Nevertheless, plans were mooted as early as 1992 to undertake a slow withdrawal of the aging type in favour of larger and more technologically advanced rotorcraft. The US Army phased out the UH-1 with the introduction of the Sikorsky UH-60 Black Hawk, although the Army UH-1 Residual Fleet had around 700 UH-1s that were to be retained until 2015, primarily in support of Army Aviation training at Fort Rucker and in selected Army National Guard units. Army support for the craft was intended to end in 2004; The UH-1 was retired from active Army service in early 2005. During 2009, Army National Guard retirements of the UH-1 accelerated with the introduction of the Eurocopter UH-72 Lakota. The final UH-1 was retired in 2016. U.S. Air Force In October 1965, the United States Air Force (USAF) 20th Helicopter Squadron was formed at Tan Son Nhut Air Base in South Vietnam, equipped initially with CH-3C helicopters. By June 1967, the UH-1F and UH-1P were also added to the unit's inventory and, by the end of the year, the entire unit had shifted from Tan Son Nhut to Nakhon Phanom Royal Thai Air Force Base, with the CH-3s transferring to the 21st Helicopter Squadron. On 1 August 1968, the unit was redesignated the 20th Special Operations Squadron. The 20th SOS's UH-1s were known as the Green Hornets, stemming from their color, a primarily green two-tone camouflage (green and tan) was carried, and radio call-sign "Hornet". The main role of these helicopters were to insert and extract reconnaissance teams, provide cover for such operations, conduct psychological warfare, and other support roles for covert operations especially in Laos and Cambodia during the so-called Secret War. USAF UH-1s were often equipped with automatic grenade launchers in place of the door guns. The XM-94 grenade launcher had been tested on Army rotorcraft prior to its use by the USAF. The unit was capable of firing 400 grenades per minute, up to 1,500 yards effective range. Into the twenty-first century, the USAF operates the UH-1N for support of intercontinental ballistic missile (ICBM) sites, including transport of security personnel and distinguished visitors. On 24 September 2018, the USAF announced that the Boeing/Leonardo MH-139 (an AW-139 variant) had won a competition to replace the UH-1Ns. U.S. Navy The US Navy acquired a number of surplus UH-1B helicopters from the U.S. Army, these rotorcraft were modified into gunships, outfitted with special