Part of the flight test program to get the F11F Tiger operational involved clearing the performance envelope for the firing of its four 20-millimeter Colt cannons, which were mounted under and aft of the air intakes
Designed as a lightweight fighter, the F11F Tiger delivered impressive performance and could reach speeds of Mach 1.1. In April 1958, the F11F-1F Super Tiger—only two of which were ever built—climbed to 76,828 feet, setting a world record. In a more extraordinary incident, a Grumman test pilot flying an F11F in 1956 accidentally shot down his own aircraft after overtaking its bullets during a dive.
As part of the Tiger’s flight test program, engineers had to verify the aircraft’s performance limits while firing its four 20 mm Colt cannons, which were installed beneath and behind the air intakes. Because the Tiger was built to be as lightweight as possible in order to get the best performance from its Wright J65 Javelin engine—a licensed version of the British Sapphire—the spent shell casings and ammunition links were discharged overboard instead of being kept inside the aircraft. During cannon-firing tests, it quickly became clear that the airflow around the jet caused the ejected casings and links to stay close to the fuselage, resulting in numerous dents and scratches. At one stage of the program, the test aircraft even had to be equipped with an armored leading edge on the horizontal stabilizer, since the damage caused especially by the links could be severe.
The fix was to redesign the ejection system and its chutes so the spent casings would be forcefully expelled through longer outlets extending into the airstream, keeping them clear of the turbulent airflow along the Tiger’s sides. A complicated internal mechanism also redirected the ammunition links into a special compartment located just ahead of the cannon ammunition boxes, since the links were more likely than the empty casings to damage the aircraft’s surface.
On September 21, 1956, Grumman test pilot Tom Attridge departed from the company airfield at Calverton, Long Island, in aircraft BuNo 138620 to carry out high-speed cannon-firing tests. This was the 41st flight for that particular F11F-1 Tiger and Attridge’s second test mission of the day. According to Robert Munro in Aero Album, Spring 1968, Volume One, the planned flight profile began at 22,000 feet, with the aircraft accelerating in afterburner during a 20-degree descent to beyond Mach 1. As he passed through 13,000 feet, Attridge was to fire a four-second burst from the cannons, pause for three seconds to let the guns cool, and then fire a second burst, completing the profile at 7,000 feet when the ammunition would be used up.
After following the planned test profile and using up his ammunition, Attridge was suddenly startled when an unknown object shattered the aircraft’s forward armored windscreen. He immediately pulled back the throttle and climbed to slow down, hoping to keep the windscreen from collapsing inward, and then turned back toward Calverton at 13,000 feet and 200 KIAS. The only visible damage he could identify was a gouge on the right intake lip, and whenever he increased engine power beyond 78 percent, the engine began to run roughly. About two miles from the end of the Calverton runway, at 1,200 feet with the landing gear down and flaps lowered, Attridge realized that 78 percent power was not enough to hold his glide path as the aircraft’s rate of descent increased. When he pushed the throttle fully forward, the engine, as he later described it, sounded like it was “tearing up,” and he immediately lost power. Three-quarters of a mile short of the runway, the Tiger came down in the trees and slid about 300 feet before stopping. Although the aircraft caught fire, Attridge escaped quickly and without injury, and the Grumman crash crew and rescue helicopter arrived in less than three minutes. Eleven minutes after the crash, the company helicopter transported him to a local hospital for evaluation.
An inspection of the aircraft revealed that it had been struck by at least three 20-millimeter rounds—one in the windshield, one on the right intake lip, and one in the nose cone. Fragments of a projectile were also discovered in the engine’s first compressor stage, along with damage to the fan blades; engineers believed the round that hit the right intake may have ricocheted into the engine. Investigators concluded that Attridge had accidentally shot down his own aircraft. During the first cannon burst, he was in a 0.5G supersonic descent and had actually passed below the path of the shells he had fired. Eleven seconds later, as he started to recover from the dive, he flew directly into the stream of projectiles from that first burst.
Later examination of the aircraft revealed that Attridge had flown the same test profile earlier that day, and what had initially been believed to be a large dent on the vertical fin caused by an ejected casing was actually a projectile strike. It appears that during his first test flight of the day, Attridge had already narrowly hit his own aircraft without bringing it down.
Photo by U.S. Navy
