<p>In 1966 the US Air Force had evaluated the Cessna O-2 in the 'little brother' mini-gunship concept, but had rejected the idea. In 1970, under the program name Credible Chase, the service again studied the concept, and proceeded to order both the Fairchild Turbo-Porter (U-23) and Helio Stallion (U-24) to evaluate the idea. The Helio H-550 design was based closely on the successful U-10 Courier, but was larger, had swept tail surfaces and was powered by a 680-shp Pratt & Whitney Canada PT6A-27 turboprop.
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<p>Fifteen Stallions were procured (72-1319/72-1333) as AU-24As, the first of which arrived at Eglin AFB in March 1972 for fitment of equipment and trials. Under the wings were four hardpoints for the carriage of light bombs, cluster munitions and rocket pods, while in the port cabin door was fitted an XM197 three-barreled Gatling-type 20-mm cannon, capable of firing at rates of 350 or 700 rounds per minute. A TVS-5 night sight was fitted. By May 1972 the initial tests were completed, with the recommendation that the system (and the AU-23) not be deployed on the grounds of its vulnerability in a combat scenario. The 15 AU-24As were dispatched to Davis-Monthan AFB for storage, although 14 of them were subsequently dispatched to Cambodia, where they were fitted with guns and weapons and used operationally.
