AC 20-146A CHG 1
5.3 FAA Orders.
If any FAA order is revised after publication of this AC, you should refer to the latest version on the Dynamic Regulatory System (DRS) at https://drs.faa.gov.
• FAA Order 8110.4C CHG 7, Type Certification.
• FAA Order 8150.1D CHG 1, Technical Standard Order Program.
5.4 FAA Advisory Circulars.
The following ACs are related to the guidance in this AC. You should refer to the latest AC version for guidance, which is available on the DRS at https://drs.faa.gov.
• AC 20-107B CHG 1, Composite Aircraft Structure.
• AC 21-25B, Approval of Modified Seating Systems Initially Approved Under a Technical Standard Order.
• AC 21-46A, Technical Standard Order Program.
• AC 21-50, Installation of TSOA Articles and LODA Appliances.
• AC 23.562-1B, Dynamic Testing of Part 23 Airplane Seat/Restraint Systems and Occupant Protection.
• AC 25.562-1B CHG 1, Dynamic Evaluation of Seat Restraint Systems and Occupant Protection on Transport Airplanes.
• AC 27-1B CHG 9, Certification of Normal Category Rotorcraft.
• AC 29-2C CHG 8, Certification of Transport Category Rotorcraft.
5.5 Technical Sources.
The following TSO is related to the guidance in this AC. You should refer to the latest TSO version for guidance, which is available on the DRS at https://drs.faa.gov.
TSO-C127c, Rotorcraft, Transport Airplane, and Small Airplane Seating Systems.
5.6 Industry Documents.
• Methodology for Seat Design & Certification by Analysis, NASA Advanced General Aviation Transportation Experiments (AGATE) Report WP3.4-034012- 077, NASA, Washington, DC, August 31, 2001.
• Performance Standards for Seats in Civil Rotorcraft, Transport Aircraft, and General Aviation Aircraft, SAE International Aerospace Standard, SAE AS8049 Revision C, SAE International, August 14, 2015.
• Analytical Methods for Aircraft Seat Design and Evaluation, SAE Aerospace Recommended Practice (ARP), SAE ARP 5765, Revision A, SAE International, December 2015.
• Instrumentation for Impact Test - Part 1 - Electronic Instrumentation, SAE Surface Vehicle Recommended Practice, SAE J211-1, SAE International, July 2007.
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• Instrumentation for Impact Test - Part 2 - Photographic Instrumentation, SAE Surface Vehicle Recommended Practice, SAE J211-2, SAE International, November 2008.
• Gowdy, V., DeWeese, R., Beebe, M., Wade, B., Duncan, J., Kelly, R., and Blaker, J., A Lumbar Spine Modification to the Hybrid III ATD For Aircraft Seat Tests, SAE Technical Paper 1999-01-1609, General, Corporate, and Regional Aviation Meeting and Exposition, Wichita, KS, April 20-22, 1999.
• Metallic Materials Properties Development and Standardization (MMPDS) Handbook, MMPDS-07, Battelle Memorial Institute, April 2012.
• Composite Materials Handbook (CMH-17), SAE International, July 11, 2012.
5.7 Other References.
• American Society of Mechanical Engineers (ASME) V&V 10-2019, Guide for Verification and Validation in Computational Solid Mechanics, 2019.
• Belytschko, T., Liu, W.K., and Moran, B., Nonlinear Finite Elements for Continua and Structures, John Wiley and Sons, Chichester, West Sussex, England, 2000.
6 DEFINITIONS.
6.1 Baseline Seat.
The seat that encompasses the common design methodology from which other seats within a family of seats can be derived. This includes items such as geometry, material, manufacturing method, and attachment methods. The baseline seat is typically the seat selected for testing based on its criticality. Other seats in the family will be similar to the baseline seat but might vary based on select details. Some of these details include, but are not limited to, leg spacing, placement of reinforcements (to support attachments), and rivet locations.
6.2 Baseline Testing.
The initial series of tests performed on the baseline seat as part of the original certification to substantiate the seat family.
6.3 Computer Modeling.
The use of computer-based finite element or multi-body transient analysis to simulate the emergency landing dynamic condition of the applicable airworthiness standard.
These codes typically follow an explicit formulation for calculation of the structural response but can follow a centered mass formulation for calculation of the occupant response.
6.4 Family of Seats.
A group of seat assemblies, regardless of the number of seat places, built from equivalent components in the primary load path.
6.5 Mass Scaling.
In finite element modeling, the process of adding nonphysical mass to the structure to increase the time step, thereby reducing the run time.
