A hybrid inner fixed structure is disclosed. The inner fixed structure may have an inner fixed structure panel having an inner fixed structure panel surface and an inset panel having an inset panel surface disposed in at least partial contact with a first portion of the inner fixed structure panel surface. In this manner, a hybrid inner fixed structure may be designed to be both lightweight and heat tolerant.

A fastener including: a sidewall oriented about a central axis, the sidewall including at least one of 1) at least one first type of deformable projection comprising a teardrop shaped radial face, or 2) an arcuate portion, a radial corrugation portion, and a circumferential corrugation portion, where the sidewall is a layered structure including a substrate and an insulating layer overlying the substrate.

A fastener assembly includes a head section with an outer profile configured to engage a tool socket. The outer profile comprises flat portions meeting at vertex regions, at least one of which has a radius of curvature selected to define a torque threshold at which the head section slips relative to the socket. The outer profile can be hexagonal, rectangular, or triangular, and may include a slot to receive a blade-type driver. Cantilevered flat portions may also be provided to flex at a predetermined torque.