AC 20-146A CHG 1
10.1.2
10.1.3
10.1.4
10.2
10.3 11
11.1 There will be instances when a modified seat design results in a structural member, in the primary load path, that reacts to a dynamic load or stress/strain greater than that reacted to during the baseline design test. Note the modified part is not necessarily the part that has increased criticality. For a non-critical structural member, that is, where the ultimate margin of safety of the baseline design is greater than or equal to 1.0, the modified design ultimate margin of safety should be greater than or equal to 0.5. Refer to paragraph 12.8, Ultimate Margin of Safety, of this AC.
For critical structural members where the ultimate margin of safety for the baseline design structural member is less than or equal to 0.5, design changes to the seat cannot result in an ultimate margin of safety that is reduced greater than 25 percent from the original margin. For critical structural members where the ultimate margin of safety is between 0.5 and 1.0, design changes to the seat cannot result in an ultimate margin of safety that is reduced greater than 50 percent from the original margin. In those cases, where a design change reduces the ultimate margin of safety, the ultimate margin of safety for the structural element in question should be greater than or equal to 0.1.
For all structural members, the ultimate margins of safety must be positive, in accordance with § 2X.305, Strength and Deformation.
Seat Installation Modification.
Analysis based on a validated computer simulation can be used to substantiate configuration changes to seat installations. The primary application is to show HIC compliance. Refer to paragraph 8.3.5, Head Injury Criteria, of this AC.
Applicability.
The material in paragraphs 10.1 and 10.2 of this AC is not applicable to changes to the seat-floor attachment structure. Significant changes to the material or mechanism of load transfer of the seat-to-floor attachments from the certificated baseline seat design, which includes the seat-to-track fitting and track substantiated under TSO-C127x, will require a new series of dynamic tests. Simple changes to the location of the seat-to-floor attachments are not included in this limitation, and they can usually be analyzed using static methods.
SEAT CERTIFICATION AND COORDINATION PROCESS.
This section focuses on the process to coordinate the use of computer modeling to generate engineering data to demonstrate compliance to § 2X.562 and follows Order 8110.4. A TSO manufacturer should coordinate the acceptable use of computer modeling with the certification branch using the applicable technical standard order program guidance in AC 21-46.
FAA Coordination.
AC 20-146A CHG 1 FAA coordination is essential to ensure the proper and timely execution of any certification program. The guidelines presented will assist in the implementation of computer modeling as a means of compliance.
11.2 Certification Plan.
The applicant and the certification branch will negotiate the use of computer modeling to generate technical data in support of establishing dynamic test conditions or instead of dynamic test conditions. If the FAA establishes a type certification board (TCB), negotiations should occur during the preliminary and interim TCB meeting. A TCB is not always required for supplemental type certificate projects. Whether there is a TCB or not, the applicant’s role is as follows:
• Acquaint the FAA personnel with the project.
• Discuss and familiarize the FAA with the details of the design.
• Identify, with the FAA, applicable certification compliance paragraphs.
• Negotiate with the FAA where computer modeling will be used and specify the intent and purpose of the analysis.
• Establish means of compliance either by test, by rational analysis (that is, computer modeling), or both, with respect to the certification requirements.
• Establish the validation criteria for the computer model relative to its application for certification.
• Prepare and obtain certification branch approval of the certification plan.
11.3 Technical Meeting.
11.3.1 The details of the computer model are defined during scheduled technical meetings between the applicant and the certification branch. The applicant should prepare a document for the FAA describing the purpose of the analysis, the validation methods, and the data submittal format. As a minimum, the following items should be contained in the document:
• A description of the seat system to be modeled.
• A description of the software to be used in the analysis. This should include the operating assumptions and limitations of the software.
• A description of how compliance will be shown.
• A description of material data sources.
• A list of validation methods, including a description and justification of the failure modes/theories.
• An interpretation of results.
• Substantiation documentation and data submittal package.
