A fusible plug is a threaded metal cylinder, often made from bronze, brass or gunmetal, which has a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached.
A fusible plug operates as a safety valve when dangerous temperatures, rather than dangerous pressures, are reached in a closed vessel. Fusible plugs are commonly installed in aircraft wheels, most typically in those of larger or high-performance aircraft. The very large thermal loads imposed by braking conditions, most apparent following a rejected takeoff, can cause already high pressure in the tyres to rise to the point that the tyre might burst. Should the critical temperature be reached in the wheel assembly, the core of the fusible plug will melt providing a relief mechanism which allows the tyre to deflate. The vented gas from the tyre, usually nitrogen, is often directed to help cool the brake assembly surfaces.