time to obtain meaningful results. Systems have to be improved. It is apparent that data on bird strikes and other kinds of impacts including hail impacts need to be collected to better understand the dynamics of the bird/hail strike problem and to set up more efficient procedures to prevent damage. It is very important to study the efficient structural implications of impact absorption.

2.2 Risk mitigation of bird strikes and hail impacts Within the context of aviation, safety is (ICAO, Safety Management Manual SMM, 2013):

The state in which the possibility of harm to persons or of property damage is reduced to, and maintained at or below, an acceptable level through a continuing process of hazard identification and safety risk management and the risk mitigation is defined as (ICAO, Safety Management Manual SMM, 2013).

“The process of incorporating defences or preventive controls to lower the severity and/or likelihood of a hazard’s projected consequence”. One can mitigate the risk of such impacts and unforeseen events with the associated losses by the application of active and passive safety measures.

Paradoxically, the mitigation and/or elimination of some threats triggers other or intensifies existing ones, such as collisions with birds. Reducing the noise in the vicinity of airports and growing traffic, comprising greater numbers of quieter aircraft, and the increase in bird populations intensifies the bird strike risk. Greater effort is required to control and monitor birds and all wildlife movements on and within the vicinity of airports. There are four ways to reduce the effect of the bird strike:

1 The aircraft can be designed to be bird strike-resistant. 2 The aircraft can have lower speed during critical phases of

flight. 3 The birds can be moved out of the way of the aircraft. 4 The aircraft can be moved out of the way of the birds.

Regarding hail impact, there are three ways to reduce this unforeseen effect: 1 The aircraft can be designed to be hail strike-resistant; 2 Efficient weather radars must be used on-board to

mitigate the hazardous regions. 3 Efficient weather radars and other systems can be used on

the ground and in the air space.

2.3 Regulatory requirements regarding bird and hail strikes There are two different strategies (Njå et al., 2012) from the regulatory side used for bird strike risk mitigation in aviation. The first strategy relates to the design of the forward-facing aircraft components to meet resistance to impact in order for them to become them resilient and less vulnerable to the incidents concentrating on how to strengthen aero-structure designs to minimize damage and ensure safety. It includes the activities related to passive safety. The second strategy is focussed on reducing aircraft exposure to bird strikes. As the problem is mainly related to take-offs and landings, the strategy concentrates on the particular conditions of airports and the need to apply the active safety solutions. The first case is under consideration in this paper. The airframe of a modern commercial aircraft (CS-25/

FAR25) is designed to be as light as possible to reduce fuel consumption. But, it must be damage-tolerant, including an impact sensitivity to ensure structural integrity to perform with a high level of safety. Among all loading modes on an airframe during a flight, the forward-facing airframe components, including wing LE, are possibly the most exposed to bird strike and hail impact. Currently, the certification specifications in relation to bird

strikes are couched in very general terms. There is no disparity between the FAR and JAR requirements applicable at present. However, the authorities only define the mass, not the species of bird to be used for testing. The requirements including bird size and impact speed are specified, and the same is applied for a wing LE. That requirement determines the kinetic energy which must be absorbed or dissipated by the LE’s skin. The problem with use of aluminium crashworthy

components is that for the fulfilment of the safety requirements, in most cases, the thickness has to be increased, causing a significant structural weight penalty. Alternatively, composite laminate materials are introduced in the high-energy-absorbing wing LE designs (Labeas and Kermanidis, 2006).

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EASA’s requirements for assurance of an airworthy wing LE for large aeroplanes in accordance with bird strike prevention in design are given in certification specifications CS-25. In detail, the CS 25.571 (titled: Damage tolerance and fatigue evaluation of structure) sets the requirements for damage tolerance (discrete source) evaluation: “The aeroplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of bird impact as specified in CS 25.631”. CS 25.631 (titled: Bird strike damage) states:

The aeroplane must be designed to assure capability of continued safe flight and landing of the aeroplane after impact with a 4 lb bird when the velocity of the aeroplane (relative to the bird along the aeroplane’s flight path) is equal to VC at sea-level or 0.85 VC at 2438 m (8000 ft), whichever is the more critical. Compliance may be shown by analysis only when based on tests carried out on sufficiently representative structures of similar design.

