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AU-24A Helio Stallion

Helio AU-24  Stallion The AU-24A is a lightweight, single-engine turboprop, high-wing monoplane. It is manufactured by Helio Aircraft Company, a division of General Aircraft Corporation. Except for fabric ailerons, the aircraft is of all-metal construction. The wing is full cantilever and contains long-span, single-slotted flaps, aerodynamically automatic full-span leading-edge slats; and leading-edge interceptors (spoilers). The interceptors augment roll control in slow flight and are mechanically inter-connected to the ailerons. Pitch control is maintained by a stabilator. The aircraft is equipped with electric and manual stabilator trim, electric aileron trim, and manual rudder trim. The AU-24A is powered by a 680-shp Pratt and Whitney turboprop engine and a Hartzell 3-bladed propeller with constant-speed, full-feathering, beta control, and reverse ranges. It has five ordnance stations, four wing pylons, and a fuselage pylon. The aft cabin is fitted to mount the XM-197 (20-mm) automatic gun system. Maximum gross weight is 6,300 pounds.

Excellent opportunities for a short take-off and landing of the Helio U-10 Courier led to the development of a slightly enlarged experimental aircraft HST-550 Stallion with a turboprop engine. The first flight took place on June 5, 1964.

It became the second, after the AU-23A Peacemaker, brought to the production of mini-gunship project. The AU-24A had four underwing wings and one ventral pendant for bombs, NURSs, light flares and smoke markers. Small arms consisted of the tri-barrel gun XM-197. Above the dashboard was located the control panel of weapons. The second prototype AU-24A was experimentally armed with four AIM-7 Sidewinder missiles, however, as pilots joked, "the only way they were used was to drop them on someone's head."

The Fairchild AU-23 Peacemaker, and the Helio AU-24 Stallion, were indeed simple when compared to aircraft such as the more advanced AC-130 gunship developed and deployed for US operations in Vietnam. The modifications that were added to make the aircraft suitable as mini-gunships, however, added a level of complexity that had not been anticipated. These additions stole from the program the most precious resource with which it could ill-afford to part: time.

The aircraft themselves were based off of their civilian counterparts with the addition of five ordinance stations, four wing pylons, and a fuselage pylon. To facilitate their role as an interdiction platform, the US Air Force also incorporated a side firing 20mm Gatling gun, a night vision sight (NVS), and a sensor collection equipment. On the surface, these modifications appear to be relatively simple additions that substantially increased the capability of the basic COTS platforms being acquired. The second- and third-order effects of these modifications, however, had a significant impact on the USAFs ability to meet the initial expectations and timeline. The militarization of the Credible Chase aircraft required additional time to modify, but also required additional aircrew training (U.S. and RVNAF) to operate these modifications.

The AU-24A, manufactured by Helio Aircraft Co., a division of General Aircraft Corp., is a high performance, lightweight single engine turbo prop, high wing monoplane. The wing is a full cantilever and all metal construction. The fuselage is semi-monocoque construction with the landing gear set well forward to permit excessive braking. The empennage is of all metal construction with ribs and spars. The AU-24A is designed for Short Take Off and Landing performance and equipped with long span single slotted flaps which span from the root section to the aileron and aerodynamically automatic full span leading edge slats. Roll control is maintained during slow flight operations by short span fabric covered ailerons, mechanically interconnected to leading edge integrated interceptors. Pitch control is maintained by a stabilator (slab tail). Pitch trim is provided by both manually operated and electrically operated controls. Aileron trim is electrically operated and rudder trim is mechanically operated. The AU-24A is equipped with air-to-ground armament delivery capabilities. Four wing pylons and a fuselage centerline bomb rack allow for rocket, bombs, target marker flares, and parachute flare delivery. The aft weapons compartment mounts an XM-197 20mm automatic gun system. Pilot control for arming and delivery of weapons is provided by the armament control panel located in the forward cabin above the instrument panel.

Helio AU-24  StallionThe AU-24A was the second aircraft evaluated for potential use in Southeast Asia as an armed light utility short take off and landing gunship. The program, designated Credible Chase, began in May 1971, and was designed to add firepower and mobility to the South Vietnamese Air Force in a relatively short time. The combat evaluation, Project PAVE COIN, of the Fairchild AU-23A and Helio AU-24A was done in June and July of 1971. The AU-24A, like the AU-23A, was found to be unsuitable for combat operations. Major problems identified included a low attack speed of about 135 knots, a low operating altitude below 5,000 feet, so "zoom" escape capability after an attack run, and an extreme vulnerability to antiaircraft fire. Further testing was recommended after the aircraft were updated to combat standards.

In January 1972, the second test phase for the AU-24A began at Eglin Air Force Base, Fla. The initial aircraft used (S/N 72-1319) was leased from Helio and retained its civilian configuration, but it allowed basic flight testing to begin. The combat evaluation of the Credible Chase program was canceled in February 1972, but the initial (stateside) evaluation was kept on the program schedule. The first combat equipped AU-24A was delivered on March 4, 1972, and operational test and evaluation began on March 17, but was delayed after a review of contractor quality control began on April 3. On April 10, the review imposed a number of flight restrictions on the AU-24A limiting maximum airspeed, dive and bank angles, and all instrument, weather and night test flights.

After some initial setbacks, Credible Chases Operational Training and Evaluation (OT&E) plan was modified. The new OT&E plan would take place from April through May 1972 and included the training of the VNAF crews in the US. Data collected from these tests would provide field commanders an assessment of STOL mini-gunship capabilities and limitations, to serve as a basis for determining VNAF requirements for such an aircraft. The revised plan was further plagued by production complications, as the arriving production aircraft quickly developed structural problems. As a result, all of the AU-23s were grounded and further deliveries were suspended until Fairchild resolved the problem with their aircraft. Manufacturing defects with the AU-24 also led to the grounding of the six production model aircraft. After isolating and resolving the problem, the AU-24 alone became the primary OT&E aircraft tested in accordance with the revised plan. The OT&E of the AU-24A was officially started over on April 22, and by May 3, the aircraft was again in trouble. This time the problem was a dynamic instability during flight. The problems were resolved by May 12, and the test program continued until completion on May 22.

Tests in the combat situation as part of the Pave Coin program showed the unusability of the AU-24A for use in combat situations. Starting on June 28, the AU-24As were flown to Davis-Monthan Air Force Base, Ariz. for storage. The Credible Chase program was canceled and no AU-24A was delivered to the South Vietnamese Air Force.

After a short time, fourteen aircraft were delivered to the Cambodian Air Force under the Foreign Military Sales program for use in border surveillance and counter infiltration roles where the threat of encountering antiaircraft fire (other than small arms) was minimal.

ModificationAU-24A
Wing span, m12.50
Length of aircraft, m12.10
Aircraft height, m2.82
Wing area, m224.80
Weight, kg
empty aircraft1080
maximum take-off2858
engine type1 TV United Aircraft PT6A-27
Power, hp1 x 680
Maximum speed, km / h352
Cruising speed, km / h257
Range of action, km715 / 386 nautical miles
Practical ceiling, m7600 / 25,000 ft.
Crew, people2-3
Armament
  • one 20 mm three-barreled M197 gun with 500 cartridges.
  • The combat load is 873 kg at 5 suspension nodes.
  • CBU-14A / cluster bomb
  • SUU-11A / containers with 7.62 mm six-barreled machine guns, LAU-32B / A 70 mm NUR.
  • light bombs weighing up to 227-kg


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    Page last modified: 07-09-2018 07:20:45 ZULU