Examining the Engineering and Maintenance of the GAU-8/A A-10 Cannon

The United States Air Force's A-10 was designed as a Close Air Support aircraft, built with a powerful 30mm armor-piercing rotary cannon known as the Avenger.

Known as the A-10 'Thunderbolt II' or, more affectionately, the 'Warthog,' the Fairchild Republic Closed Air Support (CAS) aircraft could carry a variety of ground attack munitions, but its primary weapon was a General Electric Gatling-style rotary cannon that rapidly fired depleted uranium rounds. The rapid firing, high propellant burn, numerous moving parts, jarring recoil, and large empty shells presented challenges for engineers and maintenance personnel.

In 1970, the United States Air Force (USAF) issued a Request for Proposals (RFP) for the primary weapon for CAS aircraft being developed under a program known as Attack Experimental (A-X). The weapon was an internal cannon, as lessons from the Vietnam War had led the USAF to reconsider the elimination of aircraft cannons on several fighter designs. The massive buildup of Soviet weaponry during the Cold War necessitated countering the threat with a lethal yet cost-effective solution.

Because the goal of the A-X project was for a CAS aircraft capable of destroying tanks and other armored vehicles, and the cannon would be the aircraft's primary weapon, it needed to be large enough to withstand the latest Soviet weaponry developments. The specifications called for a 30mm cannon capable of firing up to 4,000 rounds per minute (RPM) while achieving accuracy standards with high-velocity projectiles. The high rate of fire demanded a rotary cannon design, along with the need for adequate ammunition storage for the large rounds. The rotary cannon design would also increase barrel life.

Four companies responded to the RFP: Hughes, Philco-Ford, General American Transportation, and General Electric, which had previous experience building rotary cannons for aircraft with the M61 20mm cannon. General Electric and Philco-Ford were selected as finalists to build competing prototypes, with the General Electric design, ultimately designated the GAU-8/A Avenger, winning in side-by-side firing trials in 1973.

The two finalist prototype aircraft in the A-X program, the Northrop YA-9A and the Fairchild Republic YA-10A, would be designed to carry the Avenger, although both would carry the M61 while the GAU-8/A was under development. The YA-10 would be selected as the winner and designated the A-10 in production.

The General Electric GAU-8/A was mounted slightly to the left of the A-10's fuselage, occupying most of the lower center section of the fuselage. The left-side mounting allowed the firing barrel to align with the Warthog's centerline. The centerline placement of the firing barrel reduced the impact of the weapon's heavy recoil on the aircraft, keeping it aligned with the target. This contributed to the Avenger's impressive accuracy. The recoil of the gun produces 10,000 pounds of force, while the A-10's two engines each produce over 9,000 pounds of force.

The size of the GAU-8/A, along with its ammunition feeding system and drum, is clearly visible in this famous photo of the weapon next to a Volkswagen Beetle. (Image credit: United States Air Force)

To accommodate one of the largest and most powerful cannons ever mounted on an aircraft, mounted in the forward/lower fuselage area, slightly to the left, the A-10's front landing gear is positioned to the right, rather than centered, and retracts forward alongside the cannon.

Armor plates of varying thickness are placed between the aircraft's skin and the ammunition drums. This protects the ammunition from enemy fire, with the drums extending almost 20 feet (6 meters) rearward from the muzzle, almost reaching the forward fuel tank in the center of the fuselage.

GAU-8/A Avenger

As mentioned earlier, the GAU-8/A is a seven-barrel rotary cannon firing 30mm rounds. This gun follows Richard Gatling's original design principles: each barrel has its own breech and bolt, with the cocking and firing mechanisms built into the bolt, with only one barrel firing at a time. When the bolt inserts a loaded cartridge into the breech and locks into position, the cocking pin compresses the firing pin spring.

A cam releases the trigger, which then releases the firing pin, striking the cartridge's igniter, which ignites the main powder charge, causing the cartridge to fire. The bolt then opens, recoils, and ejects the spent cartridge from the breech. Driven by two hydraulic motors, the barrel continues to rotate, repeating the cycle. Initially, by operating only one motor, the rate of fire could be slowed by about half, from 4,200 to 2,100 rpm. However, the rate of fire was later fixed at 3,900 rpm.

The drum is connected to a bidirectional feed system that returns empty aluminum cartridges to the drum, rather than ejecting them from the aircraft. This prevents the empty cartridges from striking the aircraft and causing damage due to their large size, while also maintaining the aircraft's center of gravity. Once the trigger is released, the loaded live rounds in the weapon are returned to the feed system. The drum has a capacity of 1,350 unconnected live rounds.