The acceptable means of compliance with these requirements AMC 25.631 (titled Bird Strike Damage) states:

Consideration should be given in the early stages of the design to the installation of items in essential services, such as control system components, and items which, if damaged, could cause a hazard, such as electrical equipment. As far as practicable, such items should not be installed immediately behind areas liable to be struck by birds.

It was confirmed (Atkins and FERA, 2009) that given the reported level of accidents, the bird strike requirements in CS- 25 are providing an adequate level of safety. The report was commissioned in 2009 by EASA. It investigated the adequacy of the current aircraft certification requirement in relation to current and future bird strike risks on aircraft structures and windshields. The bird strike data presented in this report cover bird strikes reported in USA, Canada and UK from 1990 to 2007. Unfortunately, it has not been possible to obtain data from other countries via ICAO, but the data obtained did provide an adequate basis for analysis – approximately 11,000 incidents for which complete data on aircraft type, speed and bird species were available. The study also reviewed worldwide accidents and serious incident data given by Atkins and FERA (2009). In 2012, EASA published a certification memorandum on

compliance with CS-25 bird strike requirements. Regarding CS 25.631 requirements, the memorandum explains the phrase “continued safe flight and landing”, which may be interpreted in different ways. The effects of bird strike are also addressed in various other sections of CS-25. Regarding LE, it states CS 25.571(e), which requires that the aeroplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of bird impact as specified in CS 25.631. The AMC to 25.571 (in paragraph 2.7.2) specifies the loads associated with “get home” conditions that have to be met for this case. CS 25.629 which requires freedom from aeroelastic instability throughout the envelope described in CS 25.629 (b)(2) for any damage or failure condition, required or selected for investigation by CS 25.571 and any damage, failure or malfunction, considered under CS 25.631. From the other side the AMC25.631 draws attention to consideration of the location and installation of items, systems and equipment in relation to bird strike. In AMC 25.1309(b), a bird strike is identified as a particular risk requiring investigation as part of the common cause analysis. As demonstrated by in-service events and during several certification programmes, there is a need to further clarify

EASA’s expectations when showing compliance to the bird strike requirements, as follows. The EASA Certification Policy contained in Section 3 of this Certification Memorandum provides an overview of typical aircraft areas/zones prone to bird strike which normally are considered, including wing LE. The EASA Certification Policy also recognizes that for flaps, slats and landing gears, a lower impact speed than Vc has been accepted in the past as more appropriate for these items in the case that the deployment speed is limited because of certain (placard) restrictions (EASA CM, 2012). However, there are no existing requirements regarding the

certification with respect to how the hail impact threats to LEs (Leading Edges) of a composite wing. Although there is recognition that aircraft are under threat from hail and guidelines exist for design against impact, there is little detailed information related to the qualification requirements to demonstrate the ability to meet the hail threat. DEF STAN 00-35 provides the basics for two hail tests. The first of these uses clusters of small hailstones to determine the erosion resistance of materials. This is the only time a multi-impact test is defined in the standards reviewed. Ideally, measured data should be used to define the test parameters as much depend on the aircraft speed and global location. However, in the absence of measured data, the standard identifies that a rain intensity of 25 mm/h with a predominant particle size of 5 mm and stone density 0.9 g/cm�3 should be used. The second test is used to determine the structural performance of materials when impacted at high velocity with a single large hailstone. It is noted that a standard test cannot be defined, and that the test conditions should be defined on a case by case basis. Where actual data for the maximum hailstone likely to be encountered are not available, a default size of 19 mm diameter (considered to represent a 1 per cent risk of being encountered in flight through 160 km flights cells) should be used. The standard also describes the facilities that can undertake the work and the basic test procedures. Another hail test is defined in ASTM-310. This document defines an impact test involving a single ice ball. The standard has been designed primarily for determining the impact resistance of aerospace transparent enclosures but is recommended for wing LEs. Ice balls of diameter 0.5, 1.0 and 2.0 inches are used at velocities ranging from 200 to 2,000 ft/s, depending on aircraft type. The probability tables can be used as an estimate of the worst case probabilities of intercepting hail of different sizes for use in designing aircraft structures. The tables also indicate that multiple hits from smaller hailstones are potentially a problem that aircraft designers need to consider (Field et al., 2010).

