This paper presents a methodology for designing and evaluating lightweight, crashworthy aircraft seats that meet 21g crash safety standards and injury criteria. Four seat classes—double economy, single economy, premium economy, and business—were developed using a modular design strategy focused on part commonality (family of parts) and manufacturability. A shared family of structural components was implemented across all seat types, with dimensional modifications applied only, when necessary, due to differences in seat width or height. In such cases, the same material systems and design principles were used to ensure consistency and reduce manufacturing complexity. The designs were evaluated using finite element simulations to verify performance under aerospace crash conditions. Each seat configuration was validated against regulatory crashworthiness criteria and injury thresholds, including pelvic, lumbar, and femur compressive forces, as well as head injury criteria (HIC) values. The final designs successfully passed all crash and most injury thresholds, achieving regulatory compliance while maintaining a lightweight structure. This marked a significant improvement over a baseline seat configuration that failed to meet crash or injury requirements. By combining modularity with performance-driven design, this approach delivers a flexible and manufacturable seat family that balances safety, weight, and cost. The methodology offers broader applicability for occupant protection systems in automotive and aerospace domains, where lightweighting and regulatory compliance remain critical design challenges.