The Helio AU-24 Stallion was an American armed gunship, counter-insurgency, and utility transport developed for the United States Air Force. A total of 20 were built during the Vietnam War, with most of the aircraft being later sold to the Khmer Air Force. Design and development The Helio HST-550 Stallion was an STOL utility aircraft developed in the United States in 1963, with the first prototype flying in July 1964. Initially conceived by the Helio Aircraft Company as a turboprop-powered variant of the Helio Courier, it eventually emerged as a completely new design of the same general configuration and much of its design was initiated from components used in the Helio H-500 Twin. It was a much larger aircraft than the Courier, and Helio soon discovered that it was too expensive for the market. The United States Air Force (USAF) however, emerged as a buyer for the design, purchasing the aircraft for the Credible Chase programme as the AU-24A. This was the gunship version of the Stallion, with a PT6A-27 turboprop, equipped with an M197 three-barrel 20x102mm rotary cannon mounted in the left cargo door. It also had five underwing and fuselage hardpoints. Of the 18 aircraft purchased by the USAF, fourteen or fifteen were eventually delivered to the Khmer Air Force (KAF) between January and November 1972 under the Foreign Military Sales program for use in border surveillance and counter-infiltration roles, where the threat of encountering anti-aircraft fire (other than small arms) was minimal. Operational history Cambodia An important addition to the KAF, the AU-24A mini-gunships were assigned to a newly raised Mini-gunship Squadron (French: Escadron AU-24) stationed at Pochentong Air Base near Phnom Penh, which broadened supply convoy escort operations on the lower Mekong-Bassac Rivers corridors. Such operations had been carried out in conjunction with the Khmer National Navy (MNK) since mid-1971, when the KAF began to provide air cover to MNK convoys with their Douglas AC-47D Spooky gunships. Under Project Flycatcher, an improvement programme for the KAF, the Americans delivered among other aircraft types a single AU-24A mini-gunship before the programme was officially terminated on June 30, 1973. Although the AU-24A mini-gunships delivered to Cambodia acquainted themselves well in the river convoy escort role, they were found to be beset with a long list of technical faults, which became painfully clear on August 10, 1973, after a Stallion crashed on a rocket pass, killing its crew and