A Clamping-Based Design Approach for Optical Assemblies in Aircraft Lighting: Eliminating Fastener-Induced Cracks and Validated by Thermo-Mechanical Testing

Aircraft exterior lights are subjected to severe thermo-mechanical stresses due to aerodynamic loading, vibration, and thermal cycling. Currently, Aircraft Lights are based on Light Emitting Diode (LED) technology, where they are mounted on a Circuit Board Assembly (CBA). For some applications, Total Internal Reflection (TIR) Optics are mounted concentric over these LEDs so that light is focused in the required areas. The use of high-performance polymers such as Cyclo Olefin Polymers (COP) for these Optics provides excellent light transmission and stability. Hence, there is a need to secure these Optics to CBA. A critical challenge is securing the Optics to CBA. In the initial design, Optic was fastened directly onto the CBA using self-threading fasteners. However, due to inherent properties of COP, this fastening method proved not ideal. During assembly, frequent crack initiation was observed around the fastener locations, raising concerns over durability, reliability, and maintainability. As a design improvement, a different mounting method was developed to avoid thread cutting into optic, eliminating crack initiation while maintaining adequate clamp force for the Optic assembly. In the new method, Optic is retained in its position on CBA using a precision clamping mechanism, thereby distributing assembly loads uniformly and avoiding localized stress concentrations. COP material was retained due to its superior optical characteristics and compliance with photometric requirements for aircraft lighting applications. The new design was validated by thermo-mechanical testing. This paper presents the problem definition, the design rationale and validation testing for adopting the clamp-based mechanism. The proposed design methodology provides a pathway to enhance reliability and lifecycle performance of critical optical components in aircraft applications.