Overview of the NASA Systems Approach to Crashworthiness Program

Lisa E. Jones NASA Langley Research Center l.e.j ones @ larc..gov Hampton, VA

Abstract The NASA Aviation Safety Program was developed in response to the federal government's goal to reduce the fatal accident rate for aviation by 80% within 10 years. ACcident Mitigation is a primary element of the Aviation Safety Program. The overall Accident Mitigation goal is to provide technology to the air transport industry to enable a decrease in the rate of fatalities and injury from crash loads and from in-flight and post- crash explosion and/or fire. Accident Mitigation is divided into two main elements - Fire Prevention and Systems Approach to Crashworthiness. The Systems Approach to Crashworthiness goal is to develop and promote technology that will increase the human survival rate or reduce the fatality rate in survivable accidents. The technical background and planning, selected technical activities, and summary of future efforts will be presented in this paper.

Introduction February 1997 a national goal to reduce the fatal accident rate for aviation by 80 According to the Gore Commission final percent within 10 years. report, the worldwide demand for air travel is expected to double or triple by In response to the presidential 2017 with the requirement for $1 trillion announcement, NASA initiated the in new aircraft deliveries [1]. Without Aviation Safety Investment Strategy decreasing the accident rate, such a Team (ASIST), which sponsored four traffic volume would lead to 50 or more industry- and government-wide major accidents a year. Given the very workshops to define research needs. visible, damaging, and tragic effects of The planning effort lasted from February even a single major accident, this to April 1997, and involved over 100 number of accidents would clearly have industry, government, and academic an unacceptable impact upon the organizations. A subset of the research public's confidence in the aviation investment areas from ASIST, denoted system and impede the anticipated Human Survivability, formed the basis growth of the commercial air-travel for the technical activities within market. President Clinton announced in NASA's Aviation Safety Program (AvSP) Accident Mitigation (AM) Presented at the American Helicopter Society element. Human survivability included 58 th Annual Forum, Montreal Canada, June 11- the areas of mitigating the impact and 13, 2002. This paper is a work of the U.S. fire effects of an accident and in-flight Government and is therefore in the public domain. fire prevention. The official start of the AvSP was FY 2000. Prior to the start of the AvSP, Crashworthiness and fire focus on specific functions to meetthe prevention were included in the program objectives. The first sub- Airframe Assurance elementis crash load predictions using (AAA) program. AAA provided the finite element modeling. The second opportunity to upgrade computational sub-element, occupant protection, capabilities, increase instrumentation includesdevelopingdesignapproaches, inventories, purchase mechanical standards,and materials for protecting equipment, fund survey reports on occupants from crash loads. Crash transport,rotorcraft andgeneralaviation resistant fuels systems (CRFS) is the accidents and a fuel system report third sub-element. CRFS includes [2,3,4,5],hold workshopswith industry design approaches, standards, and andtheFederalAviation Administration materialsfor protection againstrupture (FAA), and conduct analyses and of fuel system tanks, lines, and other testing. Basedon thesesurveyreports, components. industry and FAA input, and in-house efforts,theplansfor theAvSPAM were The Systems Approach to developed. Crashworthiness plans include partneringwith customersto accomplish AvSP emphasizes not only the end goals. The partnersinclude the accident rate reduction, but also a Federal Aviation Administration, U.S. decreasein injuries and fatalities when Army, analyticalcodevendors,seatand accidentsoccur. The AvSP goal is to restraint manufacturers, airframe develop and demonstratetechnologies manufacturers and fuel system thatcontributeto a reductionin aviation manufacturers. accidentandfatality ratesby a factorof 5 by year 2007andby a factor of 10 by A more detailed description of year2022. the technical approachand work being conducted in each sub-element is To reach the goal of reducing presentedin thefollowing sections. injury and fatality rates in accidents, understandingthe crashenvironmentis Crash Load Predictions required. For occupantsto survive a crashof an aircraft, a numberof things In the last ten years, significant must happen. The majority of the advances have occurred in computing impact energymustbe absorbedby the capabilities, data acquisition systems, airframe structure and seat. The and finite element simulation. These restraintsmust function well during the technical advances provide the ability to primary and secondary impacts (eg. effectively evaluate detailed finite minimize headstrikes). The seatmust element models. For the aircraft remainattachedto the floor. An egress industry, use of finite element codes in path must remain available and there designing crashworthy structures has not must be enoughtime for the occupants proven cost effective. In addition, no to egressbefore fire and smokebecome procedures or regulatory guidelines have incapacitating. Therefore,the Systems been established for use of codes in the Approach to Crashworthinessplan is crashworthiness certification process. divided into three sub-elements that Certification for crashworthiness relies solelyon seattests[6,7,8]. A full-scale crashtestcouldbeanoptionto certify an aircraft design that utilizes the systems approach.However,thisapproachis not accepted as an economically viable method due the manufacturing cost. Also, there is a disconnectbetweenthe designer's needsand the certification process. The designerneedsloads and displacementsto sizestructureor design Figure 1. Composite helicopter full- energy absorbing structure. The scale and analytical model. certification process needs the input floor acceleration pulse and occupant responseinformation. No method to quantify the accuracyof the models is currently available. To addressthese issues,the crashload prediction efforts include best practices in analytical model development, guidelines and standards for test and analysis correlation,enhancementof theanalysis codesfor useasdesign,andcertification tools.

