Production and Manufacturer History
The Helio HST-550 Stallion was developed in the United States by the Helio Aircraft Corporation, a company with roots tracing back to the Koppen-Bollinger Aircraft Corporation founded in 1948 by Otto Koppen and Lynn Bollinger. During the development of the Stallion, Helio became part of the General Aircraft Corporation. Design work on the Stallion began in 1963, with the prototype making its first flight in July 1964. While the first production models appeared in 1966, Helio ceased production of the Stallion and the Courier family in 1974. Following this, the rights to the aircraft were acquired by Helio Aircraft Ltd. While the total number of civil HST-550/H-550A airframes is undocumented, 20 AU-24 gunship variants were produced.
Design and Technical Specifications
Designed as a single-engine turbine STOL utility transport, the Stallion utilized components from the Helio H-500 Twin program. To achieve its extreme short-field capabilities, the aircraft featured full-span automatic leading-edge slats, high-lift slotted all-metal flaps, and fabric-covered balanced ailerons. The airframe is a high-wing, all-metal construction equipped with large double cargo doors on the port side and an optional starboard door for passenger and cargo access. The H-550A civil variant could accommodate between 8 and 10 people, consisting of a pilot, copilot, and either six individual seats or eight bench seats in the cabin.
Engine and Performance
The Stallion is powered by a single Pratt & Whitney Canada (formerly United Aircraft of Canada) PT6A series turboprop. The AU-24 variant specifically used the PT6A-27, rated at approximately 680 shp (510 kW). Performance data for the H-550A indicates a top speed of 216 mph and a cruise speed of 206 mph, with a remarkably low stall speed of 42 mph. The aircraft is capable of taking off over a 50-foot obstacle in 660 feet and landing over the same obstacle in 750 feet. It has a service ceiling of 25,000 feet, a range of 445 miles, and an initial climb rate of 2,200 ft/min.
Military and Civil Service
While the HST-550 served as a civil transport, its most prominent role was as the AU-24 gunship. Under the "Credible Chase" program, the United States Air Force (USAF) utilized the AU-24A for counter-insurgency and border patrol. Between January and November 1972, 14 or 15 of these aircraft were delivered to the Khmer Air Force (KAF) in Cambodia. Based at Pochentong Air Base, the KAF's Mini-gunship Squadron used the AU-24 for convoy escort and border surveillance, and later for night bombing against Khmer Rouge rocket positions in early 1975. The aircraft's armament included a M197 three-barrel 20x102 mm rotary cannon mounted in the left cargo door, with provisions for under-wing rockets and bombs. Other operators included the Air Force of the Kampuchea Revolutionary Army (AFKRA) and the Kampuchean People’s Revolutionary Armed Forces (KPRAF), with the last operational aircraft decommissioned in 1993. Additionally, a single AU-24 was used as a liaison aircraft by the Colombian Air Force after being seized for drug-running.
Legacy
The Stallion remains a significant example of scaled-up STOL technology, bridging the gap between light liaison planes and larger gunships like the AC-130. Today, the type is extremely rare; one surviving AU-24 (serial number 001) is reported as the last flying Stallion in the world, having been upgraded with a PT6-34 engine producing approximately 750 hp.