For many years most transport aircraft throughout the world have used a common form of windshield construction with a single ply of tempered glass for pressure resistance, combined with a thick polyvinyl butyral interlayer. This interlayer is usually reinforced at the edges with a metal insert and acts as a diaphragm to provide bird impact resistance, and also serves as a secondary load path to contain pressurisation in the event of failure of the primary glass ply.