An estimated 8,000 U.S. and third-country nationals needed to be evacuated from Saigon and the shrinking government-controlled region of South Vietnam, along with thousands of "at-risk" Vietnamese who had worked for the United States during the war. Evacuation by civil and military fixed-wing aircraft from Tan Son Nhat International Airport had been taking place since early-March and continued until 28 April when PAVN artillery fire rendered the runways unusable. At this time a helicopter evacuation, code-named Operation Frequent Wind was implemented. U-Tapao and other USAF facilities in Thailand supported the evacuation.