The General Dynamics F-16 Fighting Falcon is a single-engine multirole fighter aircraft originally developed by General Dynamics for the United States Air Force (USAF). Designed as an air superiority day fighter, it evolved into a successful all-weather multirole aircraft. Over 4,600 aircraft have been built since production was approved in 1976. Although no longer being purchased by the U.S. Air Force, improved versions are being built for export customers. In 1993, General Dynamics sold its aircraft manufacturing business to the Lockheed Corporation, which in turn became part of Lockheed Martin after a 1995 merger with Martin Marietta. The Fighting Falcon's key features include a frameless bubble canopy for good visibility, side-mounted control stick to ease control while maneuvering, an ejection seat reclined 30 degrees from vertical to reduce the effect of g-forces on the pilot, and the first use of a relaxed static stability/fly-by-wire flight control system that helps to make it an agile aircraft. The F-16 has an internal M61 Vulcan cannon and 11 locations for mounting weapons and other mission equipment. The F-16's official name is "Fighting Falcon", but "Viper" is commonly used by its pilots and crews, because of a perceived resemblance to a viper snake as well as the Colonial Viper starfighter on Battlestar Galactica which aired at the time the F-16 entered service. In addition to active duty in the U.S. Air Force, Air Force Reserve Command, and Air National Guard units, the aircraft is also used by the U.S. Air Force Thunderbirds aerial demonstration team, and as an adversary/aggressor aircraft by the United States Navy. The F-16 has also been procured to serve in the air forces of 25 other nations. As of 2015, it was the world's most numerous fixed-wing aircraft in military service. Experiences in the Vietnam War revealed the need for air superiority fighters and better air-to-air training for fighter pilots. Based on his experiences in the Korean War and as a fighter tactics instructor in the early 1960s, Colonel John Boyd with mathematician Thomas Christie developed the energy–maneuverability theory to model a fighter aircraft's performance in combat. Boyd's work called for a small, lightweight aircraft that could maneuver with the minimum possible energy loss and which also incorporated an increased thrust-to-weight ratio.[13][14] In the late 1960s, Boyd gathered a group of like-minded innovators who became known as the Fighter Mafia, and in 1969, they secured Department of Defense funding for General Dynamics and Northrop to study design concepts based on the theory.[15][16] Air Force F-X proponents remained hostile to the concept because they perceived it as a threat to the F-15 program, but the USAF's leadership understood that its budget would not allow it to purchase enough F-15 aircraft to satisfy all of its missions. The Advanced Day Fighter concept, renamed F-XX, gained civilian political support under the reform-minded Deputy Secretary of Defense David Packard, who favored the idea of competitive prototyping. As a result, in May 1971, the Air Force Prototype Study Group was established, with Boyd a key member, and two of its six proposals would be funded, one being the Lightweight Fighter (LWF). The request for proposals issued on 6 January 1972 called for a 20,000-pound (9,100 kg) class air-to-air day fighter with a good turn rate, acceleration, and range, and optimized for combat at speeds of Mach 0.6–1.6 and altitudes of 30,000–40,000 feet (9,100–12,000 m). This was the region where USAF studies predicted most future air combat would occur. The anticipated average flyaway cost of a production version was $3 million. This production plan, though, was only notional, as the USAF had no firm plans to procure the winner. The U.S. Air Force initially ordered 15 full-scale development (FSD) aircraft (11 single-seat and four two-seat models) for its flight test program, but was reduced to eight (six F-16A single-seaters and two F-16B two-seaters). The YF-16 design was altered for the production F-16. The fuselage was lengthened by 10.6 in (0.269 m), a larger nose radome was fitted for the AN/APG-66 radar, wing area was increased from 280 sq ft (26 m2) to 300 sq ft (28 m2), the tailfin height was decreased, the ventral fins were enlarged, two more stores stations were added, and a single door replaced the original nosewheel double doors. The F-16's weight was increased by 25% over the YF-16 by these modifications. The FSD F-16s were manufactured by General Dynamics in Fort Worth, Texas, at United States Air Force Plant 4 in late 1975; the first F-16A rolled out on 20 October 1976 and first flew on 8 December. The initial two-seat model achieved its first flight on 8 August 1977. The initial production-standard F-16A flew for the first