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8.3.4.2 To account for the testing uncertainty, conservatism can be incorporated into validation and model use via a factor of safety. Repeated testing of seat cushions shows a typical test variability in lumbar load of about ±125 lbs, which gives a data spread of 250 lbs in lumbar load when testing parameters are tightly controlled. Assuming uncertainty is normally distributed, this data spread gives a standard deviation of approximately 42 lbs Based on this standard deviation, there is a 95 percent confidence that the true load is below the regulatory limit of 1,500 lbs, if the measured or simulated load is no greater than 1,430 lbs.
Note: TSO-C127x references SAE AS8049 and SAE ARP 5526C as the source of minimum performance standards (MPS) for seating systems.
SAE ARP5765A defines a means of assessing the credibility of computer models of aircraft systems.
8.3.4.3 Setting the cumulative probability of 95 percent equal to 1,500 lbs for normally distributed data using a standard deviation of 42 lbs results in a mean value of 1,430 lbs for the distribution. Therefore, only seat configurations with dynamic test data that yield spine loads below 1,430 lbs should be used for validation. Likewise, for model use, it is recommended that only models that produce a lumbar load below 1,430 lbs be used.
8.3.4.4 In the validation phase, models can exceed 1,430 lbs. In situations where the numerical spine load under-predicts the dynamic test data, the numerical spine load should not exceed 1,430 lbs minus the magnitude of the under-prediction for the Hybrid II; there is no adjustment made for under-prediction for the FAA Hybrid III.
8.3.4.5 Table 1 below provides an example of validation under-prediction and validation over-prediction and the corresponding model use recommended limits. In the first case, the model under predicts by 50 lbs, so the model use is limited to 1,380 lbs (1,430 lbs minus 50 lbs) for the Hybrid II, and there is no adjustment made for the FAA Hybrid III. In the second case, the model over predicts and the model use is simply limited to 1,430 lbs.
AC 20-146A CHG 1 Table 1. Example Peak Lumbar Loads Example Peak Lumbar Loads Validation Hybrid II Model Use FAA Hybrid III Model Use Model under-predicts Test = 1,400 lbs, Model = 1,350 lbs Model = 1,380 lbs or less Model = 1,430 lbs or less Model over-predicts Test = 1,400 lbs, Model = 1,450 lbs Model = 1,430 lbs or less Model = 1,430 lbs or less
8.3.5 Head Injury Criteria (HIC).
8.3.5.1 Section 2X.562 or TSO-C127x defines the certification requirements for HIC. The applicant can use the results of computer modeling to show compliance with these requirements, within the limitations summarized below. However, an installation change that results in a significantly higher head strike velocity will likely require testing.
8.3.5.2 The regulation specifies calculating HIC during the duration of the head impact, with a maximum allowable HIC limit of 1000 units. The selected time interval should correspond to the duration of the major head impact on aircraft interior features.
8.3.5.3 The profile, that is, the shape and peak “g” of the acceleration time-history plot, as well as the average “g” loading for resultant head accelerations obtained in the analysis, should correlate to the results of the dynamic test.
The acceleration is measured at the head center of gravity for an ATD.
Occupant simulation software should be checked for the reference location of the head acceleration output. Sensor locations should be documented in the v-ATD calibration report.
8.3.5.4 Given two dynamic tests with the same desired deceleration profile, the maximum HIC values will likely vary. Therefore, a precise match between the test derived HIC and the analytical HIC is not realistic. However, the maximum analytical HIC value should correlate to within 100 HIC units of the maximum test derived HIC value unless a different level of correlation was determined prior to the initiation of any program. The FAA encourages generation of conservative HIC prediction models. One method to add conservatism to the process is to incorporate test uncertainty as a factor of safety in validation and model use. Using the same process as in paragraph 8.3.4.2 of this AC, and assuming a typical data spread of ±200 HIC units, the 95 percent confidence HIC value is 890 HIC units. Therefore, the FAA recommends that only seat configurations with dynamic test data that produce a HIC value below 890 HIC units 17
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8.3.5.5
8.3.5.6 should be used for validation. Likewise, for model use, the FAA recommends that only models that produce a HIC value below 890 HIC units be used.
