Preliminary Safety Assessment of Circular Variable Nacelle Inlet Concepts for Aero Engines in Civil Aviation

Preliminary safety assessment of circular variable nacelle inlet concepts for aero engines in civil aviation

S. Kazula, D. Grasselt, M. Mischke & K. Höschler Brandenburg University of Technology, Cottbus, Germany

ABSTRACT: A safe design process and its application are introduced to a concept study for circular variable aero engine inlets. The paper highlights the tasks of inlets, the compromise in designing them and how using variable inlets could solve this compromise and allow for faster and more efficient commercial aircraft. However, high safety and reliability requirements bring up disadvantages. Tackling these disadvantages, a systems engineering approach is complemented by a safety assessment process, according to Aerospace Recommended Practice ARP 4754A. Safety methods that are applicable during early phases of the product development process are presented and applied to develop feasible variable inlet concepts. Hence, safety requirements, potential failure events and resulting failure modes are systematically identified, assessed and mitigated. The mitigation of a failure condition by the means of redundancy within the adjustment control system is presented.

NOMENCLATURE systems is the inlet that supplies the aero engine with air. The geometry of these inlets is designed as AMC Acceptable Means of Compliance a fixed compromise regarding aerodynamic drag. ARP Aerospace Recommended Practice Significant work on the optimisation of the sur- CCA Common Cause Analysis face geometry of rigid inlets has been carried out CMA Common Mode Analysis (Luidens et al. 1979, Pierluissi et al. 2011, Albert CS Certification Specification & Bestle 2014, Schnell & Corroyer 2015), however, DD Dependence Diagram a rigid inlet can only achieve the best compromise EASA European Aviation Safety Agency between optimal geometries for different flight FAST Function Analysis System Technique conditions. FHA Functional Hazard Assessment Using aero engine inlets with a variable lip and FMEA Failure Modes & Effects Analysis duct geometry for different flight conditions is FTA Fault Tree Analysis perceived as a possibility to reduce aerodynamic MA Markov Analysis drag and therefore to have a positive effect on air- PRA Particular Risk Analysis craft efficiency and speed (Baier 2015). Therefore, PSSA Preliminary System Safety Assessment studies, e.g. (Kondor & Moore 2004), da Rocha- RTO Rejected Take-off Schmidt et al. (2014) and Ozdemir et al. (2015), SAE Society of Automotive Engineers have been conducted on concepts for variable SSA System Safety Assessment inlets for commercial aircraft. Additionally, first TRL Technology Readiness Level patents for variable inlets, e.g. US 4075833 and US US United States 5000399, exist. VDI Verein Deutscher Ingenieure/ The only commercial aircraft that used variable The Association of German Engineers inlet systems are the Concorde and the Tupolev ZSA Zonal Safety Analysis Tu-144. Their variable inlet systems consisted of movable ramps and flaps for supersonic flight. Compared to subsonic aircraft both aircraft mod- 1 INTRODUCTION els had huge deficits concerning range, efficiency and noise, which is why they have been retired. Improvement of efficiency and thus emissions as Within the scope of this study, in contrast to well as travelling speed are major goals in civil inlets with ramps and flaps, variable inlets with aviation (European Commission 2001, European a closed contour that can adjust the lip and duct Commission 2011). These goals also depend on the geometry are investigated. From the beginning, the aero engine and its subsystems. One of these sub- development process is accompanied by a safety

