Tigers, Dives and 20 mm Cannons

On Friday the 21st, 1956, one of Grumman’s in-house test pilots, Thomas W. Attridge Jr. climed into a borrowed Navy Grumman F11F-1 Tiger (BuNo 138620), taxied out, throttled up and lifted off from the main runway on Grumman’s test facility near Calverton, NY. His test flight parameters required him to fly over the Atlantic Ocean to a gunnery range where he’d be testing out the F11’s four 20 mm cannons by firing upon the ocean’s surface. Pretty easy day for a very experienced former naval aviator who flew all over the Pacific Theater in WWII as a fighter pilot, right?

The F11F-1 Tiger involved in the incident. This particular Tiger was the 15th production aircraft delivered to the Navy. (US Navy Photograph from the US Navy National Museum of Naval Aviation Archives)
The F11F-1 Tiger involved in the incident. This particular Tiger was the 15th production aircraft delivered to the Navy. (US Navy Photograph from the US Navy National Museum of Naval Aviation Archives)

After reaching the range 20 miles or so away from the coast, Attridge was at 20,000 ft and ready to initiate the test. Pushing the stick forward gently, he put his Tiger in a shallow dive and triggered a four-second burst of around 70 rounds at 13,000 ft, and then another four-second burst at 7,000 ft to clear the ammo belts on the cannons. Almost immediately after finishing the second burst, the Tiger shook violently as if it had been hit by something. Complicating matters was the fact that the front windshield had somehow buckled inwards. Unable to figure out what exactly just happened to his aircraft, Attridge diagnosed the Tiger’s behavior as being the result of a particularly vicious bird strike and rolled back power to 200 knots while pointing his nose towards base. Communicating with the tower, he reported that he noticed three marks of damage: 1. the forward cockpit glass was cracked all over, 2. there seemed to be a large gash on the right engine’s intake and 3. he could only push his throttle to 78% until the engine started to sound like “a Hooever vaccum cleaner picking up gravel from a rug”, according to Attridge.

Unable to climb and unable to increase his speed by much, Attridge neared the airbase which had crash vehicles readied up on the apron for the stricken Tiger’s arrival. Now lined up for his approach at about 1,200 feet, Attridge was nearly home when his engine finally sputtered its death song and gave out. Fighting to control the dead aircraft (because without an engine, a fighter jet like the Tiger essentially becomes a gliding brick), Attridge’s jet tore into the wooded area behind the runway, leaving a 300 ft-long swath of destruction. When the jet finally settled, the aircraft was ablaze and remaining ammunition started cooking off while horrified rescuers rushed to save the pilot’s life. Even with a broken leg and three fractured vertebrae, Attridge freed himself from the former Tiger and was picked up by a Grumman-registered Sikorsky S-58 helicopter, which descended low enough to have its blades evenly sheared by contact with the trees, almost adding onto the number of the crashes of the day. Attridge was flow away to Riverhead, NY and was hospitalized for a long recovery.

The investigation kicked off soon afterwards. Test pilots typically have a lot to say about the aircraft they fly, and Attridge was no exception to the rule. His insight was valuable, and he was among the first to be interviewed. Grumman and Navy engineers examined the wreckage of the aircraft and to their shock, they found 20 mm bullet entry markings in both the windshield and right engine intake, corroborating Attridge’s observation of a long gash on that same part of the aircraft and. Finally analyzing the engine of the Tiger, a bruised and deformed 20 mm bullet was found, lodged firmly in the first compressor stage. As it turns out, Attridge had somehow managed to shoot himself down! But how?

Sorry folks, but it’s time to do a little physics here. First, the 20 mm Colt Mk12 cannons dished out their shells with a muzzle velocity of around 3000 feet per second. That with the speed of the aircraft factored in pushed the velocity of the bullets to around 4,300 feet per second. The Tiger, already supersonic at 880 miles per hour, releasing these bullets at that speed meant that the bullets encountered extreme air resistance and slowed down fairly quickly. Had Attridge kept his jet flying on that same angle and at the same speed, the bullets would have never made contact. However… the Tiger entered a steeper dive and intersected with the bullets’ trajectories. That’s when they hit. The dead engine was the result of one of those 20 mm rounds ricocheting around the inside of the powerplant, breaking fan blades and scratching chamber surfaces as it went. The jet was just about as fast as the bullets at that point, and with all those variables coming together (i.e. speed of the jet and the bullets, air resistance, angles, etc.), the bullet strikes were scientifically inevitable. Thus, the Grumman F11 became the first fighter jet in history to shoot itself down. Though the Navy repeatedly told its fighter pilots that it was really an extraordinarily coincidental occurrence, they still advised pilots to either bank away or pull up after firing their cannons. TigerBulletPath

Oddly, that wasn’t Grumman’s only instance with a self-shootdown of an aircraft. On the 20th of June, 1973, an F-14 Tomcat flying out of Point Mugu, CA (piloted by a Grumman test pilot and radar intercept officer) was downed after its own AIM-7E Sparrow pitched up during its deployment and tore a hole in a fuel tank. Unable to recover control of the jet, the pilot and RIO ejected safely and lived to fly another day.

Tom Attridge was returned to flight status a little under six months after the incident. His career with Grumman rose to new heights in subsequent years after he became the project manager for LEM-3, a lunar module that flew with Apollo 9. He then became vice president of Grumman Ecosystems which, interestingly enough, helped bring about the digital camera. Sadly, he passed away in 1997 at the age of 74.


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