3. Random nature of bird and hail strikes

The bird strike and hail impact must be considered as random phenomena due to the differences in size, shape, geometry and material properties of projectile and target. Interactions between projectile and target, weather conditions, parameters of projectile as its weight, anatomy/structure, angle of impact, impact speed, location with respect to the target, phase of flight – all these factors influence on damage severity of the structure and on aircraft’s ability to fly and to land safely. There are many uncertainties which contributed in a global uncertainty. Additional bird strike collision can involve one or more birds and

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hail strike is always multi-impact. Figure 1 shows the random features of bird strikes. Hail strike warning and protection is mainly performed by

weather radar implementation that allows to move the aircraft from the hazardous regions. Uncertain regions that are difficult to identify can exist. Future aircraft may be equipped with radar capable of warning of a certain type of potentially hazardous icing conditions at high altitude (NASA official website, 2016). It is inevitable that an aircraft will pass through a hailstorm at

some stage during its lifetime and therefore will be subjected to some level of hail impact damage which depends not only upon the mass, velocity and impact angle of the hailstone (and indeed the number or intensity of hailstones) but also upon the geometry and material of the structure that is being impacted (Field et al., 2010).

4. High-velocity impact characterization

At normal speeds, during a perfectly inelastic collision, an object struck by a projectile will deform, and this deformation will absorb most or all of the energy due to the force of the collision. Viewed from a conservation of energy perspective, the kinetic energy of the projectile is changed into heat and sound energy, as a result of the deformations and vibrations induced in the struck object. However, these deformations and vibrations cannot occur instantaneously. A high-velocity collision does not provide sufficient time for these deformations and vibrations to occur. Thus, the struck material behaves as if it were more brittle than it would otherwise be, and the majority of the applied force goes into fracturing the material. Another way to look at it is that materials actually are more brittle on short time scales than on long time scales: this is related to time- temperature superposition. Impact resistance decreases with an increase in the modulus of elasticity, which means that stiffer materials will have less impact resistance. The impact process on a soft body consists of four main phases (Figure 2). First phase is shock at contact and second is impact shock decay,

where the peak pressure is observed (Hugoniot pressure: PH = r0u0uS, uS = c0 1 kup, where r0, u0, uS, c0, k and up are the initial density of the impactor, the projectile’s initial velocity, the shock velocity, sound speed, constant for material and particles velocity, respectively), third is steady flow (steady-flow pressure: PS ¼ 12 r0u20) and phase pressure decay (Wilbeck, 1978; Heimbs, 2011). The high-velocity impact process generates stresses by

deceleration of the projectile that greatly exceeded the yield stress. This behaviour is typical for hydrodynamic regime, and the projectile can be treated as a fluid. In that case, the material density dominates the behaviour of the parts instead of material strength. The impact energy that must be dissipated in the collision is

currently considered approximately the relative kinetic energy of the impactor. Therefore, the speed of the aircraft is much more important than the size of the impactor when it comes to reducing energy transfer in a collision. The body density of the impactor is also a parameter that influences the amount of damage.

5. Numerical simulation of bird and hail impacts on wing leading edge

As previously discussed, the collisions with the birds or hailstones during flight can lead to serious damage of the forward-facing components, including wing LE. It is important to design the wing LE to be resistant to critical damage, and any damage must be easy detectable and repairable. Additionally, the aviation authorities require that wing LEs need to prove a certain level of bird strike resistance in certification tests before the aircraft is allowed into service. Certification for bird strikes is mainly achieved by experimental full-scale test. For these tests, real birds have to be used, typically dead or sedated

Figure 1 Random nature of bird strike resulting in multisite damage as a result of multi-bird strikes during approaching phase of flight