2459 and reliability process. The aim of this study is to 2006). The objective of the internal airflow is to achieve technical feasible concepts for this kind of supply the aero engine during each operating con- variable inlets, related to Technology Readiness dition with the correct quantity of air at a desired Level (TRL) 3. flow velocity (Rolls-Royce Plc 2015). The axial flow Although those variable inlets have been inves- velocity that is required by the compressor system tigated in first research studies they did not find of the aero engine is around Mach 0.5 (MacIsaac & its way in modern civil aviation yet. This can be Langton 2011). Hence, a deceleration of the inter- justified by the trade-off between the positive and nal airflow is required at flight speeds above Mach negative effects of the usage of variable inlets on 0.5 to ensure a highly efficient and safe operation commercial aircraft. Besides additional weight and of the compressor system. Thus, the inner contour production costs, negative effects are the increased of the inlet duct has to be designed as a diffuser complexity and thus potential safety and reliabil- (Farokhi 2014). The efficiency and operational sta- ity issues. These issues are related to the numerous bility of the compressor system also depends on requirements and boundary conditions regarding the uniformity of the airflow. Therefore, flow sepa- inlets. rations should be avoided under all conditions, as In contrast to earlier studies, it is reasonable they can lead to vibration excitations, rotating stall to supplement the utilised systems engineering and engine surge concatenated by a loss of thrust approach for product development with a safety and reduced aero engine durability. assessment process to identify and fulfil the safety Moreover, the fan and compressor induced requirements in aviation right from the beginning. noise emissions have to be reduced by the inlet, Thusly, possible events as well as resulting failure which is achieved by integrating acoustic treatment conditions can be identified and possible safety into the diffuser wall, see Figure 1. issues remedied during early phases of the product Furthermore, probes for pressure and tempera- development process. Such an approach has been ture measurement at the fan level can be part of carried out successfully within a project concern- the inlet. Hence, these measured data must be pro- ing coupled actuation systems for thrust reversers vided to a flight control system. and variable nozzles of aero engines (Grasselt & Additionally, the inlet has to protect itself and Höschler 2015, Grasselt et al. 2017). As a result, the compressor system from icing and its conse- variable aero engine inlets could be utilised in civil quences, e.g. impact damage and flow separation. aviation, whereby these aircraft could be more efficient and faster, while maintaining high safety and reliability. Therefore, this paper deals with the safety assessment process within concept development and preliminary design of variable inlets in civil aviation. First, the inlet system and its necessary functions are described. Afterwards, the utilised safety assessment process referring to ARP4761 is introduced and applicable safety analysis methods, e.g. Functional Hazard Assessment (FHA) and Fault Tree Analysis (FTA), are presented. Some selected results of the applied methods are shown and discussed. These results contain the identification of safety-relevant requirements, failure conditions and events. Finally, the influence of this assessment on the design of the investigated concepts is shown.

2 AERO ENGINE INLETS

2.1 Tasks and implementation The main purpose of nacelle inlets for aero engines is to divide the free stream in front of the aero engine, depending on the capture stream tube as a function of operating conditions, into an internal and an external airflow. The external airflow shall flow over the nacelle surface while avoiding flow Figure 1. Typical design of a rigid subsonic nacelle separation and other sources of drag (Mattingly inlet.

2460 An anti-icing system is installed in the inlet to ensure this. Most commonly, electrical or bleed air anti ice systems are used (Rolls-Royce Plc 2015). Bleed air anti ice systems transfer hot air from the compressor to the inlet lip to prevent icing. Aluminium is typically used for the inlet lip, due to its good heat conductivity. Furthermore, aluminium is light and resilient to foreign object damage, sand erosion, hail and bird strikes. It is to prove during certification that thrust can be maintained to a certain level to assure that the flight can be safely continued after a single bird strike (Hedayati & Sadighi 2016). The outer planking as well as the inner boundary with the acoustic linings is made of compos- ites, which minimises weight. Thereby, the inlet should withstand stipulated loads and be robust against damage (EASA 2016). This way, incidents like the Air France Flight AF-66, where the fan and the engine inlet were separated from the aircraft during flight, should occur at a very low rate to minimise the risk for passengers and crew (Aviation Herald 2017).

2.2 Trade-off in design and variable inlets Ensuring reliable operation during all flight phases must be unified with aerodynamic requirements during the geometric design process of nacelle inlets (Seddon & Goldsmith 1999). On the one hand, the inlet should be highly efficient at high flight velocities above Mach 0.8 during cruise operation. On the other hand, it is necessary to avoid flow separations and hazardous events dur- Figure 2. Tendencies of optimal inlet contours for dif- ing take-off and climb operation up to Mach 0.3. ferent flight phases and velocities. Figure 2 presents, optimal aerodynamic contours of an inlet for different flight conditions. Optimal efficiency at high flight velocities can be achieved by a thin or sharp lip contour combined flight, while ensuring reliable operation during with a small entry area A1 to minimise wave and take-off and climb. However, it needs to be con- spillage drag (Farokhi 2014). As the entry area is sidered that the variation of the inlet presents an reduced, a longer diffuser is required at high veloci- additional function, which can entail further reli- ties to avoid flow separations. Sharp inlet lips can ability and safety issues (SAE Aerospace 2010). be used for flight Mach numbers up to 1.6 without Therefore, variable nacelle inlets for Mach num- significant losses (Farokhi 2014). However, at low bers up to 1.6 are investigated focussing on safety aircraft velocities, where high angles of incidence and reliability in the context of an internal research and crosswind can occur, a sharp or thin lip con- project at the chair of Aero Engine Design at the tour is sensitive to flow separations and its potential Brandenburg University of Technology. Within negative consequences. For these operating condi- the scope of that project, a methodical safe design tions, a round and thick inlet lip with a large inlet approach is developed and utilised to perform a area is optimal. Such a ‘blunt’ lip geometry causes feasibility study for concepts for variable inlets higher drag and thus less efficiency during opera- up to TRL 3. The safe design approach for vari- tion at higher flight Mach numbers (Bräunling able inlet concepts has been presented in Kazula 2015). Hence, conventional rigid subsonic inlets can & Höschler (2017). These concepts can be divided only accomplish a compromise geometry that pro- into three geometry adjusting mechanism groups: duces increased losses during high flight velocities. movement of rigid segments, deformation of elas- Using a variable inlet, which applies the optimal tic surface material and boundary layer control. contour for each flight condition, can improve effi- The preliminary safety assessment in chapter 4 ciency and maximum aircraft speed during cruise focusses primarily on the first group.