AC 20-146A CHG 1
11.3.2 The document, referred to as the certification plan, should be developed in conjunction with the seat design evaluation phase and approved by the FAA as early in the certification process as possible.
12 DOCUMENTATION REQUIREMENTS FOR COMPLIANCE.
12.1 Overview of a Validation and Analysis Report (VAR).
Section 21.21(b) requires an applicant to submit type design, test reports, and computation necessary to show compliance to the applicable airworthiness requirements. Section 21.603(a)(2) requires an applicant for a TSO authorization to submit a copy of the technical data required in the applicable TSO. This section describes the minimum data and information necessary for substantiation of compliance with § 2X.562 or TSO-C127x when computer modeling is submitted as engineering data. The applicant and the certification branch should negotiate any additional data necessary. The document containing this information will be referred to as the VAR in this AC.
12.2 Purpose of Computer Model.
The VAR lists the certification regulations or TSO requirements relevant to the certification of the seating system. The VAR should state how the computer model would be used to demonstrate compliance for each stated requirement. The applicant should define the purpose of the computer model as either:
• Application of computer modeling in support of dynamic testing (refer to section 9 of this AC), or
• Application of computer modeling instead of dynamic testing (refer to section 10 of this AC).
12.3 Validation Criteria.
The VAR should document the appropriate validation criteria identified in sections 7 and 8 of this AC.
12.4 Overview of Seating System.
The VAR should contain an overview of the design of the seating system. This overview will describe the seat layout in the aircraft, the occupant restraint type, and the attachment method of the restraint. If applicable, the VAR will describe the adjustment positions required during takeoff and landing. In addition, the VAR will contain a description of any special occupant protection features included in the seat/restraint system design.
12.4.1 Seat Structure.
The VAR should provide a description of the seat’s critical components, primary load paths, energy absorbing features, the seat attachment hardware, and the floor attachments or seat tracks. The VAR will describe the material properties of the primary structural and energy absorbing components, along with the method of fabrication. Give 24
special attention to describing which primary structural members are designed to displace, deform, elongate, or crush to dissipate kinetic energy.
12.4.2 Restraint System.
The VAR should provide a description of the restraint system, including part number, and any other devices designed to restrain the occupant in the seat or reduce the occupant’s movement under emergency landing conditions. This might include the shoulder and lap belts, load limiting devices, belt locking devices, pre-tensioners, and inflatable restraints. The VAR should also describe how the restraint system and its devices are anchored and list the material properties of the restraint system.
12.4.3 Unique Energy-Absorbing Features in the Installation.
Unique energy-absorbing features are components, other than the seat and restraint system, that are designed to limit the load imposed on the seating system or occupant.
Examples include energy-absorbing subfloor structure or inflatable devices mounted on the airframe but not considered a part of the seat/restraint system.
12.5 Software and Hardware Overview.
The VAR should contain a brief description of the software and hardware used to perform the analysis, including the following information:
• CPU type and instruction set of computer hardware.
• Operating system.
• Auxiliary software components (message passing interface (MPI) product, etc.).
• Finite element binary specifics (version, revision, precision, parallel application, etc.) if applicable.
12.6 Description of Computer Model.
The VAR should contain a detailed description of the computer model, including the input data.
12.6.1 Engineering Assumptions.
12.6.1.1 The applicant should document assumptions used for the analysis.
Assumptions can include, but not be limited to, simplification of the physical structure, the use of a particular material model, methods used for applying boundary conditions, failure theories, and the method of load application. The VAR should document a rational support for the use of each assumption. The FAA may require the applicant to demonstrate that the assumptions do not negatively affect the analytical results.
12.6.1.2 Those components that are not critical to the performance of the seating system and do not influence the outcome of the analysis can be omitted from the model. However, the mass of the system must be preserved. The VAR should list all components excluded from the analysis and provide justification for the exclusion of those components from the model.
