When the A-10 was about to be introduced, USAF leadership used the exact same arguments to prevent that as they are using now in an effort to kill it. They saw merely a clunker that flew at 300 knots or less, an anachronistic dud unfit to operate on the modern battlefield where it was to kill Russian tanks. In fact, the A-10 would never had been introduced if the USAF was not engaged in the budgetary battle against the US Army. Army was about to introduce the new attack helicopter, the Cheyenne. Cheyenne was a compound helicopter, designed to overcome the inability of normal helicopters to achieve higher speeds when necessary, and its high price would see financial resources redirected away from the US Air Force and into the Army’s purse. USAF would have none of it, and it decided to finally take responsibility for the close air support mission it was supposed to do anyway, and so introduced the A-10. Technical requirements were outlined mainly by Pierre Sprey after talks with surviving US and German pilots who carried out close air support in World War II and the Vietnam war, while the overall effort was directed by the Colonel Avery Kay. More heavily armed, survivable and less expensive, A-10 easily killed off the Cheyenne, and the USAF never placed any orders beyond the first batch. In fact, the A-10 was the first and the last US fighter designed for close air support.
Before being finally accepted into service, A-10 had to pass one final test against A-7 Corsair II. Tests, carried out during April and May of 1974., had shown that the A-10 is far superior to Corsair II in carrying out the Close Air Support mission, particularly in low-visibility conditions. Testing of GAU-8/A Avenger cannon was finished by September, and in October, testing of TF-34-100 turbofan engine was likewise done. First preserial production A-10A flew on 15th February 1975., and second on 26th April, after which two prototype YA-10A were retired. It was noticed that A-10A was slightly overweight, but this was deemed not to degrade performance by an unacceptable amount. First serial production A-10A flew on 21st October 1975., and was delivered to USAF on 5th December 1975.
Primary goal of design was high survivability in constested airspace. Analysis of air operations in Vietnam and Arab-Israeli wars has revealed that 62% of losses of single-engined aircraft was caused by fuel fire, 18% was caused by the pilot being disabled, 10% by damage to control surafces, 7% by engine loss and 3% by structural damage (further reading on topic of single vs twin engined aircraft here). Primary danger for the new attack aircraft was considered to be fire by antiaircraft guns up to 57 mm caliber, so all A-10s systems were designed to withstand a single direct hit by a shell of that caliber. Light ground-to-air missile systems (MANPADS) were not considered a significant danger, whereas defenses against SA-6 were primarily active jamming systems, as armor protection against missiles would degrade performance too much.
Aircraft hull is of a half-monocoque structure, and is made from aluminum legures 7075 and 2024, known for their resistance to corrosion and stresses. Hull construction contains four longerons, multiple frames, while aircraft skin is riveted. A-10s design allows the interchangeability of components mounted on each side of the hull (vertical stabilizers, main wheels, hull panels, weapons hardpoints) which simplifies maintenance and reduces aircraft price. Pilot is surrounded by a titanium bathtub with plate thickness of 12,7 to 38,1 mm, whose sides also serve as parts of the airframe. Side panels of the tub are designed to withstand a direct hit by an armor piercing incendiary projectiles up to 23 mm in caliber. Bathtub weights 544 kg, which is 47,1% of 1.023 kg allocated to armored protection of the A-10. Additional 37,4% goes for protection of fuel systems, 9,7% for protection of 30 mm ammunition drum, and remainder for other components. Windshield is made out of bulletproof glass and can withstand hits from ammunition up to 23 mm in caliber.
Flight control is done by a double hyraulic systems. Unlike the F-105 Thunderchief, whose dual systems were right next to each other and so would get destroyed by a single hit, A-10s systems are spaced from each other and protected with armored plating. If both are destroyed, a portion of flight surfaces can be controlled manually. Special attention was given to fuel systems. Due to A-10s large wing surface, wings were made dry with the exception of small inner section up to the wheel. Main fuel tanks are located inside the main hull. All fuel tanks are self-sealing, and filled with porous foam, to stop the fuel leaks. Individual fuel tanks as well as the fuel tank section itself are sealed off from each other and from other systems by fireproof panels. Self-sealing fuel lines are fitted so that they pass through the fuel tanks. In the worst-case scenario, main fuel tanks can be isolated from the rest of the aircraft, in which case two small fuel tanks located between engines are used for fuel supply, allowing a range of 370 km to return to base.
