In February 1968, Military Assistance Command, Vietnam (MACV) established a requirement for an airborne platform along the demilitarized zone (DMZ) area which would intercept enemy communications, including voice and Morse code signals, and use direction-finding equipment to locate enemy installations. After several false starts, three LEFT JAB platforms were fielded in December 1970 and assigned to the 138th Radio Research Company at Phu Bai, not far from the DMZ. The most distinguishing characteristic of the system was an oval antenna mounted under the belly of a U-21, a twin-turboprop fixed-wing aircraft. Originally designed for ground-based systems, the antenna could be extended after take-off.

On March 4, 1971, the 224th Aviation Battalion received word that a LEFT JAB aircraft was missing in action. The fate of the craft and its 5-member crew was later confirmed by a report of the North Vietnamese News Agency, which claimed that a surface-to-air missile had downed a US plane just inside North Vietnam. Killed in action that day were pilot CPT Michael W. Marker, co-pilot WO1 Harold L. Algaard, SP6 John T. Strawn, SP5 Rodney D. Osborne, and SP5 Richard J. Hentz and SP5 Rodney D. Osborne. This incident was the last of three Army Security Agency (ASA) aircraft lost in Vietnam, and the only fixed-wing; the other two incidents involved helicopters. However, these losses did not ground flights near the DMZ. On the contrary, during the enemy's 1972 Spring Offensive, airborne communications collection efforts in the area took on an even greater significance.

On January 27, 1973, President Nixon ordered a cease-fire. All remaining US forces were directed to depart Vietnam within 60 days. During the cease-fire, ASA crews continued to fly collection missions that, despite the truce, continued to be subjected to antiaircraft fire. On 16 February 1973, a crew of the 138th Aviation Company completed the Army's final ARDF mission in the vicinity of Pleiku. Less than a month later, on 7 March 1973, the 509th Radio Research Group was discontinued, and the handful of its remaining soldiers boarded the last plane for home, bringing ASA's 12-year tour of service to a close.

A memorial honoring Army Intelligence personnel killed in aviation-related incidents over the past 50 years is located at the Aviation Memorial Park on Fort Huachuca, at the corner of Hatfield and Irwin Streets. There are also six interpretive panels highlighting the contributions of Intelligence Aviators from the Civil War through the War on Terror, and static displays of several different kinds of planes used by Army Intelligence. Please visit the Air Park, which is free and open to the public, or see it on the Virtual Tour by going to and clicking on Aviation Memorial Park.