12.6.2 Modeling of the Physical Structure.
12.6.2.1 The VAR should provide a description of the structure. If the model is constructed with finite element techniques, this documentation should include a description of the finite element mesh. It should describe how the critical components of the structure were modeled and provide the rationale for the selection of the element types that were used to represent the structure. In addition, the applicant should describe the limitations of the mesh element used in the analytical modeling. If the mesh element is either unconventional or is a new element, the VAR should provide the mathematical formulation of that element, engineering assumptions made during the element’s formulation, and any limitations that apply to its usage.
12.6.2.2 The choice of element, material models, and other model features can greatly affect the accuracy of a finite element solution. For example, a membrane element carries in-plane loads only, while a shell element also carries perpendicular loads (bending). Depending on the loading conditions present, a membrane element could produce significant errors that would not cause the solution to abort. Likewise, an elastic material model cannot predict plastic behavior. The applicant should justify the choice of elements, material models, and other model features for all components that are in the critical load path.
12.6.2.3 If the model is constructed with multi-body dynamics, this documentation should include a description of the properties of the multi-bodies and details on the connectivity, joint properties, and any contact functions.
12.6.3 Material Models.
12.6.3.1 The applicant should document the material models used in the analysis.
This may include a list of the materials used by the analysis software and a general description of the material properties. In addition, the applicant should identify the source of the material data.
12.6.3.2 When standard material property data are used, the guidelines outlined in the MMPDS and CMH-17 should be followed. (Refer to the Metallic Materials Properties Development and Standardization Handbook and Composite Materials Handbook.) It is acceptable to use S-basis or typical properties in the model when conducting validation simulations. A-basis material values for components in the structural load path, and B-basis 26
material values for all other structural parts, should be used when the model is used for certification purposes without supporting dynamic tests.
The material values are explained in the referenced handbooks.
12.6.3.3 Any material data acquired through in-house tests should be supported by appropriate documentation that describes the basis of such test, test methods, and results. When applicable, material strength and material variability properties must comply with §§ 23.613, 25.613, 27.613, and 29.613. This includes proprietary data.
12.6.4 Constraints.
Constraints are boundary conditions applied in the model. This includes single and multi-point constraints, contact surfaces, rigid walls, and tied connections. The applicant should document the boundary conditions applied in the model and discuss how the model boundary conditions correspond to the test conditions. The VAR should also provide a description of contact definitions and nodal constraints. Finally, the VAR should document the values used to represent frictional constants and the validity of such values.
12.6.5 Load Application.
Loads that are applied in the computer model may include concentrated forces and moments, pressure, enforced motion, and initial conditions. The VAR should contain a description of how external loads are applied to the model and should list all nodal points affected by the load application. The VAR should also provide a copy of the acceleration/deceleration profile time history.
12.6.6 Occupant Simulation.
The VAR should document the v-ATD including the version number and the calibration data. The v-ATD calibration should state whether it meets SAE ARP 5765, Revision A, or other means, and it should document any usage limitations.
12.6.7 General Analysis Control Parameters.
12.6.7.1 General analysis control parameters are features of a program that control, accelerate, and terminate an analysis. This may include parameters that enhance the performance of the software for reducing the computational time or the use of subroutines that facilitate the post-processing of results.
12.6.7.2 The VAR should include a summary of the control parameters used for a particular analysis. There should be ample justification for parameters that may influence the outcome of the analysis. As an example, the analyst should show that the artificial scaling of mass for reducing computational time is acceptable and does not negatively influence the results of the model. Paragraph 6.5 of this AC provides a definition of mass scaling.
12.7 Result Interpretation.
This paragraph contains guidance and recommendations for the output, filtering, and the general methods of reporting numerical data. The purpose is to achieve uniformity in the practice of reporting numerical results. The use of the following recommendations will provide a basis for a meaningful comparison to test results from different sources.