Figure 2 Typical pressure curve for normal soft body impact on a rigid plate with stages according to Wilbeck model, based on previous studies by Wilbeck (1978) and Heimbs (2011)

Hugoniot Pressure

Shock Wave

Release

Steady-flow Pressure

time

I II III IV P

re ss

ur e

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chickens (ASTM standards), but the use of real birds is not ideal because of the large scatter between individual tests. Computational methods have been used since the early 1980s for bird strike-resistant designing, as they are more efficient than the expensive physical certification tests with real birds. But their accuracy has to be assessed regarding all simplifications and assumptions for modelling of an impactor, a target and dynamic reactions, as well as verification and validation, by using experimental test data. The acceptable level of uncertainty for certification is equal to the accuracy understanding here as an adequacy that is demonstrated in the validation process. The accuracy is appropriate if the results are between credible mathematical model and the approved simulation model.

5.1 Historical tests of soft body impact on rigid target The current validations of the subjected simulations are based on very limited test data. It is very important to underline that in almost all cases, the experimental studies of Barber et al. and Wilbeck from the late 1970s (Wilbeck, 1978) have been used for validation of the simulation models. These impact tests have been conducted on a rigid plate with pressure transducers mounted on the surface, using different impactors such as real birds, beef, neoprene, rubber, gelatine and porous gelatine. The impactors of different weights, except the real birds, had a cylindrical shape and velocities of 100-300 m/s. The pressure– time curves, measured by the centre pressure transducer with a sampling frequency of 300 kHz, were taken as the reference curves for numerous model validations. However, uncertainties of these 30-year-old test data arise from the fact that the piezoelectric quartz pressure transducers used in these studies were not designed for transient impact loads as they had no adequate acceleration compensation with noise occurring in the curves. Furthermore, their resonance frequency was about 300 kHz, leading to resonance-induced over shots in some of the curves. The accuracy of the measurements was also limited by the finite frequency response of the transducers, which prevented measurements of rise times of less than 5 ms. Also, the distance between the gas gun and the pressure plate is not known, and no information is given regarding whether the geometrical consistency of the fluid-like impactor was maintained at the time of impact or if the impactor was already distorted during the time of flight. Lavoie et al. (Heimbs, 2011) stated that the experimental pressure data by Wilbeck (1978) should only be seen as a reference for the general behaviour rather than as a tool for evaluation.

5.2 Modelling of a soft impactor The bird species of the projectile may be varied (having different body shapes or densities) because only mass is defined by the certification regulations. Typical artificial substitutes for birds have a simplified regular geometry such as a cylinder, a cylinder with hemispherical ends, an ellipsoid or a sphere, representing the torso of the bird. The impact tests of bird strike were conducted with different bird-like materials (soft material substitutes that have the specific gravity of water, producing loading profiles similar to those of real birds) such as wax, foam, emulsion, beef, rubber and neoprene. Based on the tests of comparison between the behaviour of each of the substitutes with real bird, it was concluded that gelatine, or

porous gelatine, reproduced the behaviour of a real bird with a high degree of accuracy. For this reason, gelatine is the most frequently used bird material substitute in both experimental tests and numerical simulations (Heimbs, 2011). Many studies have been conducted to develop constitutive models for birds to improve the numerical simulation results summarised previously (Liu et al., 2014). Advanced research in the influence of bird geometry from primitive to that based on CT scans of real mallards, multi-element and multi-material models have been discussed (Hedayati and Sadighi, 2016). The closest results of the Hugoniot and the steady pressure distributions with Wilbeck experimental data were demonstrated for the mallard bird model. Different body parts of the bird and its orientation have significant influence on the impact process. For the bottom-side impact, the high pressurized area is distributed over a larger area that shows its higher danger in bird strike events. The head-side impact and the tail-side impact are the second and third most dangerous scenarios for the mallard bird model, respectively, and the wing-side impact shows the lowest pressures. Interestingly, in the wing-side impact, the existence of wings has led to central pressure peaks even lower than other points of the target. Nonlinear finite element method (FEM) codes have the

