9th Tactical Reconnaissance Squadron comprising 4 RB–66Bs and 2 EB–66Cs arrived at the base. The RB–66Bs were equipped with night photo and infrared sensor equipment and began reconnaissance missions over South Vietnam, while the EB–66Cs began flying missions against North Vietnamese air defense radars. By the end of May, two more EB–66Cs arrived at the base and they all then redeployed to Takhli Royal Thai Air Force Base. In mid-May 1965, following the disaster at Bien Hoa the 10 surviving B-57 bombers were transferred to Tan Son Nhut AB and continued to fly sorties on a reduced scale until replacement aircraft arrived from Clark AB. In June 1965, the B-57s were moved from Tan Son Nhut AB to Da Nang AB. On 8 October 1965 the 20th Helicopter Squadron equipped with 14 CH-3 helicopters was activated at the base, it moved to Nha Trang Air Base on 15 June 1966. 33rd Tactical Group On 8 July 1963 the units at the base were organized as the 33d Tactical Group, with subordinate units being the 33rd Air Base Squadron, the 33rd Consolidated Aircraft maintenance Squadron and the Detachment 1 reconnaissance elements. The Group's mission was to maintain and operate base support facilities at Tan Son Nhut, supporting the 2d Air Division and subordinate units by performing reconnaissance. 505th Tactical Air Control Group The 505th Tactical Air Control Group was assigned to Tan Son Nhut on 8 April 1964. The Unit was primarily responsible for controlling the tactical air resources of the US and its allies in South Vietnam, Thailand, and to some extent Cambodia and Laos. Carrying out the mission of providing tactical air support required two major components, radar installations and forward air controllers (FACs). The radar sites provided flight separation for attack and transport aircraft which took the form of flight following and, in some cases control by USAF Weapons Directors. FACs had the critical job of telling tactical fighters where to drop their ordnance. FAC's were generally attached to either US Army or ARVN units and served both on the ground and in the air. Squadrons of the 505th located at Tan Son Nhut AB were: 619th Tactical Control Squadron activated at the base on 8 April 1964 It was responsible for operating and maintaining air traffic control and radar direction-finding equipment for the area from the Mekong Delta to Buôn Ma Thuột in the Central Highlands with