6.6 Occupant.
In this AC, occupant is not synonymous with the word passenger; occupant refers to the anthropomorphic test device (ATD). The occupant model is used to correlate the behavior of the ATD, as opposed to human biodynamic behavior.
6.7 Seating Configuration.
The interior floor plan of the aircraft, which defines the seating positions available to crew and passengers during taxi, takeoff, landing, and in-flight conditions.
6.8 Seat Primary Load Path.
The components within the seat that carry the load from the point of load application to the structure that reacts the load from the seat system or sub-system. The seat primary load path varies depending on the parameter being evaluated, as follows:
• Structural—from seat belt/harness to fittings attaching seat system to aircraft structure.
• Lumbar—from bottom cushion to fittings attaching seat system to aircraft structure.
• Row-to-row head injury criterion (HIC)—from point of ATD head contact to the attachment of seat primary structure.
• Head path (for example, front row or large pitch seats)—from seat belt/harness to fittings attaching seat system to aircraft structure.
6.9 Seating/Restraint System.
A system includes the seat structure, cushions, upholstery, safety belt, shoulder harness, and attachment devices.
AC 20-146A CHG 1 7 VERIFICATION OF EXPLICIT CODES.
The verification process determines the computational model accurately represents the underlying mathematical model and its solution (reference paragraph 5.6 of this AC).
Verification of explicit codes precedes validation, as it is important to minimize errors before progressing. Verification consists of two components, code verification, and calculation verification. Calculation verification is further divided into temporal and spatial convergence. The following sections discuss how this process could be applied for explicit codes. In certain situations, implicit codes may be used, and verification methods should be applied.
7.1 Code Verification.
7.1.1 Code verification is the process of determining that the numerical algorithms are correctly implemented in the computer code and identifying errors in the software.
(Refer to ASME V&V 10-2019 listed in paragraph 5.6 of this AC). Verification helps ensure the implementation of the mathematical model and solution algorithms are working correctly in the code, and that the code solution predicts the analytical solution to a certain extent.
7.1.2 Computer analyses used for certification purposes are typically conducted on a commercially available computer code. It is the applicant’s responsibility to select and assess that computer code produces valid results. The applicant or their software provider performs code verification on the hardware and software environment of their design. In addition to performing code verification, the applicant or their software provider should also include copies of their quality assurance policies and procedures in the verification data submitted for certification.
7.2 Calculation Verification.
Calculation verification, also called solution verification, is the process of determining the solution accuracy of a particular calculation (refer to ASME V&V 10-2019). The goal of calculation verification is to show that discretization errors due to insufficient spatial or temporal discretization is small, and that the model converges to a unique solution with spatial and temporal refinement, respectively. The convergence errors in the numerical model outputs for the system response quantities of interest should be minor compared to the differences allowed in validation comparisons.
7.2.1 Temporal Convergence.
7.2.1.1 The dividing of the total time of a simulation into smaller segments is called temporal discretization. Each segment is typically referred to as a time step, denoted as Δt below. The stability of explicit time integration methods depends on the time step. If the time step is too large for a given element size and material, the method fails, either because of stability issues or poor accuracy. If the time step size is getting too small, the solution time becomes impractical, thus diminishing the effectiveness of the method. The critical time step for a given finite element according to 7
AC 20-146A CHG 1 the Courant stability condition is Δtcr = 2/ω = min(L/c). In this equation, ω is the natural frequency of the finite element, L is the characteristic length of the finite element, and c is the sound wave speed (a function of material stiffness and density) as described by Belytschko, et al. (reference paragraph 5.6 of this AC). The time step used for the calculation is the smallest Δtcr of all finite elements in the model. The time step selected for the analysis should be smaller than the time for the sound wave speed to cross the smallest element in the finite element mesh. Otherwise, numerical instability might develop and cause the solution to diverge.
Belytschko addresses the inherent numerical instabilities encountered with explicit dynamic analysis codes.
7.2.1.2 In theory, the most numerically efficient solution is obtained when an integrating time step equivalent to the stability limit is chosen.
Commercial codes attempt to offset the problems of numerical instability by regulating and constantly updating the time interval used throughout the analysis. The simulation uses a time step based on the element size, material, and element type. The time step is recomputed at each cycle based on the mesh size and stiffness of the materials.
7.2.1.3 Occasionally, the simulation time step is controlled by only a few small or stiff elements in the model. When this happens, it is typically useful to remesh the controlling elements. In the case of extreme compression, such as with foams consolidating, some codes can automatically remove elements when the length becomes a small fraction of the initial length to avoid extreme time steps or instabilities.