The GAU-8/A itself weighs about 620 lb, but the addition of the feed system and a loaded drum magazine increases the weight to over 4,000 lb. The barrel is 90.5 inches long, and the entire system is 19 feet 10.5 inches long.

The Avenger round consists of a training round that fires an aluminum projectile partially encased in a steel casing. The armor-piercing incendiary (API) round, known as the PGU-14B, fires a projectile made of an aluminum body over a smaller-caliber depleted uranium core, with a total projectile weight of approximately 14 ounces (6,096 grains).

The PGU-13/B is a high-explosive/incendiary round, consisting of a projectile made of a steel casing filled with a high-explosive/incendiary mixture and an impact fuse at the tip. This projectile weighs approximately 13.3 ounces (5,833 grains).

This projectile has a plastic driver band embedded in it, originally intended to reduce the glare of the muzzle flash, but it was found that the band reduced muzzle wear, so the band was incorporated into the bullet. The cartridges are made of aluminum alloy, which reduces weight and increases ammunition capacity by 30%. When fully loaded, Avenger rounds are 11.4 inches long and weigh 1.53 pounds or more.

Avenger Technical and Maintenance Issues

The large 30mm cartridges contain a significant amount of propellant, and the placement of the cannon in the nose of the aircraft means that a large amount of soot, smoke, and unburned propellant accumulates in the nose of the aircraft during firing. This creates several maintenance issues.

Sergeant Major Thomas Barjaktarovich of the 127th Maintenance Group conducts a 36-monthly required in-depth inspection of a GAU-8/A Avenger 30mm Gatling gun used by an A-10 Thunderbolt II of the 107th Fighter Squadron, at Selfridge Air National Guard Base, Michigan, Feb. 5, 2017. (Image credit: United States Air National Guard/MSgt. David Kujawa)

First, soot from the rapid fire of the cannon will accumulate on the front of the aircraft, including the windshield. This sometimes obstructs the pilot's view. The solution was found to be a window-cleaning system consisting of a cleaning solution and air bled from the aircraft's propulsion system.

Another problem caused by the gases from the cannon fire is that they flow over the wings and are drawn into the aircraft's turbofan engines. Because they are largely devoid of oxygen, these gases can choke and shut down the engines. Various attempts have been made to counteract these gases, but with varying results.

Engineers devised a solution by connecting the engine's ignition system to the cannon's trigger, so that each time the trigger is fired, the engine's ignition system immediately fires. The engine will restart immediately if it stalls due to smoke.

The GAU-8/A cannon is inspected and cleaned every 36 months or every 25,000 rounds by trained U.S. Air Force maintenance personnel. Removable panels in the Warthog's nose and lower forward fuselage allow access to the cannon. The Avenger and its components can be completely removed from the aircraft and disassembled for cleaning, inspection, replacement of components, and lubrication.

The USAF stipulates a minimum barrel life of 21,000 rounds per seven-barrel set, allowing worn barrels and gun components to be replaced as they deteriorate. Precessed and threaded barrels are inspected with a barrel scope. The hydraulic drive motor, ammunition drum, and feed system are also inspected, cleaned, and lubricated as needed. Damaged, corroded, or worn components are replaced.

The recoil adapter must be inspected and adjusted to the proper tension. This adapter helps reduce the effects of the extreme recoil and vibrations generated by firing the GAU/8A, helping to prevent other components of the gun and aircraft from becoming loose, damaged, or cracked. The tension balancer should also be checked to ensure the slack remains constant on the Avenger during firing. A safety wire is installed to ensure the bolt does not loosen due to the extreme vibrations the aircraft experiences during firing. Loose components can cause misfires, damage, or even destroy the aircraft.

Once the cannon is reassembled, a dummy round is loaded into the cannon to ensure all components are properly aligned and timed. The dummy round simulates the weight and shape of a real round, but allows for safe testing and adjustments. Live-fire tests can also be conducted without the cannon on board. If everything works, the cannon and its components are reassembled on board the aircraft, returning it to service and ready for duty.

Although the GAU-8/A was designed for the A-10, variations of this cannon have been used in naval applications. The A-10 is scheduled for retirement soon, so the iconic "BRTTTTT" sound produced by the Avenger may soon be a thing of the past.