capabilities of predicting the loads and deformations of both the soft impactor and the complex aircraft components with acceptable levels of accuracy. Benchmark based on Wilbeck’s physical model and literature simulations’ models are used to assess the accuracy and the precision. The results must be between the test results and approved simulations. Currently, there are few professional commercial explicit FEM codes dedicated for researchers. These codes are predesignated to use for the analysis of highly nonlinear behaviour of materials with inelastic strains, high strain rates and large deformations, similar the situation of a crash, and they offer different approaches to bird modelling by using mesh-based methods such as the Lagrangian approach, techniques based on Eulerian or arbitrary Lagrangian Eulerian approach and more recent solvers based on meshless smoothed particle hydrodynamics (SPH) method (Dede and Kayran, 2015). The studies of a soft body impact on composite structures using the SPH method combined with a material law for hydrodynamic solid to simulate the bird strikes or hail impact, have achieved the best correlations with tests in comparison to all above mentioned methods. As gelatine was used for the impactor material, the elastic-plastic contribution to the material behaviour was neglected and the material model reduced to the equations of state for pressure. Prato et al. (2014) and Prato et al. (2015) evaluated the

hybrid mesh-meshless approach. The investigation of the effect of the use of different approaches to model the soft impactors has a relevant rule in the study of the optimal method to be used to reduce real impact event. Comparison of FEM with SPH model with and without the activation of the failure of Lagrangian elements has been investigated, and the observed behaviour in researched cases is almost the same as if a failure criteria is not used, but it seems that using hybrid mesh- meshless model has to be more effective, regarding the comparison with real test if a failure criteria is instead applied. Instead of considering a failure mode, a pick of contact force has been observed during the first instant of impact. That has

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been probably caused during the switch from Lagrangian to SPH when the SPH contact force produced by the particles appeared higher than the one produced by the simple SPH model (Prato et al., 2014). A proper calibration of the hail definition has been postulated (Prato et al., 2015), and a correct correlation can be achieved.

5.3 Finite element method packages characterization Analysing a physical problem in the FEM packages is done in three main steps: 1 pre-processing, where the FEM model that is going to be

solved is prepared; 2 solution, where the prepared FEM model is solved;

additional controlling parameters are usually used in this step to avoid solution instabilities; and

3 post-processing, where the quantities of interest are obtained from the solved problem, such as stress, strain, displacements, velocities, accelerations, resultant forces and visualization.

The solution step is usually completely handled by the FEM package, and therefore, the user does not have to deal with it. In some FEM packages (e.g. ANSYS and ABAQUS), all these three steps are contained in a single graphical user interface (GUI), whilst in others, the user has to use different programmes for each step, (e.g. in LS-DYNA, which is the most prevalent programme in bird strike analysis, the solution is separated from the pre- and the post-processing). In some cases, users may prefer other more professional programmes for the better handling of a specific step. For instance, some users implement the programme HyperMesh for the more efficient discretization of very complex geometries. In a pre-processing procedure, the two- or three-dimensional computer-aided design (CAD) model of the physical object has to be performed. In very complex geometries, the geometrical model can be prepared in a professional CAD programme and then exported into the FE pre-processing programme (Hedayati and Sadighi, 2016).

5.4 Validation of the numerical simulation Although the final certification test must be performed on the real structure, the optimisation design process could contain the numerical simulations (Figure 3). Simulation programmes must be verified and validated (Figure 4). Validation is the process of determining whether the conceptual model accurately represents the real system. Verification is the process of determining whether a computer programme (chosen or built for simulation) works as intended (sometimes it means debugging of the computer programme). Numerical simulations provide an invaluable tool to evaluate

design requirements and to reduce the costs of the physical tests. The results demonstrated in the bibliography regarding the

numerical simulations are generally in good agreement with the experimental values, demonstrating the robustness of the developed simulations in supporting the design of bird strike- and hail strike-resistant aircraft structures. But current validations are based on very limited test data. In almost all cases validations use, available as open source, the impact tests based on experimental studies of Barber et al., see (Wilbeck, 1978), as

discussed in Section 5.1. Although more data were generated by the bird strike group of the Group for Aeronautical Research and Technology in Europe (Heimbs, 2011), those results are not available to the public.