Forward part of the aircraft is designed around the GAU-8/A gun and 30 mm amunition drum. Engines are mounted in armored gondolas on the aft part of the hull. This engine configuration was chosen for several reasons. It allowed simpler construction and access to the engines, allowed air intakes to be high above the ground so as to avoid ingesting debris; taking out one engine will not take out both; infrared signature of the engine exhaust is reduced; engines are separated from the fuel; engines can keep working while the aircraft is being rearmed.
Wings are straight and thick, optimized for maneuverability at low speeds and altitudes and generating lift during takeoff and landing. Low position also allows quick rearmament, as well as carriage of heavy weapons near the centre of the aircraft, resulting in reduced inertion during the roll. Wing construction has three wing spars, and outer wing has a dihedral of seven degrees. Wings have both slats and flaperons; slats were mounted after testing revealed negative interaction between the wing and the engine at high angles of attack (stall that developed at mid-wing would stop the air flow into the engine, leading to compressor stall). Ailerons on outer wing edges consist of two sections which can separate and serve as air brakes. All control surfaces are hydraulically driven. Wing tips are drooped to reduce vortex flows and improve aileron effectiveness at slow speeds. Plating of the lower portion of the wing had to be strenghtened due to cracks which appeared during the low-level flight in Europe’s turbulent atmosphere.
Main wheels of undercarriage retract into small aerodynamically shaped bays located on the lower inner section of the each wing. About half of the wheel protrudes when the undercarriage is fully retracted, allowing belly landings with minimal damage to the aircraft. Nose wheel had, due to the gun location, to be moved to the right. All three wheels retract forward, allowing air current and gravitation alone to deploy them should the hydraulic system fail; if this does not happen, aircraft can still belly land as described. Wheels are widely separated, making takeoff and landing from provisional airstrips easier.
Twin tail surfaces of the A-10 hide the engine exhaust from ground-based IR sensors, as well as – along with the hull and main wings – protecting them from weapons fire from the ground. A-10 is also equipped with an auxilliary power unit which provides power for the engine start. Electrical systems consist of two 30/40 kVa 115/200 V alternating current generators, batteries and converters. Climatisation system uses the engine compressor air for purposes of pressurizing cockpit and pilot’s G suit, defrosting the canopy, fuel transfer, cleaning of fuel lines. Propulsion group consists of two turbofan TF34-GE-100 engines, originally developed for the S-3A Viking aircraft. All main components of the engine can be removed without removal of auxilliary ones, and compressor blades can be removed individually without the need to take apart the entire engine. TF-34 has shown itself to be resistant to use in sand conditions, as well as to bird ingestion.
Thanks to its construction, A-10 can get back to the base without half a wing, half a tail, a single engine, with complete hydraulics failure, or with several of the listed damages at the same time. During the static testing of aircraft’s construction, a sections of aircraft fuselage and wing – complete with full fuel tanks – were simply riddles with fire from a Russian 23 mm anti-air gun, using both armor-piercing and high-explosive incendiary ammunition. Around 430 shots were fired at the cockpit, 250 into fuel tanks and 60 into ammunition drum. Additional 108 pieces of various calibres were fired into front canopy panel. Testing has revealed that the area in which a single 23 mm shot was lethal was equivalent to 1/10th of the area on a smaller, unprotected aircraft. Foam in fuel tanks has also demonstrated excellent ability to stop any fuel leaks.
Actual live combat has revealed A-10s ability to survive hits from larger shells of 57 mm in caliber, as well as MANPADS and SAMs, much of the time. In one incident A-10 got hit by four 57 mm shells, including one below the cockpit, but the aircraft made it back and the pilot was unharmed.
One almost always underapriciated aspect of aircraft survivability – especially by stealth proponents – is aircraft’s on-ground survivability. Conventional air bases are huge built-up areas, easily detected, identified and attacked. Even if the aircraft survive – and even with armoured shelters, there is no guarantee of that – cratering of the asphalt / concrete air strip will place aircraft at the base out of the action for the time being. This is doubly true for stealth aircraft, which require specialized controlled atmosphere hangars for maintenance. A-10s ability to fly from forward bases, dirt strips and open fields makes it significantly less vulnerable to such attacks than other aircraft in the NATO inventory.
Hrvatski Vojnik, Broj 64., Godina IV, 20. Svibnja 1994. (Croatian Soldier, No.64, Year IV., 20. May 1994.)