The A-7 Corsair II is a carrier-based subsonic light attack aircraft design that was introduced to replace the A-4 Skyhawk in US Naval service and based on the successful supersonic F-8 Crusader aircraft produced by Chance Vought. The A-7 was one of the first combat aircraft to feature a head-up display (HUD), doppler-bounded inertial navigation system (INS), and a turbofan engine. It initially entered service with the US Navy during the Vietnam conflict and was then adopted by the United States Air Force to replace their A-1 Skyraiders that were borrowed from the Navy as well as with the Air National Guard.