7.2.1.4 When updating the mesh is not practical, mass scaling can be employed.
Mass scaling is the process of adding nonphysical mass to the structure to increase the time step, thereby reducing the run time. Choose a minimum time step (Δt) that is greater than the time step required to satisfy the Courant criteria. The software adds mass in all the elements with a critical integration time step smaller than this given value. A scale factor is applied on the critical time step to take into account that the time step for the current configuration is computed based on the previous configuration (t−Δt). A scale factor applied to the time step can be used to prevent the instability by enforcing the time integration algorithm to use a smaller time step. Depending on the quality of the mesh, and strain rate properties a different scale factor may be used. Typically, the scale factor varies from
0.6 to 0.9.
7.2.1.5 Mass scaling should only be used on the smallest elements contained in the model, so as not to affect the overall mass of the system. The FAA recommends that the overall mass scaling should not exceed 5 percent for critical seat components. For non-critical seat components, an increase in mass of up to 10 percent is acceptable. For quasi-static simulations, it is 8
AC 20-146A CHG 1 acceptable to increase the mass scaling up to 10 percent since the kinetic energy is small. Rigid body techniques do not use mass scaling.
7.2.2 Spatial Convergence.
7.2.2.1 The finite element analysis technique divides a continuum into finite elements, volumes, surfaces, and line segments that are, interconnected at a discrete number of points, called nodes, and solve the boundary-value problem. The number of elements and the types of elements used greatly affect the accuracy of the result. For example, a coarse mesh can produce erroneous results. Construction of a model includes a trade-off between the accuracy of the solution resulting from the mesh, and the amount of time it takes to run the simulation. Typically, one uses the coarsest mesh that produces a sufficient level of accuracy. As such, evaluation of spatial convergence is necessary in components that are in the critical load path.
7.2.2.2 An estimate of the spatial convergence can be generated based on two or more mesh refinements. If the results of the numerical solution do not change significantly from the refinement, one can expect to be close to the asymptotic region. For example, the compression of a square cushion via a weight can be computed for differing mesh sizes. (See figure 1 below.)
The results have converged at the fourth mesh, but the third mesh may be acceptable. Exact calculation of the spatial convergence error of an explicit structural analysis is a non-trivial pursuit that is an ongoing research activity. Adaptive meshing techniques and extreme compression of elements or removal of elements will have unknown effects on the spatial convergence.
Figure 1. Mesh Refinement Example 9
AC 20-146A CHG 1 8 COMPUTER MODEL VALIDATION.
8.1 Overview.
8.1.1 The validation process determines the degree to which a model is an accurate representation of corresponding physical experiments from the perspective of the intended uses of the model (refer to ASME V&V 10-2019). The ability of the model to represent a physical phenomenon is evaluated by comparing the model predictions with physical test data. This process relies on high-quality test data and a quantitative comparison of test and simulation results. Validation is not an iterative process of refining the model to reduce discrepancies between the experimental systems and model outcomes. Refining the model is called calibration; it does not provide confidence that the model can accurately predict the system’s response to untested conditions.
8.1.2 This AC provides guidance on the numerous parameters that deserve consideration when comparing the results of numerical analysis to actual test data. However, in performing the validation of a numerical model, there is no substitute for good engineering judgment. An applicant should not use the guidance in this AC as a substitute for communicating and coordinating with the applicable certification branch when validating a numerical model. Sections 9 and 10 of this AC define the conditions where an applicant may use the results from computer analysis for certification purposes.
8.1.3 Any virtual anthropomorphic test device (v-ATD) used in the analysis should simulate the same type ATD that was used in the physical testing, which is defined in § 2X.562, TSO-C127x, or FAA policy. The applicant should demonstrate that the v-ATD is calibrated to behave similarly to the physical ATD or that it meets the guidance in SAE ARP 5765, revision A or later. Before any of the validation criteria are calculated, check the occupant simulation software for the reference location of all sensors of the v- ATD compared to the corresponding locations of the sensors on the ATD.
8.2 General Validation Acceptance Criteria.
8.2.1 The model should be validated against the original dynamic tests in accordance with § 2X.562(b) or TSO-C127x. The FAA will not accept a model validated against another validated model.
8.2.2 Model extrapolation should be limited to conditions that are similar to the model validation conditions. Occupant trajectory and head injury criteria detailed in appendix A of this AC. The current seat analysis should be similar to the test and analysis used to validate the numerical model, including loading conditions, seat type, and worst-case conditions. For example, the applicant should not use test results from a four-legged seat to validate a three-legged seat model. As another example, a model validated against longitudinal test data should not be used to evaluate vertical test conditions.