Note: Models can exceed 890 HIC units in the validation phase.
It is also unlikely that the analytical head deceleration time history function will perfectly match the test-generated head deceleration time history function. Therefore, the initial and final integration times, t1 and t2, as defined and used in § 2X.562 or TSO-C127x, will likely vary between test and analysis. These time values should differ by no more than 5 ms (milliseconds). In addition, the contact velocity should match within 10 percent and location of the contact should match.
Note: Regardless of the validation of the model and the specific configuration, the predicted HIC must always remain below 1,000 HIC units for the data to comply with the § 2X.562.
The following is not an exhaustive list, but the applicant may choose to use computer modeling under the following circumstances:
1. The predicted occupant head strike envelope will satisfy the above stated requirements by showing that no contact with adjacent seats, structure, or other items in the cabin will occur.
2. To evaluate a modified seat installation where the potential head impact surfaces are identical, only the geometric strike envelope has changed. Original HIC values exceeding 890 HIC units will typically not support numerical substantiation.
3. The applicant has performed dynamic testing in the presence of a rigid structure. The applicant would then reposition the seat in the aircraft where the head strike will be on a less rigid structure but with equivalent head strike velocities.
4. For scenarios that meet the guidelines in table 2 below. This includes situations where the head impact surface has changed.
Table 2. Example HIC Values Example HIC Values Validation Hybrid II Model Use FAA Hybrid III Model Use Model under-predicts (in HIC units) Test = 850, Model = 800 Model = 840 or less Model = 890 or less Model over-predicts (in HIC units) Test = 850, Model = 900 Model = 890 or less Model = 890 or less 18
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8.3.6 Femur Compressive Load (Part 25 Airplanes Only).
Section 25.562(c)(6) or TSO-C127x defines the certification requirements for axial compressive loading of the femur. The maximum allowable limit is 2,250 lbs. The femur compressive load is usually not an issue in the testing of part 25 seats. However, if the certification branch or the applicant determines it should be evaluated, the load time-history profile for the compressive femur load obtained in the analysis should correlate to the dynamic test data. The peak load value, as determined by the analysis, should correlate to within 10 percent of the dynamic test results unless a different level of correlation was determined prior to the initiation of any program.
8.3.7 Non-Critical Criteria.
8.3.7.1 The applicant should validate parameters that are important to the particular application of the analysis. Depending on the purpose of the analysis, it may not be necessary to meet all validation criteria in this AC.
However, the FAA cautions that gross discrepancies in the model (such as unrealistic load paths or failure modes) may impact the ability of the model to predict parameters of interest. The following examples are offered to illustrate non-critical criteria for the specified model use condition:
8.3.7.1.1 Lumbar loads for the horizontal test, refer to § 2X.562(b)(2) or TSO-C127x, with a two-point restraint are usually not critical. It is unlikely the applicant will have to evaluate the ATD lumbar load in this test condition.
8.3.7.1.2 The upper torso restraint for a side-facing seat, where the occupant is adjacent to a structural barrier, typically carries small loads. It may be of little value for the applicant to correlate the analytical upper torso restraint loads to the test data.
8.3.7.1.3 For the horizontal test required by § 2X.562(b)(2) or TSO-C127x, lateral (y-axis) floor reaction loads are small compared to the vertical or horizontal reaction loads. It is not reasonable to expect the applicant to correlate the model to all three loads (that is, vertical, lateral, and horizontal) reported by each load cell. It is more reasonable to require validation for those loads critical to the application of the model.
8.3.7.1.4 Restraint loads in a § 25.562(b)(1) or TSO-C127x test are essentially zero throughout the critical portion of the test. It is unlikely the applicant will have to evaluate these loads.