2461 3 SAFE DESIGN APPROACH in ISO 26262 for automotive industry, propose safety processes for different areas of application. 3.1 Systems engineering When designing complex systems, it is favourable 3.3 Safety process in aviation to utilise a systems engineering approach, e.g. In aviation, failures could lead to fatal accidents. The Design for Six Sigma and VDI Guideline 2221. risk for accidents can be reduced by improvements Methodical design approaches allow for improved in the areas of airplane design, flight operations, requirements, interface and risk management, com- maintenance, air traffic management, regulations plexity reduction as well as more efficient solving and design methodologies (Hasson & Crotty 1997). of the design task. Furthermore, weaknesses dur- Since the Chicago Convention in 1944, local avia- ing development can be minimised. Most methodi- tion authorities have been publishing regulations cal design approaches are based on a common to ensure a safe operation. The European Avia- iterative structure: tion Safety Agency (EASA), for instance, releases - starting with analyses concerning requirements Certification Specifications (CS), e.g. CS-25 – and functions of the desired product, Large Aeroplanes. Paragraph CS-25 AMC 25.1309 - continuing with the allocation of solution prin- describes the safety assessment process in aviation ciples to functions, preliminary design and that is based on the process in ARP 4754A (SAE preselection of potential solution architectures Aerospace 2010) and the methods in ARP 4761 - and concluding with detailed design, as well as (SAE Aerospace 1996). The design approach for validation and evaluation of the design. variable inlets of Kazula & Höschler (2017) utilises safety methods of the ARP 4761, see Figure 3. As the desired product functions in modern industries become increasingly complex, particular safety efforts, such as safety assessments and tests, 3.4 Safety methods for variable inlet development should be considered to ensure safety and reliabil- The first of the methods presented in Figure 3 is ity (Bertsche & Lechner 2004). the preparation of a requirements document that is as complete and accurate as possible. Therefore, all requirements that could be introduced by the dif- 3.2 Safety and reliability engineering ferent stakeholders should be identified and quan- The safety and the reliability of a product play tified (Sadraey 2013). A product with high safety important roles in various industries for reasons of and reliability as well as low development costs can efficiency and business sustainability up to social be achieved during early stages of development by acceptance. These industries, all of them using a focussing on the Type Certificate Program and its separate safety approach, include the power, rail, entailed requirements. These requirements are set shipping, automotive and aviation industry (Verein by the aviation authorities and have been consid- Deutscher Ingenieure 2000). The most effective ered within this study. However, the creation of a time to improve product safety and reliability is requirements document is an iterative process as during early development phases (Bertsche & Lech- some safety requirements are only identifiable dur- ner 2004). On the one hand, this can be achieved ing the safety assessment. by using a mature design approach according to To comply with a requirement, a product must design guidelines, which contains a precise and fulfil a function. A function is defined as the con- complete requirements document and early testing. version of input material, energy or data into On the other hand, analytical methods can be uti- desired output (Roth 2001). Within the scope of lised to forecast reliability and to find weaknesses in the development process it is usual to perform a design. Analytical methods are divided into quali- functional structure analysis. This way, the neces- tative methods and quantitative methods. Whereas sary main and secondary functions are identified quantitative methods, e.g. Markov Analysis are and broken down up to elementary functions like used to predict probability of faults, qualitative ‘convert’ or ‘increase’ (Koller & Kastrup 1998). methods like the FHA are utilised to identify and A detailed functional structure can contribute to assess potential failure events and resulting condi- a high product safety by preventing design flaws, tions. The safety and reliability of a product can however, a too detailed breakdown should be be positively influenced by utilising appropriate avoided, as this limits the solution variety during safety and reliability methods during each step of development (Verein Deutscher Ingenieure 1993). the development process. For this purpose, multiple Hence, it is reasonable to start with a simple func- authors, e.g. Bertsche & Lechner (2004) and Meyna tional structure and iteratively increase the level of & Pauli (2010), as well as organisations, e.g. Inter- detail within the development process (Roth 2000). national Organization for Standardization (2011) The most common means to create a functional

