The US Air Force’s (USAF) F-35 Joint Strike Fighter Integrated Test Force has started the first phase of testing the F-35A Lightning II aircraft’s four-barrel Gatling gun at Edwards Air Force Base (AFB), California, US.

Known as the GAU-22/A, the gun fires 25mm rounds integrates internally to the F-35A, unlike the US Marine Corps and Navy variants, where the gun is mounted on the outside as a pod.

Undertaken at Edwards AFB’s Gun Harmonizing Range, the ground testing uses a target practise round, the PGU-23/U, which fires from the gun but does not explode on impact.

The trials are designed to answer questions like whether the gun door opens correctly, spins up and down correctly, is the air flows through the vent, and is it adequate to clear the flammable gasses, amongst others.

The GAU-22/A was evaluated as a standalone before integration into the F-35A, and was also flown during test points without firing to ensure that the flight envelope would not overstress the gun mounts.

Designated as AF-2, the testing airframe is a modified flight sciences aircraft, and completed four months of instrumentation modifications and had a line production gun installed for this project.

461st Flight Test Squadron operations assistant director and the project test pilot major Andrew Rollins said: "(AF-2) is the only aircraft in the world that can get us this data.

"It’s been highly instrumented in order to get us the information we need to proceed to the next test point and ultimately to get the gun to its full envelope."

"(AF-2) is the only aircraft in the world that can get us this data."

Comprising personnel from the USAF, Lockheed Martin, Pratt & Whitney and Northrop Grumman, the test team expects to finish ground testing sometime during next month and start the airborne phase in late-September.

During the airborne tests, the team will watch for potential effects of having the gun mounted internally, like vibrations, acoustics and airflow.

Rollins said: "While we’ll be targeting very specific objectives, the pilot will also be observing more qualitative effects such as muzzle flash, human factors and flying qualities."

An operational gun capability is scheduled to be added with a future block of software, which is in the beginning stages of testing at the air base.

Image: An F-35A Lightning II aircraft fires a burst of rounds down range at the Edwards Gun Harmonizing Range, on Edwards Air Force Base, California, US. Photo: courtesy photo/Darin Russell.