8.2.3 Conservative simulation results are encouraged, but not required. In the context of simulation, conservatism relates to the model over-predicting a particular response or 10
AC 20-146A CHG 1 under-predicting a particular strength. For example, if in the test, the calculated HIC was 700, but the model denoted 725, then the simulation is conservative.
8.2.4 The level of correlation required should not be more stringent than the level of accuracy of the test data, which is dependent on the test instrumentation.
8.2.5 The FAA considers the computer model validated if an acceptable agreement can be shown between the analysis and test data for those parameters critical to the application of the model. The calculation methods are detailed in appendix B of this AC. Test data used to validate the model should be included as an appendix in the validation and analysis report (VAR), refer to section 12 of this AC.
8.2.6 When precise occupant trajectory information is not required, a visual comparison of the event can be sufficient.
8.2.7 In addition to the general validation criteria above, the applicant might need to validate the model to some or all of the application specific criteria defined in paragraph 8.3 of this AC to ensure that the design requirements of the computer model are correct.
8.3 Application-Specific Validation Criteria.
The applicant should validate relevant parameters to the application of the model. The applicant and the certification branch should identify and agree on the validation criteria specific to the application, and the certification plan should list those criteria. The applicant and the certification branch should negotiate any additional validation criteria not listed in this AC. The following paragraphs provide guidance on the validation parameters to consider and suggested correlation levels. However, the final levels should be coordinated with the certification branch prior to the initiation of any program. If no acceptable rationale is available to make this determination, then the details listed in the following paragraphs may be followed:
8.3.1 Occupant Trajectory.
8.3.1.1 The general occupant trajectory should correlate to the test data. The trajectory should be specified by time history plots under SAE J211-2.
Occupant trajectory describes the overall translational and rotational motion of the occupant. SAE AS8049 defines use of the seat reference point (SRP) as the datum to determine occupant trajectory or position. The trajectory of the occupant can include head path, pelvic displacement, or torso displacement. When the pelvis is not visible, the knee can be used as a surrogate. If there is a concern regarding femur injury, occupant trajectory can include leg motion as well.
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8.3.1.2 Head path trajectory can be, in and of itself, a validation item. For example, if the applicant conducted a validation effort to support a claim that no head contact occurs, head path is a unique validation item. It can also be used to support another parameter, such as HIC.
8.3.1.3 The numerically derived occupant trajectory should be compared to high-speed video obtained from dynamic tests. The ability of the computer model to predict an occupant trajectory can be established by comparing time-history plots, for example, x-position versus time, and z-position versus time to calibrated photometric data obtained from the baseline dynamic test. Acceptable limits for model validation may be based on criteria established between the applicant and the FAA.
8.3.1.4 Quantitative evaluation of extremity flail is typically not required, specifically for forward- and aft-facing seats. However, arm and leg interaction with other seats or monuments can have a significant effect on the motion of the head and pelvis and, therefore, should not be completely ignored.
8.3.1.5 For side-facing seats, the applicant can use computer modeling to demonstrate that only incidental body-to-body contact will occur when the occupants are exposed to the accelerations and velocities pursuant to § 2X.562. This assumes that the seat structure and occupant motion have been validated to a baseline side-facing seat test.
8.3.1.6 Appendix A of this AC illustrates some of the items to consider when evaluating occupant trajectory and HIC. Although appendix A is an example of those items to consider in this type of validation, it is not a universal example. Other situations might require more or less stringent validation efforts.
8.3.2 Structural Response.
Quantifying the structural response of the computer model includes evaluating the internal loads and structural deformations of the seat. Validation of the computer model should include a comparison of the structural performance criteria.
8.3.2.1 Internal Loads.
8.3.2.1.1 The applicant and the certification branch should work together to establish what floor reaction loads, if any, are critical to the application of the analysis. Correlation of the floor reaction loads demonstrates a properly modeled load path from the occupant to the restraint system to the floor. The peak critical floor reaction loads between the analysis and test data should correlate to within 10 percent unless a different level of correlation was determined prior to the initiation of any program. In addition, the applicant should provide data showing that the critical floor 12
AC 20-146A CHG 1 load reactions correlate to the test results as described in appendix B of this AC.
8.3.2.1.2 There may be times when the applicant introduces a unique design for a primary load path member. The applicant may choose to install test instrumentation to monitor the internal loads or strains on this member.
This instrumentation is not required for certification but may be useful in validating the numerical model. In this instance, the applicant and the certification branch should work together to determine how or if these loads will be used to validate the model.
