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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, Fla., 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 propaganda leaflet dispensers.
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, Ariz., for storage.
The AU-23As were eventually supplied to Thailand under the Military Assistance Program for use in border surveillance and counter-infiltration roles.
Armament: One XM-197 20mm side-firing cannon, plus up to 1925 lbs. of external stores on five pylons -- two on each wing (1400 lbs. of stores maximum) and a center fuselage pylon (525 lbs. of stores maximum). In combat evaluations, the maximum ordnance load was about 1300 lbs. The aircraft was also evaluated with side-firing XM 93 7.62mm minigun, XM 59 .50-cal. machine gun and XMU-470 20mm fixed side-firing gun pods. Ordnance tested included SUU-11 gun pods, 2.75 inch rocket pods, BDU 33 with 25 lbs. bomblets, MK 81 250 lbs. bombs, MK 82 500 lbs. bombs, BLU-118 500 lbs. napalm canister, CBU-55 500 lbs. cluster bomb unit, MK-24 flares, ADU-272 canisters, smoke grenades and propaganda leaflet dispensers.
Engine: Garrett TPE 331-1-101F turboprop of 650 hp
Maximum speed: 148 knots at take-off power, 5,000 feet altitude, 6,000 lbs. gross weight
Cruising speed: 142 knots at maximum continuous power
Combat cruise speed: 129 knots
Range: 420 nautical miles
Endurance: 4.84 hours
Combat radius: 162 to 201 nautical miles depending on mission
Span: 49 ft. 8 in.
Length: 36 ft. 10 in.
Height: 14 ft. 4 in.
Weight: 6,100 lbs. maximum gross
Crew: Three (pilot, co-pilot, gunner); in transport configuration, the aircraft could carry six passengers or five troops with field gear or one litter patient, three ambulatory patients and one medical attendant
Serial numbers: 72-1304 to 72-1318
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