Near the end of the 1950s the USAF Tactical Air Command put together its requirements for a future attack aircraft. TAC wanted an airplane that could do Mach 2.5 at altitude and Mach 1.2 at low level, where, if necessary, it could fly 400 miles without slowing down. It would have to be able to take off and land on airstrips as short as 3,000 feet and to fly un-refueled across the Atlantic Ocean with an ordnance load of up to 30,000 pounds.
Meanwhile, the U.S. Navy also happened to need a new fighter—to be based on aircraft carriers and serve as fleet defense. The coincidence turned out to be one of the most unhappy and expensive chapters in the history of interservice collaboration. Shortly after John F. Kennedy took office in 1961, Secretary of Defense Robert McNamara tried to save the new administration money by ordering the Air Force and the Navy to develop one aircraft that would carry out both missions. In 1969 the Navy finally abandoned the joint venture, dubbed TFX for Tactical Fighter Experimental; by then, McNamara's idea had ended up wasting $377 million.
From the start, the two services banged heads over just about everything. The Navy wanted side-by-side seating for the two-man crew, long-range radar, long loiter capability at upper altitudes and subsonic speeds, and a gross weight of under 50,000 pounds for carrier operations. The Air Force wanted tandem seating and, focusing on low-level, supersonic capabilities, some sort of terrain-reading radar—not yet invented—and a gross weight of 75,000 pounds. The only point of agreement was the need for a variable-geometry, or swing, wing: Both services wanted maximum lift for short takeoffs, along with high-speed capability once airborne. (Although the F-111 was the first operational aircraft to use a variable-geometry wing, the concept was not new. The Bell X-5 experimental aircraft tested the configuration in the early 1950s, and those tests were based on research reports "imported" from Germany at the end of World War II.)
General Dynamics built the F-111A for the Air Force, and Grumman built the F-111B for the Navy. Both versions had the variable-geometry wing, turbofan engines, side-by-side seating, and a crew escape module (instead of the usual ejection seats). But only seven Bs ended up being built. The Navy withdrew from the program after concluding that the aircraft could never be brought under weight restrictions for carriers. But having gathered experience in producing swing-wing aircraft, Grumman later embarked with the Navy upon another program that proved far more successful—the F-14 Tomcat.
The Air Force soldiered on with the F-111A, which turned out to be a revolutionary aircraft, exemplifying, according to the U.S. Air Force historian Richard P. Hallion, two great aeronautical advances then under way. One was the breakthrough in high-speed flight made possible by the maturation of turbojet technology. The second was an explosive growth in the use of electronics, which led to the development of the first "systems" airplanes. "The first category of those airplanes were the air defense interceptors," says Hallion. "But the second category was the sophisticated air-to-surface attack aircraft, and coming out of that was the notion of all-weather attack. There were two great early systems airplanes in the air-to-surface arena. One was the F-111 and the other was the [U.S. Navy's] A-6."
The electronics package in the F-111 included the hair-raising terrain-following radar, built by Texas Instruments and integrated with a targeting radar, an inertial navigation system, and other sophisticated avionics. The result was an unprecedented autopilot that would enable high-speed flight at extremely low altitude at night and in all kinds of weather. The pilot and weapon systems operator could be guided with electronics to strike a target they couldn't see. At least that was the plan.
The F-111A saw its first action with the 428th Tactical Fighter Squadron in Vietnam during the spring of 1968. Three crashed within a month of the first combat sortie, killing two crews. A fourth crashed at Nellis Air Force Base in Nevada that May. The Air Force suspected that in at least two of the crashes, the cause was TFR failure. At the same time, -111s were suffering engine compressor stalls, during which airflow reverses because of pressure fluctuations at the inlet. The Air Force grounded the aircraft and, along with NASA and General Dynamics, worked to redesign the engine inlets.
The sink rate of a variable-sweep-wing aircraft on final approach was a challenge to pilots. "We lost one airplane and one crewman at Edwards Air Force Base as a result of that," says Hallion. Then during spin-and-departure trials, the test model went out of control and crashed.
The RAAF took delivery of its first F-111C in July 1968. During static testing in September, a wing failed, and General Dynamics suspended delivery of the 23 remaining aircraft.
"The F-111 was a very difficult airplane for the Air Force to deal with," says Hallion with a sigh. The critical areas were the pivot joint, where the wings attach to the fuselage, as well as a fitting called the wing-carry-through box (WCTB), wherein the loads from the wings pass through the fuselage. Hairline cracks were causing catastrophic failure of the pivot joints, while an investigation of the WCTB traced the failures to defective manufacturing. Over the next several years all USAF F-111As (and the RAAF's one C) were retrofitted with strengthened pivot joints and properly manufactured WCTBs. The compressor stalls were fixed by expanding the inlet and inserting a new adjustable inlet device that maintained proper airflow into the compressor. But the modifications were driving up the price of the aircraft, first from the original $4.5 million per unit to $6.3 million, then later much higher.