5.5 Structural health monitoring in certification tests and operation Safety and maintenance cost plays a significant role for manufacturers and operators. Both aspects should be under consideration, sometimes, they should be traded-off. Maintainability must include detectability and repairability. Inspection and maintenance account for a large portion of total life cycle costs of an airliner. The cost is estimated from 17 per cent even to 29 per cent and depends on the type of aircraft, strategy of air operator and implemented technology. They generate the efforts of aviation industries and academia to improve the diagnostic systems integrated with the aircraft structures, which gives the possibility identifying the hazard of damage based on the structural health monitoring (SHM) and damage prognosis (DP). They generate the evaluated processes into repair techniques, too. Effective assessment during flight is possible by using smart technology based on embedded optimal sets of the sensors of SHM and DP. Monitoring the propagation of guided waves is effectively used to assess the health of the airframe structure. Rayleigh–Lamb waves are particularly suitable for thin-walled structures such as a shell of wing LE. Piezo sensors/actuators can pitch and catch such waves; therefore, they can be effectively used in the benchmark for assessment the health of the structure. The cross-industry guidelines for the implementation of

SHM for aerospace applications have been created as a SAE

Figure 3 Process of performing the simulation model of the real system of bird and hail strikes on wing LE (prepared for AfloNext, AFLONEXT, 2013)

• SIMULATION MODEL

• PHYSICAL SYSTEM

• REAL SYSTEM (Bird and Hail Strikes

on the Wing

YSTEM Hail Strikes

Wing

IMPACT PROCESS

• PHYSICAL SSSSYSTEM

PROJECTILE (BIRDs , HAILSTONEs)

/TARGET (WING LEADING EDGE)

INTERACTIONS

GROUND-BASED DEMONSTRATOR

ULATION ODEL

VALIDATION & VERIFICATION

EKSPERIMENTAL TESTING

Leading Edge)

Figure 4 Process of validation and verification of the simulating programme (AFLONEXT, 2013)

Real System Bird and Hail Strikes on the Wing Leading Edge

Conceptual Model

Simulation Program

Validation Verification

Validation

Verif

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International Aerospace Recommended Practices document: SAE ARP 6461 “Guidelines for Implementation of SHM on Fixed Wing Aircraft” (SAE ARP, 6461, 2013). These guidelines have brought together manufacturers, operators/ users, systems integrators, regulators, technology providers and researchers to produce information on the integration of SHM into aircraft maintenance procedures, generic requirements and advice on verification, validation and airworthiness.

5.6 Verification and validation benchmarks Benchmarks are useful tools that can effectively aid verification and validation (V&V) processes of a simulation model. Strong- sense benchmarks can be viewed as engineering reference standards with a high-enough quality (Oberkampf and Trucano, 2007). Benchmarks are performed as GUI in Matlab®. Matlab is one of the most advanced and most popular high-order programming languages. Also, it is user- friendly for creating own code and exporting it out of the programme environment. It can effectively support the work with the other professional software, such as Abaqus and LS- Dyna. As it has been expressed in introduction to this section, there

are a lot of sources of uncertainty in the process of creating simulations. Each source of uncertainly must be verified using appropriate benchmarking tests. The numerical simulation of bird strike and hail impact on

composite wing LE is recorded using high-speed camera, which will be next used for image recognition and then for pattern recognition in SHM system and for benchmark V&V. The benchmarking tests, available by use GUI developed in Matlab® in accordance with the algorithm shown below, consist of techniques for verification and when the model is verified, then techniques for validation can be applied. The techniques for verification should include good

programming practice for writing and debugging the codes by using “structured walk-through”, “trace” and the comparison of final simulation output with analytical results based on the open literature described in the previous sections. Next, the three-step approach for developing a valid and

credible model has been created. As input data for validation process the data from physical tests on the GBD will be used. The requirement for the model is according to ASTM standards. Data for the final validation are obtained from the high-speed cameras and SHM system based on passive-active piezo-sensors that are recommended to use. The minimum of an effective frames’ number for validation of the simulation model of the projectile with known impact process duration can

