A methodology for aircraft seat certification by dynamic finite element analysis

Abstract

Dynamic aircraft seat regulations are identified in the Code of Federal Regulations 1(CFR), 14 CFR Parts 23.562 and 25.562 for crashworthy evaluation of a seat in a dynamic environment. The regulations specify full-scale dynamic testing on production seats. The dynamic tests are designed to demonstrate the structural integrity of the seat to withstand an emergency landing event and occupant safety. The Society of Automotive Engineers (SAE) standard, AS8049, supports detailed information on dynamic seat testing procedures and acceptance criteria. Full-scale dynamic testing in support of certification is expensive and repeated testing due to failures drastically increases the expense. Involvement of impact environments, flexibility in interior configurations and complexity of seat engineering designs make analysis of these problems quite complex, so that classical hand calculations are practically impossible. Efforts have been made to improve the occupant safety and to reduce the testing costs through substantiation via computer modeling analysis techniques. Development in Dynamic Finite Element Analysis (DFEA) methodology helps the aircraft industry in designing and certifying seats and other interiors more economically and confidently. The objective of this study is to provide a methodology for aircraft seat certification by using DFEA techniques. The goal of the Finite Element Analysis (FEA) in product development is not only to design a seat but also to substantiate the certification tests or replace the certification tests. The case of a certification by substantiation tests increases the necessity of validation of the Finite Element model. The US government Advisory Circular (AC) 20-146 demonstrates the methodology for dynamic seat “Certification by Analysis” for use in Parts 23, 25, 27 and 29 airplanes and rotorcraft. This AC provides guidance on how to validate the computer model and under what conditions the model may be used in support of certification or Technical Standard Order (TSO) approval/ authorization and same validation process utilizes in this research work. In the methodology of this dissertation, the seat certification process for business jet aircraft is defined in 3 stages: 1. Evaluation of seat critical options by using a simple FE – one-dimensional (1D) seat model, 2. Non linear FE analysis of seat worst loading condition and validation with the test results, 3. Substantiation of certification test or tests by using validated FE model and updation of the certification plan. Best FE modeling practices for dynamic aircraft seats, material testing procedures and component validation are presented here using nonlinear FE codes such as the LS-DYNA, with two case studies of aircraft passenger seats. Case 1 - Combined Vertical / Longitudinal velocity change dynamic test condition (15g side facing passenger seat – Part 23). Case 2 - Longitudinal velocity change dynamic test condition, (16g forward facing business jet passenger seat – Part 25). Comparisons of the DFEA and test results indicate reasonable correlations, establishing confidence in the DFEA methodology.