12.7.1 Energy Balance.
If the numerical analysis uses an explicit finite element formulation, the applicant should evaluate the presence of hourglass modes, also known as zero energy modes, to determine if they are located at critical structural components. If this evaluation determines that these modes are present, the applicant should assess the hourglass modes to quantify their influence on the accuracy of the analysis. The applicant should correct the model if it does not attain the appropriate energy balance. The VAR should contain a summary of the ratio of initial energy to final energy and provide a comparison of hourglass energy to total energy.
12.7.2 Data Output.
12.7.2.1 The transient analysis should generate data at a minimum frequency of 10,000 Hz for electronic data and 1,000 Hz for trajectories. This will maintain an equivalent practice with the instrumentation and data requirements specified in SAE J211, and it will allow for a meaningful comparison between numerical data and test data.
12.7.2.2 If the output of the data channels is dependent on the integration time step of the analysis, and its sample rate is higher than the rate of the test data, the reported simulation data should be subsampled for comparison. If any post-analysis filtering is to be applied, the generated data may need to be reported at a higher frequency than 10,000 Hz (or 1,000 Hz) to remain consistent with the requirements outlined in SAE J211. The VAR should document any deviations to this practice.
12.7.3 Data Filtering.
The filtering practices of SAE J211 apply for all applications.
12.8 Ultimate Margin of Safety.
12.8.1 The ultimate margin of safety represents the ultimate strength of the structure in relation to the strength required to carry the ultimate load. In this AC, it is presented as a decimal value defined as:
MS ultimate = ([Ultimate Strength / Ultimate Load] - 1)
12.8.2 For the structural substantiation of the seat/restraint system and attachment structure, the ultimate margin of safety must show a positive value. The VAR should document the ultimate margins of safety for those structural elements the applicant or the FAA identifies as critical.
Appendix A Appendix A. Occupant Trajectory and Head Injury Criteria (HIC) A.1 OVERVIEW.
This appendix serves as an example of items to consider and document when validating occupant trajectory and HIC. Refer to paragraphs 8.3.1 and 8.3.5 of this AC for validation criteria regarding occupant trajectory and HIC. This is not meant to be a universal example. Other situations may require more or less stringent validation efforts.
A.2 TEST SETUP.
A.2.1 The scenario is a front-row seat with a foam-covered bulkhead 25 inches forward of the occupant. Refer to Figure A-1 below. The launch seat has been represented by a rigid fixture for simplicity. The seat cushion is one-half inch thick stiff foam covered in fabric. The occupant is represented by a 50th Percentile Male (Hybrid II) ATD and is restrained by a standard nylon lap belt. The impact condition is from § 25.562(b)(2), with no yaw or floor misalignment. The anthropomorphic test device (ATD) has been instrumented with three linear accelerometers in the head (Endevco model 7264B).
Based on the specific geometry of this aircraft configuration, it was determined that femur compression was not an issue. Therefore, no femur load cell was installed (refer to paragraph 8.3.6 of this AC). Webbing transducers were attached to both ends of the lap belt to record belt loads (Denton model 3255).
A-1
Appendix A Figure A-1. Test Setup (xz-plane) A.2.2 Pre-test measurements included the head center of gravity (CG), hip-point (H-point), knee, and ankle of the ATD, along with the SRP, belt anchor location, multiple points on the lap belt, points on the floor, foot stop, bulkhead (including the foam), and targets required for photometric scaling and validation.
A.2.3 The model was generated using MADYMO 6.1. The seat is represented by rigid planes and the seat cushion was excluded from the model. Refer to figure A-2 below. The friction coefficient between the ATD and seat was defined to represent the effect of the fabric-covered foam. The occupant model is the MADYMO Hybrid II 50th Percentile Male (p572 version 3.3). The lap belt is represented by simple springs with material properties derived from high rate tests of the webbing. The foam on the bulkhead is modeled by finite elements, and the material properties were determined by load frame tests. The sled pulse from the physical test was applied to the ATD in the x-direction and gravity was applied in the z-direction. No penetration contacts were defined between the ATD and all rigid surfaces, bulkhead, seat, floor, and foot stop, as well as between the individual parts of the ATD. A kinematic contact was defined between the ATD and the foam on the bulkhead.