be estimated from the data available in the open literature (Table I). The total duration tD of the impact can be estimated as the time needed for the impactor to flow through its own length L by the relative impact velocity, i.e. tD ¼ Lu0. Next, the benchmarking test is used to choose the best SHM

system for pressure and after impact damage for an assessment. For assessment, the pressure passive system of piezo-sensors is verified in accordance with the standard SAE ARP 6461 as the suitable guidelines for implementation of SHM on wing LE. The three-step approach for developing a valid and credible

model starts with the development of the model with high face validity that seems reasonable to people who are familiar with the system under study. A cornerstone for establishing the credibility of a computer simulation is defined as an effective communication between the builder of a simulation model and its user (Schlesinger et al., 1979). This step has been supported by the discussions with system experts. In the second step, the system is observed and intuitively verified with the interactions with the stakeholders on a regular basis throughout the process. Performing the so-called structured walk-through the conceptual model and the sources of possible uncertainties correlated with assumptions were created. Testing the assumptions of the model was assessed empirically. In this step, the assumptions made in the initial stages of model development are quantitatively tested. The theoretical distribution based on the available models in Matlab® from the performed data is fitted to the observed data, and graphical methods and goodness of fit tests will be used to test the adequacy of the fit. Sensitivity analysis can be used to determine if the output of the model significantly changes when an input distribution or when the value of an input variable is changed. If the output is sensitive to some aspects of the model, that aspect of the model must be modelled very carefully. The last step consists of the assessment of how representative simulation output data are determined. The most definitive test of a model’s validity is determining how closely the simulation output resembles the output from the real system. The Turing test can be used to compare the simulation output with the output from the real system. In the classic Turing test, it is proposed that the computer needs only to imitate a human to be a “thinking machine” regardless of how these outputs are generated. This method is classified as an artificial intelligence and is popular and available in the professional software dedicated for researchers. The output data from the simulation can be presented to knowledgeable persons about the system in the same exact format as the system data. If the experts can differentiate between the simulation and the system outputs,

Table I The estimated number of effective frames for validation process of bird strike and hail impact and for real and substitute materials

Performance of the high-speed camera (no. of frames per second)

Real bird Artificial substitute of bird Real hailstone Substitute of hail

Duration (ms)

No. of effective airframes

Duration (ms)

No. of effective airframes

Duration (ms)

No. of effective airframes

Duration (ms)

No. of effective airframes

4,000 8-12 32-48 4-20 16-80 �1,2 4 1-3 4-12 6,000 48-72 24-120 7 6-18 10,000 80-120 40-200 12 10-30

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their explanation of how they did that should improve the model. Statistical methods are available for comparing the output

from the simulation model with that from the real-world system. Two simple approaches for comparing the outputs from the real-world system with the simulation outputs are recommended: the inspection approach and the confidence- interval approach, which are the most suitable statistical methods for comparing observations of the GBD impacted by birds and hailstones with performed simulation output data. The inspection approach runs the simulation model with

historical system input data, including actual observed inter- arrival and service times instead of sampling from the input probability distributions, and compares the system and model output data. The system and the model experience exactly the same observations from the input random variables. This approach results in model and system outputs being positively correlated. Inspection approach, so-called trace driven method (available in Matlab, LS-DYNA), may provide valuable insight into the adequacy of the simulation model for some simulation studies. In fact, this may be the only feasible approach because of severe limitations on the amount of data available on the operation of GBD system. Second statistical approach, the confidence-interval approach,

is referred to as lower/upper confidence bounds or limits and is a more reliable approach for comparing a simulation model with the GBD system but requires a large amount of data. Essentially, a confidence interval is calculated for one of the model’s response variables, and if that confidence interval contains the known or observed value for the simulant for the same response variable, the model is considered valid for that response variable (Petty, 2017). In this approach, one can collect m independent sets of data from the system and n independent sets of data from the model, and m and n can be equal. In this approach, the random variable Xj is the average of observations in the j-th set of system data with mean mX = E(Xj) and random variable Yj is the output from the j-th replication of the simulation model with mY = E(Yj). Next step has an objective to build a confidence interval for subtraction of the mean values, which is equal: z = mX – mY. In the case of correlated outputs, where Xj is correlated with Yj (e.g. using trace driven simulation), m = n and pair Xj ‘s and Yj’s. For the subtraction of the random variables Xj and Yj, i.e. Zj = Xj �Yj (for j = 1, 2 . . ., n), and for these subtractions, each Zj’s is an independent and identically distributed random variable than mean value E(Zj) = z . An approximate 100 (1 – a) per cent confidence interval which includes the true value of the population parameter is a set described as follows:

Z nð Þ 6 tn�1;1�a=2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Var Z nð Þ

� �q , where �Z nð Þ5

Pn j51

Zj=n and

Var �Z nð Þ � �

5 Pn j51

Zj � �Z nð Þ � �2

=n n � 1ð Þ. If the confidence interval does not include a zero, then the observed difference between mean values mX and mY is statistically different at level a.

6. Conclusions

Experiments and/or numerical simulations are typical design tools in optimizing aircraft structure against the bird and hail strikes. Nowadays, advanced engineering software allows modelling highly nonlinear phenomena. This paper is primarily

focussed on summarizing current research trends in bird strike and hail impact simulations on LE and on expectations of all stakeholders in civil aviation that safety must be increased. Aviation authorities require that wing LEs need to prove a certain level of bird strike resistance in certification tests before the aircraft is permitted to go into service. Certification for bird strikes is mainly achieved by experimental full-scale test using chickens in accordance with ASTM standard. These tests can be partially replaced by credible computer simulations. The cornerstone for establishing the credibility of a computer simulation is defined as an effective communication between the builder of a simulation model and its potential user. That is why the simulation model must be verified and validated at each step of modelling based on benchmarking tests. However, current validations are based on very limited test data, often using open sources of the experimental studies from the late 1970s. Although more data have been generated, the results are not available to the public. Predictive numerical modelling of the impact on LEs is an

extremely challenging problem. There is an increasing need to reduce the weight of composite airframes while improving its penetration resistance to enhance aircraft performance and protection. The composite LE resistant to bird and hail impacts, the smart LE (self-diagnosed and easy-repaired) and new certification standards based on the benchmarking tests are of relevance and have been discussed in this paper, especially in the aspects of risk and costs reduction. The costs (both time and money) are significant and estimated at about 20 per cent of the life cycle costs and have been discussed in this paper too. The bird strike and hail impact must be considered as

random phenomena due to the differences in size, shape, geometry and material properties of projectile and target, interactions between them and environmental influence, weight, anatomy/structure, angle to flight path, impact speed, location to target of the bird or hail, region and phase of flight when the bird or hail strike occurs and the effect of the damage on the aircraft’s ability to fly and to land safely. The mesh and meshless methods used in modelling of soft impactors possess some pros and cons. It was found in benchmarking test by comparison mesh, meshless and hybrid models that the hybrid mesh-meshless model is the most effective. The evaluation of this model is needed, and a proper calibration of the hail definition has been postulated, and then a correct correlation is expected to be achieved. The sources of data for benchmarking tests, except “in

house” physical tests, can be taken from open literature. The algorithms of benchmarking tests for verification and validation have been described in detail. Statistical methods for comparing the observations of GBD with simulation output data are used in the validation process because of random nature of the data. Data for the final validation are obtained from the high-speed cameras and smart diagnostic system. The problem of the effective frames’ number for validation of the simulation model has been discussed. The validations while designing, based on two statistical approaches of inspection approach and confidence-interval approach, are recommended. These statistical approaches are simple but computational cost-effective and can help validate simulation model during designing the resistant to bird and hail impacts

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with acceptable accuracy. The accuracy is understanding here as adequacy. The accuracy is appropriate if the results are between credible mathematical model and the approved simulation model. Some recommendations for improving the regulation

requirements should concern the problem of detailing the dimensional and material properties for the anatomy of birds and set the validation benchmarks of simulations. It is valuable, because gathering or/and improving unified data for benchmarks mitigate uncertainty of certification tests and indirectly increase flight safety.

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