Best Practices

The best practices in analytical model development work is based on modeling of complete aircraft as presented in Figure 1.[9], fuselage sections shown in Figure 2. [10], outside customer projects [11], and small-scale lab experiments [12]. The structures are composed of composite, metallic, or hybrid materials. Data are being compiled for a report on the effective methods for developing detailed finite element crash models. The information will be updated as necessary. These documented data are intended to help the user develop the best model in a reasonable time by avoiding approaches that do not work. Examples of best Figure 2. Energy absorbing fuselage test practices to be included in the document article and analytical model. range from how to prevent geometric discontinuities, to the importance of Historically, when full-scale accurate material properties [13]. aircraft or sections of aircraft were Ultimately, these practices should be tested, instruments were mounted to give usedto develop a full aircraft model information on the performance of a used for designing and certifying concept. As the test and analysis crashworthyaircraft. One approachfor correlation requirements and this would requiretestingbut thetesting expectations change, the instrumentation couldbelimited to a fuselagesectionsor of the test article must evolve to provide critical components of the aircraft the data needed. For example, there are structure. Experimentaldatafrom these usually many accelerometers located on component tests would be compared the floor structure and on large masses. analytical results. If the correlation However, these data give little validatesthecomponentanalysisandthe information on the load path through the full aircraftmodelis developedwith the structure below the floor. The ability to samepractices,confidenceis increased correlate the predicted and test loads in for the validity of the full aircraft main structures in the wing box area simulation. would be advantageous to the designer. This information could be used to Test Analysis Correlation (TAC) identify structural members that need to be modified. However, obtaining this Guidelines and standards for test information from a test is not analysis correlation are needed to straightforward. Strain gages, though quantify the accuracy of the model excellent for use in quasi-static testing, simulations. Along with the offer little useful information in a full- computational capabilities, significant scale crash test. Part of the TAC effort advances have occurred in data is focused on development of acquisition systems. Digital data instrumentation for use in the crash acquisition systems are capable of environment. Other TAC efforts involve recording multiple channels and millions using the results as a of data points at rates exceeding 10khz. guide for instrumentation layout, Analytical models with tens of efficient data reduction processes and thousands of elements are common in automated data handling for performing crash simulations and the computational correlation studies in a timely manner. resources used to run the simulations on desktop personal computers. However, The main TAC goals are to oftentimes the assessment of the identify what data should be correlated correlation accuracy is based on the and to develop methods to quantify the qualitative comparison of time history accuracy of the analytical predictions. response. These comparisons provide While investigating typical comparisons valuable information with regard to the of accelerations, it was found that the global response of the aircraft [10]. filtering frequency affects the correlation Unfortunately, the global responses do accuracy. The standards for filtering not meet all of the information data were established before the digital requirements of the aircraft designer and data acquisition systems were available. the certification process. Standards for selecting the filtering frequency need to be addressed[14]. Memorandum of Agreement [15], Or, anotherapproachbeing investigated NASA and the FAA are working involvesacquisitionof datathat areless together to define the expectations of a noisy or that do not needfiltering, such process to certify an aircraft for asdisplacements.Again, this approach crashworthiness by analysis. The FAA will require instrumentation is actively funding efforts in crash development. analysis [16]. Regularly scheduled meetings and teleconferences with the In addition to pursuing new FAA on certification issues are planned. methodsto correlatesimulationand test Validation of the simulations has been data, literature searchesand trials of identified as a critical issue by NASA. other methodsoutsidethe crashfield of Demonstrations of the processes in study are being conducted. The final actual test and simulation correlation TAC product should: identify the data studies are expected to be the resolution to be correlated;defineprocessesfor the for many of the concerns. A full-scale collectionandpost-processingof thetest crash test of a Fokker 28 aircraft is data; provide methodsto retrieve and scheduled for March 2004. A photo of post-processdata from the simulation; the test article is presented in Figure 3. anddefine a methodfor comparisonsof Modeling and testing of Fokker 28 thedatathatincludecalculationsandthe fuselage sections are planned to presentation of the correlation in a demonstrate the previously discussed logical, quantifiable, and simplified methods and processes. The successful manner. methods and processes will be used in the full aircraft model. The FAA will be Code Enhancements informed and involved with this test series. Results from this work are Enhancement of analysis codes expected to guide the follow-on work. includes identifying deficiencies in the codes and new capability requirements, identification of errors in theory or computing, and identification of post- processing or output file anomalies. An example of a deficiency would be the lack of a typical element type. An example of a post-processing anomaly is aliasing resulting from the insufficient sampling of time history results. These are a few of the types of enhancement needs encountered by the in-house Figure 3. Fokker 28 aircraft scheduled researchers. to be crash tested in 2004.

