Guns on a Fighter - How Hardware Problems Became Software Problems

In my professional guise as a Software Test Engineer, I sometimes run across stories that make me wish I could spend a few hours picking the brains of the test lead on some project or another. The recent articles about the F-35B/C’s gun pod, and last year’s articles about the F-35A’s internal gun issues were one such example. How far we’ve come in a hundred years or so, that the function of a fighter aircraft’s most basic weapon can be impeded by software problems.

The Fokker Eindecker, the first successful fighter fitted with Interrupter gear. Despite its spindly appearance, it gave rise to the "Fokker Scourge". Source

It certainly wasn’t always this way. In World War I, when pilots abandoned their handguns, grenades, and rifles in favor of fixed machine-guns, the first problem was where to mount them? Two-seat aircraft mostly began by giving the weapons to the observer. Single-seaters tried mounting them above the wing, firing clear of the propeller arc, or placing the engine and propeller at the rear to allow a clear forward firing position.

Largely through trial-and-error, it was discovered what seems obvious in hindsight: that for a single-seat, maneuverable fighter, the best place for guns to be placed was in the direct line of sight of the pilot, firing over the nose of the aircraft. “All” the pilot had to do was point the nose of the aircraft where he wanted the bullets to go. Unfortunately, since most aircraft of the time also hung their single engine and propeller on the nose, placing machine-guns behind the propeller tended to have negative consequences once bullets started impacting the spinning propeller blades.

Solving this clearance problem was a hardware issue, not software. Interrupter gear, a clever mechanical system of synchronizing a machine-gun and propeller so that the gun fired only when the bullet would not hit a propeller blade, was the ultimate answer. Other options were tried, however, particularly by the British and French forces. Most famously, Roland Garros equipped his Morane-Saulnier Type L with steel wedges on the propeller to deflect bullets which otherwise cause damage. This worked as well, but the degraded propeller and engine performance meant that the solution was abandoned for synchronization gear once it became available.

The gun remains an integral part of almost every fighter and attack aircraft’s arsenal. The experiences of the Vietnam War, where gun-less F-4 Phantoms found themselves in knife fights with cannon-armed MIGs seared the requirement into the institutional memory of the U.S. Air Force. No USAF fighter is likely to be without a gun for a very long time to come.

Despite being primarily a attack bomber, the gun equipped F-105 scored 24 of its 27 kills in Vietnam with its internal 20mm cannon. Source

That being said, it does seem that the gun has become less vital than ever. Where the F-16 has the capacity for up to 511 rounds for its M61A1 20mm cannon, the F-35A carries only 182 for its 25mm GAU-12. This decrease in capacity means that every shot needs to count.

And thus we come back to the software problem. Where WWI pilots used a basic aiming reticle and fought at slow speeds and point-blank ranges, the F-35 uses a helmet mounted sight and integrated aiming to assist with accuracy. That takes targeting software. The gun itself has to be hidden behind a small door when not in use, that too requires a software solution, not mechanical, to ensure that the shutter is always opened before the gun fires.

F-35A near a refueling tanker. The internal cannon is hidden inside the bulge on the upper fuselage ahead of the wing root. Source

That’s a lot of unit testing. That’s a lot of systems integration testing. That’s a whole lot of regression and halo testing, because the targeting system is concerned with a lot more than just the gun, and the flight computers have to be involved too because the position of the gun causes a slight yawing motion when fired.

My hat’s off to the QA department at Lockheed Martin.

Research aided by Wikipedia, and F-16.net