8.3.7.2 These examples do not constitute an exhaustive list. They are simply meant to illustrate that engineering judgment and the particular application of the model should guide the applicant and the certification branch to the proper validation criteria.
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8.4
8.4.1
8.4.2
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9.1
9.1.1
9.1.2
9.2 Discrepancies.
Failure to satisfy all validation criteria does not automatically preclude the model from being validated. The applicant and the certification branch engineer should evaluate whether the deviations impact the ability of the model to predict credible results and determine if deviations from the validation criteria are acceptable.
In addition, the applicant may present evidence to show the deviation is within the inherent reliability and statistical accuracy of the test measurements. The applicant should quantify any discrepancies between the results obtained from analysis and the dynamic test data for those parameters that are critical to the application of the analysis.
Computer Hardware and Software.
Any analysis should be conducted on a platform, which has been shown to be reliable and acceptable. Certification data produced by a computer model should be performed on the same hardware, compute environment (operating system, auxiliary software, components, etc.), and software version on which the validation was conducted. If a different software version, hardware platform, and/or compute environment is used, the applicant should revalidate the model using the new configuration.
APPLICATION OF COMPUTER MODELING IN SUPPORT OF DYNAMIC TESTING.
General.
There will be occasions when the applicant wants to determine the critical loading scenario for a particular seating system. This paragraph provides guidance on those items to consider when the applicant performs trade studies to identify the most critical configuration/installation. A final certification test is required to certify the critical configuration/installation to the requirements of § 2X.562 or TSO-C127x.
Paragraphs 9.2 through 9.4 of this AC specify the conditions when a computer model can be used to provide engineering analysis and rationale in support of dynamic testing.
These conditions do not form an exhaustive list of items to consider, but they are the most common.
Determination of Worst-Case Scenario for a Seat Design.
After the computer analysis is complete, the results from the simulation can be used to determine the worst-case or critical loading scenario for a particular seating system.
This may include the following:
• Identifying components of seat structure that are critically loaded.
• The selection of the critical seat tracking positions, such as seat adjustment positions.
• An evaluation of the restraint system, such as critical attachment location.
Note: The restraint system is not limited to the actual belts; it also includes the required anchoring attachments. Computer modeling may be used to analyze the effect of anchoring the restraints at different locations on the seat frame or in the aircraft. The restraint system would also include any inflatable devices used for restraint or occupant protection. This AC does not address the issues that may be necessary to correctly model and validate their operation; however, the general principles discussed here apply.
• An evaluation of the yaw condition to address loading on the seat frame and movement of the occupant out of the restraint system.
• The number of seat places occupied.
• The selection of the worst-case seat cushion build-up.
9.3 Determination of Worst-Case Scenario for Seat Installation.
Results of a validated computer model may be used to select the worst-case seat system installation as a candidate for dynamic testing. In determining the most critical seat installation, each seating system should be analyzed in its production installation configuration. For example, an analysis to determine a worst-case seating system may include seating systems installed at different positions in the fuselage, which will result in various restraint anchor positions relative to the occupant and seat structure.
9.4 Determination of Occupant Strike Envelope.
The results of the computer analysis can be used to determine the occupant strike envelope with aircraft interior components. Each seating system should be analyzed in its production installation configuration. The occupant strike envelope will determine if a potential for head strike exists and, if so, which items are required in the test setup during the HIC evaluation tests.
10 APPLICATION OF COMPUTER MODELING INSTEAD OF DYNAMIC TESTING.
There will be occasions when the applicant wants to certify a seat that is based on a certificated design concept, such as a family seat design, that differs from the certificated design. If the applicant intends to use the results of computer modeling to provide engineering/certification data instead of dynamic testing for a modified design, then the results from this validated model can be applied to the modifications specified in paragraphs 10.1 and 10.2 of this AC.
10.1 Seat System Modification.
10.1.1 Analysis based on a validated computer simulation may be used to substantiate seat designs or installations that have been modified from a certificated configuration. These modifications may include changes to primary and non-primary load path structural members.
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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.
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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.