Certification Issues Occupant Protection

The certification issues of a The occupant protection systems approach aircraft design are functions are to reduce the injury being actively worked. Under a joint frequency and severity and to increase the proportion of occupantsescapinga survivable accident. This function is hardware development and injury mechanism identification. Hardware development includes: material characterization testing; energy absorbingstructuralconcepts;controlled aircraft break-up technology; floor structural integrity; restraints; seat technology; aircraft layout; and sensor technologyfor collection of dataduring accidents. NASA develops energy absorbing materials and structures technology for aircraft and space applications [17,18,19]. An energy absorbingsubfloordesignis presentedin Figure 4. An energyabsorbingconcept for a Mars SampleReturn structure is shownin Figure5.

In addition to the in-house efforts,NASA is utilizing othermeansto motivatethe industryanduniversitiesto be involved in crashworthiness technology development: Small BusinessInnovativeResearchcontracts; andNationalResearchAnnouncements.

Occupant protection activities include providing information to the Figure 4. Energy absorbing subfloor designeron crash physics, biometrics, structureduringdynamictest. current seat and restraint design considerations,delethalization of the interior, and post-crashfactors. This informationwill bein theform of design guidessimilar to thosedevelopedby the U.S. Army [20]. Two designguidesare planned. The first is a generalaviation design guide that is to be published in 2002. The secondis a transportdesign guide that is a deliverableat the end of the program. It is expectedthat these documents will be updated as technologyprogresses. Figure 5. Energy absorbing Mars samplereturnconcept. To assist the efforts in injury methods. Work in CRFS is being mechanism identification and as initiated through the National Research previously mentioned, surveys of Announcement process with the FAA transport,generalaviation androtorcraft providing oversight. Design and accidentsweregenerated[2,3,4]. In the manufacturing expertise of aircraft fuel caseof thetransportsurveyof survivable systems resides with the FAA and accidents, the data collected at the industry. NASA will work with the accidentsitesand medicalfacilities did FAA and industry to provide structural not provide complete information on expertise for new concepts. NASA injuries. It is critical to crashworthiness plans to provide the Fokker 28 for efforts that the typesof injuries andthe concept evaluation testing of wet wing mechanisms that caused them be designs. identified. For example, if lower leg fractures are occurring due to seat collapseand are preventingor slowing Concluding Remarks egressandtheresultsarefatalitiesdueto smokeinhalation,this informationneeds The purpose of this paper is to to be reportedin the post-crashsystem. review the historical and technical However, the National Transportation background of the Systems Approach to SafetyBoard(NTSB) doesnot focuson Crashworthiness element of the Aviation this information. The charter of the Safety Program. The paper summarizes NTSB is to focus on the causeof the the basis for the planning and execution accident. Therefore, NASA and the of the technical efforts. The aircraft FAA arecollaboratingwith theNTSB to structure forms a system in which all enable the acquisition of this components work together to protect the information. occupant in a survivable crash. To be able to design an aircraft by this method, it must be cost effective. A three-part Crash Resistant Fuel Systems approach has been developed. The first part is the crash loads predictions and The crash resistant fuel systems has the potential to benefit all classes of (CRFS) functions are to decrease the aircraft. NASA is working on chance of a post-crash fire and/or slow developing tools, methods, instruments, the post-crash build-up of fuel fire, and processes that will make is feasible smoke and heat to increase time for to produce a finite element model and egress. A study of transport airplane crash simulation that is acceptable for crash resistant fuel systems was certification and has design capabilities. documented [5]. In the study, a list of The second part is investment in the recommendations was generated. The technologies that can be tailored for use recommendations include: a need for a in the system being designed. Materials, crashworthiness rating system for fuel structures, seats and restraints are a few systems; materials improvements; use of of the hardware research areas. Also, it breakaway and self-sealing technology; is necessary to have injury mechanism development of fuel bladder technology information that is accurate and suitable for transport aircraft; and complete. The third part is crash development of frangible fastening resistantfuel systems. Fire, smokeand Commuter Category Airplanes," heatarethreatsin all accidents. Section 23.562 Federal Aviation Administration, Washington, D.C. The Systems Approach to 7. "Federal Aviation Regulations, Part Crashworthinesswork is a teameffort. 25, Airworthiness Standards: The teamincludesNASA, FAA, Army, Transport Category Airplanes," service contractors, general aviation Section 25.562, Federal Aviation manufacturers,rotorcraft manufacturers, Administration, Washington, D.C. transport industry, suppliers, and 8. 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