2462 Figure 3. Suitable safety and reliability methods for the separate phases of the methodical design approach. structure are the FAST-diagram (Function Anal- by utilising the Fault Tree Analysis (FTA), the ysis System Technique), the function net and the Dependence Diagram (DD) or the Markov Analy- function tree. While all of these methods are quite sis (MA), supplemented by a Common Cause similar, function trees are the recommended choice, Analysis (CCA) (SAE Aerospace 1996). While because of their structure that synergises well with the most commonly used FTA, which is an itera- the Functional Hazard Assessment (FHA) (Verein tive top down method, presents the relationship Deutscher Ingenieure 2000). between failures through logic gates, the DD uses The FHA is a qualitative method that should be paths and the MA time dependant probability performed at the beginning of the safety process. functions. On the one hand, FTA, DD and MA At this point, it is reasonable to carry out qualita- have many advantages, which are discussed for tive safety methods, as they support the systematic instance in Kritzinger (2016) and SAE Aerospace investigation of failure conditions, causes and con- (1996). On the other hand, all of these methods sequences. During later phases of the development lack a systematic that assures completeness (Verein process these qualitative methods can be replaced Deutscher Ingenieure 2000). by quantitative methods to investigate reliability in Therefore, within the System Safety Assess- more detail (SAE Aerospace 2010). The main objec- ment (SSA) it is reasonable to combine the top tive of the FHA is to systematically assess functions down method FTA with the bottom up method of a systems and to identify and classify failure con- Failure Modes and Effects Analysis (FMEA), that ditions as well as their effects. It is performed on is simple to use but iterative and time consuming Aircraft, System and if required subsystem level. (Kritzinger 2016). The SSA evaluates the compli- Similar to the function tree, a disadvantage of this ance of the investigated system with the safety method is that it can be difficult for inexperienced requirements from the PSSA by applying analyses users to apply this method appropriately as it allows and test methods. In addition to the FMEA, the for an easy deployment of enormous tables and as SSA includes quantitative FTAs and a CCA. A the classification of hazards can be rather subjec- CCA comprises of a Zonal Safety Analysis (ZSA), tive. Kritzinger (2016) describes the advantages of a Particular Risk Analysis (PRA) and a Com- an FHA, one of them being the provision of top mon Mode Analysis (CMA). A PRA is utilised to level events for the following Preliminary System identify external events and a ZSA to determine Safety Assessment (PSSA). individual failure modes that can cause hazards. The PSSA is used to analyse which single or A CMA is used to verify independence of func- multiple system, subsystem or component failures tions, as this is not done within the FHA (SAE lead to the functional hazards that have been iden- Aerospace 1996). Finally, tests must be performed tified within the FHA. This way, safety related to validate the results from earlier analyses and to design requirements can be determined and concept comply with the requirements set by the aviation designs can be evaluated. A PSSA is performed authorities. It is reasonable that the less experience

2463 the product developer has, the more tests should be The function tree that is presented in Figure 5 performed for a successful certification. contains a few important functions of the nacelle inlet system. Different flight phases require for different geometries to achieve minimal drag and 4 SELECTED RESULTS an internal airflow with high uniformity and the appropriate velocity. An inlet geometry adjust- 4.1 Function trees ment system can improve the achievability of these The Aircraft has to fulfil different 1st level or top functional requirements. Additionally, an adjust- level functions according to SAE Aerospace (2010) ment system could be used to prevent negative and Kritzinger (2016), one of them being ‘control effects of icing by detaching ice. Furthermore, it thrust’, see Figure 4. Thrust control is achieved could utilise existing systems for data and energy by generating, adjusting, ensuring and determin- transfer. The influence of the variability on other ing thrust. The inlet system influences all of these inlet subsystems has to be investigated, as the inlet functions, e.g. to generate thrust, the inlet system would not be a rigid enclosed structure anymore has to provide the aero engine with an airflow. and the installation space for acoustic treatment

Figure 4. Simplified aircraft function tree.

