Why are steel bushings used in spar bolt holes?

Steel bushings are used in spar bolt holes primarily to prevent crushing when bolts are tightened. In aircraft structures, the spars bear significant loads, and when bolts are installed tightly, the pressure exerted can lead to deformation or crushing of the materials surrounding the bolt holes, especially if those materials are softer than the bolts themselves. The presence of steel bushings provides a harder surface that can better withstand the compressive forces without deforming, thereby maintaining the integrity of the structure and ensuring that the bolts remain properly secured over time.

Additionally, using bushings can enhance the overall durability and longevity of the connections within the aircraft by distributing the load more evenly and protecting the surrounding material from wear and tear. The bushings also provide a consistent hole size for the bolts, which contributes to more reliable assembly and maintenance practices.

While options related to aesthetic appearance, friction reduction during assembly, and weight reduction may have their relevance in specific contexts, they are not the primary functions served by steel bushings in spar bolt holes. The critical role of preventing structural damage under load makes the use of steel bushings a vital aspect of aircraft design and safety.