A-2
Appendix A Figure A-2. Numerical Model Setup (xz-plane) A.2.4 MADYMO 6.1 is a commercially available rigid body dynamics and finite element simulation code. As part of the development process and distribution to its customers, the developer provides a theory and reference manual to assist the user in applying the code and material models appropriately. No formal statement of code conformity or code verification was available from the developer (as discussed in paragraph 7.1 of this AC); however, the user attests that the code is acceptable for its intended use.
A.2.5 The bulkhead foam was modeled using eight-node solid elements that carry tensile, compressive, and shear loads. This type of element provides good accuracy for simple, rectangular shapes. The load frame test used to generate material properties was simulated to show that the mathematical implementation of this foam block is correct.
The time step was controlled by the software, updating as necessary to maintain compliance with the Courant criteria (as discussed in paragraph 7.3.1.1 of this AC). No mass scaling was used in this model. The mesh resolution was evaluated as described in paragraph 7.2.2 of this AC.
A-3
Appendix A A.2.6 The v-ATD selected is the MADYMO Hybrid II 50th Percentile Male (p572 version 3.3). This v-ATD predates the publication of SAE ARP 5765, Revision A, Analytical Methods for Aircraft Seat Design and Evaluation, and, therefore, has not been evaluated by the developer to show compliance with the dynamic calibration paragraph of the ARP (as discussed in paragraph 8.1.3 of this AC). The developer has shown compliance with the head drop test. During end-user evaluation of the head path from the SAE ARP dataset, this v-ATD had acceptable agreement up to 30 inches of head x-direction travel. For this current simulation, this v-ATD can be considered to be conditionally compliant with the limitations that the v-ATD be used for a § 25.562 Test 2 pulse with limited head excursion.
A.2.7 For this test setup, there are two primary occupant trajectory items to consider: head path and pelvic motion. For brevity, only xz-plane head motion is examined here. If there is significant lateral trajectory, the applicant will likely be required to validate the y-axis motion. Y-axis motion is not considered in this example. Figure A-3 of this AC shows the xz-plane motion of the head CG for the test and simulation. The figure also shows the location of the wall and front edge of the foam. The origin of the figure is the SRP. Qualitatively, the impact location was similar, with the measured height and estimated lateral location of initial head contact occurring at roughly the same location between test and simulation. Because the impact surface is completely homogenous, this estimation is sufficient to meet the impact location requirement in paragraph 8.3.5.6 of this AC. Differences exist between the initial position of the head and the depth the head travels into the foam. In general, it is appropriate for the FAA to ask the applicant to explain discrepancies and to present data to defend these explanations. This is not the same as allowing the applicant to rationalize the differences. An explanation can be supported with data. A rationalization cannot usually standup to this type of scrutiny.
Calculation of the curve shape error requires that the x-position versus time and zposition versus time be evaluated separately; curve shape error on the x-position is 5.1 percent and 8.5 percent on the z-position. The magnitude error is 0.3 in on the peak xposition with the simulation under-predicting the motion. For this impact scenario, the maximum position in the z-axis does not have significance. As referenced in paragraph
8.3.3 of this AC, lap belt loads should be evaluated separately from occupant trajectory, omitted for brevity.
A-4
Appendix A Figure A-3. Comparison of Head Path (xz-plane) A.2.8 Before evaluating HIC, aspects of the head CG resultant acceleration should be considered, as illustrated in figure A-4. The test peak is 82.3 g, and the simulation peak is 83.8 g. The difference is 2 percent, well below the 10 percent limit, and the simulation result is conservative. The curve shape error is 9.8 percent. Depending on the impact scenario, the FAA may also ask the applicant to evaluate the components of the resultant acceleration separately. The head resultant velocity at impact was 38.5 feet per second (fps) in the test and 37.7 fps in the simulation (magnitude error of 2.0 percent).
A-5