Figure 5. Simplified aero engine nacelle inlet system function tree.

2464 could be reduced. The essential functions of an inlet adjustment system are change and locking of the inlet geometry as well as gathering, transfer- ring and processing geometry data.

4.2 FHA The function trees can be used to determine the input for the FHA. For clarity reasons, only the function ‘adjust inlet geometry’ and its impact on the aircraft is presented in the following. After identifying the functions, associated failure conditions and their effects must to be determined regarding single and multiple failures during normal, e.g. take-off, climb and cruise, as well as special conditions, e.g. windmilling. The main failure condition of the adjustment system is that an undesired geometry is adjusted, what could affect a single engine or all engines. A further categorisation is possible concerning: Figure 6. Simplified fault tree for the inlet adjustment - which geometry is adjusted, system. - is the system either incapable to maintain the desired geometry or to adjust the geometry on time or in general, - is the failure occurring either announced or unannounced, either abruptly or anticipated. Then, the effects of these failure conditions must be identified and classified. Possible effects of the presented failure conditions are ‘loss of thrust’ or ‘reduced efficiency’. The classification is conducted according to CS-25 AMC 25.1309 regarding the severity of an effect on aircraft, crew or occupants and is divided into ‘catastrophic’, ‘hazardous’, ‘major’, ‘minor’ and ‘no safety effect’. Each of these classifications entails a specific probability requirement concerning occurrence of a failure mode. Table 1 presents the failure classification for an undesired, abrupt but announced

Table 1. Failure classification for the event ‘undesired geometry is adjusted’ on a single engine.

Probability requirement

Flight phase Classification Events per flight hour

Start No Safety Effect No Probability Requirement Idle No Safety Effect No Probability Requirement Taxi No Safety Effect No Probability Requirement Take-off Minor <1.0E-03 RTO No Safety Effect No Probability Requirement Climb Minor <1.0E-03 Cruise No Safety Effect No Probability Requirement Descent No Safety Effect No Probability Requirement Approach No Safety Effect No Probability Requirement Go-around Minor <1.0E-03 Landing No Safety Effect No Probability Requirement Brake No Safety Effect No Probability Requirement Figure 7. Control flow chart for a variable inlet system.

2465 adjustment of a sharp thin cruise geometry on a It should be considered that some safety regula- single engine. Commercial aircraft must be able tions, guidelines and expert opinions could be too to fly with one engine inoperative. A failure of conservative and thus limit the solution variety of the adjustment system of a single engine can only new technologies. Furthermore, safety and reliabil- cause surge and therefore loss of thrust on a sin- ity is often neglected during academic studies. gle engine. Hence, this event only leads to a slight Due to the safe design approach of this study, reduction of the aircraft functional capability and function trees, an FHA and an FTA have been a slight increase in workload for the flight crew. described in this paper. This results in the identi- However, a failure of the adjustment system on fication of safety requirements and design faults. multiple engines could lead to loss of thrust on These faults can be mitigated by the means of more than one engine. For instance, during take- redundancy within the adjustment control system. off this can result in a rejected take-off (RTO). This way, the negative effects of variable inlets This is classified as a hazardous mode with a prob- can be reduced during early stages of the prod- ability requirement of less than 1.0E-07 events per uct development. Hence, variable inlets could be flight hour (EASA 2016). applied in commercial aviation. This could allow for faster and more efficient aircraft, while maintaining safety and reliability. Moreover, the safe 4.3 FTA and failure prevention design approach could be applied to other prod- The fault tree in Figure 6 shows how the hazardous uct developments in aviation or other industries, as event ‘loss of thrust’ is based on the loss of thrust there is currently no general safe design approach. on all engines. This is caused by the incapability to provide air sufficiently due to the adjustment of an undesired inlet geometry. This could be caused by REFERENCES a single control system failure. Single failure conditions can be avoided by a Albert, M. & Bestle, D. 2014. Automatic Design Evalu- redundant design. This can be achieved by design ation of Nacelle Geometry Using 3D-CFD. 15th AIAA/ISSMO Multidisciplinary Analysis and Optimi- modifications. Therefore, the adjustment system zation Conference. Atlanta, GA. is protected by a locking system, which is inde- Aviation Herald 2017. 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