8.3.2.2 Structural Deformation.
8.3.2.2.1 A comparison of the planar space plots of structural deformation obtained from the analysis and photometric data obtained from dynamic test can help validate the model. Acceptable agreement should be obtained between the shape and magnitude, within 10 percent as described in appendix B of this AC unless a different level of correlation was accepted prior to the initiation of any program, for members that are critical to the overall performance or structural integrity of the seat or seating system.
Not all safety margins or all modes of failure need to be examined, only those the certification branch and applicant believe will be critical. The applicant can use visual comparisons between the test and analytical data for non-critical structural members.
8.3.2.2.2 If analysis of dynamic tests requires the determination of post-test permanent deformation such as that required for a structural test, then the analysis should differ by no more than 10 percent, unless a different level of correlation was determined prior to the initiation of any program, from the test and should not exceed any allowable deformations.
8.3.2.3 Failure Modes.
Failure modes of components may be validated through full-scale dynamic tests or component tests following a building block approach. See figure 2 of this AC. Modeling of tests that show structural failure can provide confidence in the model’s ability to capture relevant failure modes. If the model is only validated against tests with no failure or permanent deformation, there is little confidence that the model is capable of predicting failure. The conceptual framework of component and subcomponent level tests typically included in the building block approach can be adapted to the seat structure. The large quantity of tests needed to provide a statistical basis comes from the lowest levels, coupons, and elements (which are used to calibrate the numerical model), and the performance of structural details are validated in a lesser number of subcomponent and component tests. Detail and subcomponent tests may be used to validate the ability of analysis methods to predict local strains and failure modes. Added statistical considerations—for example, 13
repetitive point design testing and/or component overload factors to cover material and process variability—will be needed when analysis validation is not achieved. The static strength substantiation program should also consider all critical loading conditions for all critical structure. Refer to AC 20-107B, CHG 1 for further information.
Figure 2. Schematic Diagram of Building Block Approach
8.3.2.4 Joints and Fittings.
Joints and fittings are typically highly loaded seat structural elements. In general, they possess indeterminate load paths and contact (free-play) nonlinearities and might be difficult to model mathematically. Changes in the load path or material properties of these elements can affect the structural integrity and performance of the seat. Therefore, these parts of a seat structure should be modeled with care, paying particular attention to all possible failure mechanisms. Some changes to fittings and joints are acceptable to substantiate with modeling. However, significant changes to the material or mechanism of load transfer require tests.
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8.3.3 Restraint System.
8.3.3.1 With few exceptions, such as aft-facing seats and side-facing seats bounded by a wall or divider, the restraint system contributes significantly to the retention of the occupants and is part of the primary load path from the occupant to the seat. In this case, the test-simulation correlation should be evaluated for the restraint loading time history and the maximum value.
Maximum values that correlate to within 10 percent, unless a different level of correlation was determined prior to the initiation of any program, will ensure the computer model will predict the inertial force transfer from the occupant to the seat. For a seating configuration with a shoulder belt, conservatism is not always straightforward. Over-estimation of the shoulder belt load can lead to under-estimation of the ATD head x-motion and vice versa.
8.3.3.2 Additional parameters, such as belt payout or permanent elongation, when present, are not required for seat certification, but should be taken into account in order to provide additional confidence that the model is capturing the pertinent physics. Another factor to consider is any belt tightening that can occur after an inertial reel lock.
8.3.3.3 Belt payout is a term used to describe how much of the shoulder harness restraint is released before locking of the inertia reel. In a sudden deceleration, it is unlikely the shoulder harness instantly locks in place.
There is a finite length of time where the harness is free to release from the reel. The amount of restraint released from the reel is the belt payout.
8.3.3.4 Occupant trajectory and restraint system loads are closely related functions. The necessity of validating the restraint system loads does not negate the necessity of validating the occupant trajectory in situations where the restraint system loads are required. It is not acceptable to show compliance to occupant trajectory instead of restraint system performance or to validate restraint system performance at the expense of occupant trajectory.
8.3.4 Spine Load.
8.3.4.1 Section 2X.562 and TSO-C127x defines the certification requirements for lumbar spinal loading. The maximum allowable load is 1,500 pounds (lbs) of compression. The computed spine load should be correlated with the test data when appropriate. This includes, but is not limited to, the initial validation of a test scenario or situations where a design change could affect this parameter such as a seat cushion change. The spine load time-history and maximum spine load obtained in the analysis should correlate to within 10 percent of the dynamic test data unless a different level of correlation was determined prior to the initiation of any program and the numerical spine load should not exceed 1,500 lbs.