Fairchild AU - 23 Peacemaker The Fairchild AU - 23A Peacemaker was an American armed gunship , counter - insurgency , utility transport version of the Pilatus PC - 6 Porter for the U . S . Air Force . 15 were built under license in the USA by Fairchild Industries , for use during the Vietnam War in the early 1970s , all aircraft were later sold to the Royal Thai Air Force . Design and development . In May 1971 the Aeronautical Systems Division at Wright - Patterson Air Force Base , Ohio , began work on a project to evaluate the potential use of armed light utility short takeoff and landing aircraft in Southeast Asia . The program , named Credible Chase , was designed to add mobility and firepower to the South Vietnamese Air Forces in a relatively short time . Two commercial aircraft were selected for testing : the Fairchild Porter and the Helio Stallion . Initial performance testing was conducted with leased aircraft at Eglin Air Force Base , Florida , and was successful enough to warrant a combat evaluation . The Porter , designated AU - 23A , was fitted with a side - firing 20mm XM - 197 Gatling cannon , four wing pylons and a center fuselage station for external ordnance . The 20mm cannon was essentially a three barrel version of the M61 Vulcan 6 - barrel 20mm cannon . The aircraft could carry a variety of ordnance including forward firing gun pods , 500 and 250 pound bombs , napalm units , cluster bomb units , flares , rockets , smoke grenades and Airborne leaflet propaganda dispensers . Operational history . The combat evaluation , PAVE COIN , was done in June and July 1971 . The AU - 23A was tested for eight possible missions : armed escort of helicopters , close air support , hamlet defense , STOL airlift and resupply , armed reconnaissance , border surveillance , forward air control , and counter infiltration . USAF crews flew 73 missions ( 94 sorties ) and RVNAF crews flew 68 missions ( 85 sorties ) . Several types of weapons were test dropped / fired including 2 . 75 inch rockets ( explosive and smoke ) , cluster bomb units ( CBU - 14 ) , MK 6 Mod 3 flares , and MK 81 , 82 and 106 practice bombs . More than 8 , 000 rounds of 20mm ammunition was fired also — both high explosive incendiary and target practice tracer types . Several problems were discovered during the PAVE COIN program , the most serious was the extreme vulnerability of the aircraft to all but the lightest antiaircraft fire ( below 12 . 7mm ) . Despite the problems , the USAF continued with the development program and ordered 15 AU - 23As for further testing . The 4400th Special Operations Squadron ( Provisional ) was created to complete the operational test and evaluation of the Credible Chase aircraft . The first AU - 23A ( 72 - 1306 ) was delivered to the 4400th SOS on Jan . 2 , 1972 , followed by two more aircraft ( 72 - 1304 & 72 - 1305 ) at the end of the month . Testing continued until Feb . 4 , when the three aircraft were grounded because of cracks in the rudder assemblies . The first three aircraft were returned to Fairchild for repair and delivery of new aircraft resumed in late April 1972 . On May 10 , 1972 , an AU - 23A ( S / N 72 - 1309 ) crashed after an in - flight engine failure . The pilot was not hurt , but all AU - 23As were grounded until May 22 , during the accident investigation . The last AU - 23A was delivered on June 7 and testing was completed on June 28 . The 4400th recommended the aircraft not be used in combat without a major upgrade program . Specific problems identified included a slow combat speed ( 135 knots ) , a low working altitude , no capability for " zoom " escapes after delivering ordnance and a complete lack of armor protection for the crew and vital aircraft systems . On June 30 , 1972 , the 4400th SOS ferried the AU - 23As to Davis - Monthan Air Force Base , Arizona , for storage . The AU - 23As were eventually supplied to Thailand under the Military Assistance Program for use in border surveillance and counter - infiltration roles .