time on 7 August 1978 and its delivery was accepted by the USAF on 6 January 1979. The F-16 was given its name of "Fighting Falcon" on 21 July 1980, entering USAF operational service with the 34th Tactical Fighter Squadron, 388th Tactical Fighter Wing, at Hill AFB in Utah, on 1 October 1980. On 7 June 1975, the four European partners, now known as the European Participation Group, signed up for 348 aircraft at the Paris Air Show. This was split among the European Participation Air Forces (EPAF) as 116 for Belgium, 58 for Denmark, 102 for the Netherlands, and 72 for Norway. Two European production lines, one in the Netherlands at Fokker's Schiphol-Oost facility and the other at SABCA's Gosselies plant in Belgium, would produce 184 and 164 units respectively. Norway's Kongsberg Vaapenfabrikk and Denmark's Terma A/S also manufactured parts and subassemblies for EPAF aircraft. European co-production was officially launched on 1 July 1977 at the Fokker factory. Beginning in November 1977, Fokker-produced components were sent to Fort Worth for fuselage assembly, then shipped back to Europe for final assembly of EPAF aircraft at the Belgian plant on 15 February 1978; deliveries to the Belgian Air Force began in January 1979. The first Royal Netherlands Air Force aircraft was delivered in June 1979. In 1980, the first aircraft were delivered to the Royal Norwegian Air Force by SABCA and to the Royal Danish Air Force by Fokker. During the late 1980s and 1990s, Turkish Aerospace Industries (TAI) produced 232 Block 30/40/50 F-16s on a production line in Ankara under license for the Turkish Air Force. TAI also produced 46 Block 40s for Egypt in the mid-1990s and 30 Block 50s from 2010. Korean Aerospace Industries opened a production line for the KF-16 program, producing 140 Block 52s from the mid-1990s to mid-2000s (decade). If India had selected the F-16IN for its Medium Multi-Role Combat Aircraft procurement, a sixth F-16 production line would have been built in India. In May 2013, Lockheed Martin stated there were currently enough orders to keep producing the F-16 until 2017. The F-16 is a single-engine, highly maneuverable, supersonic, multi-role tactical fighter aircraft. It is much smaller and lighter than its predecessors but uses advanced aerodynamics and avionics, including the first use of a relaxed static stability/fly-by-wire (RSS/FBW) flight control system, to achieve enhanced maneuver performance. Highly agile, the F-16 was the first fighter aircraft purpose-built to pull 9-g maneuvers and can reach a maximum speed of over Mach 2. Innovations include a frameless bubble canopy for better visibility, a side-mounted control stick, and a reclined seat to reduce g-force effects on the pilot. It is armed with an internal M61 Vulcan cannon in the left wing root and has multiple locations for mounting various missiles, bombs and pods. It has a thrust-to-weight ratio greater than one, providing power to climb and vertical acceleration. The F-16 was designed to be relatively inexpensive to build and simpler to maintain than earlier-generation fighters. The airframe is built with about 80% aviation-grade aluminum alloys, 8% steel, 3% composites, and 1.5% titanium. The leading-edge flaps, stabilators, and ventral fins make use of bonded aluminum honeycomb structures and graphite epoxy lamination coatings. The number of lubrication points, fuel line connections, and replaceable modules is significantly lower than preceding fighters; 80% of the access panels can be accessed without stands. The air intake was placed so it was rearward of the nose but forward enough to minimize air flow losses and reduce aerodynamic drag. Although the LWF program called for a structural life of 4,000 flight hours, capable of achieving 7.33 g with 80% internal fuel; GD's engineers decided to design the F-16's airframe life for 8,000 hours and for 9-g maneuvers on full internal fuel. This proved advantageous when the aircraft's mission changed from solely air-to-air combat to multi-role operations. Changes in operational use and additional systems have increased weight, necessitating multiple structural strengthening programs. The F-16 has a cropped-delta wing incorporating wing-fuselage blending and forebody vortex-control strakes; a fixed-geometry, underslung air intake (with splitter plate) to the single turbofan jet engine; a conventional tri-plane empennage arrangement with all-moving horizontal "stabilator" tailplanes; a pair of ventral fins beneath the fuselage aft of the wing's trailing edge; and a tricycle landing gear configuration with the aft-retracting, steerable nose gear deploying a short distance behind the inlet lip. There is a boom-style aerial refueling receptacle located behind the single-piece "bubble" canopy of the cockpit. Split-flap speedbrakes are located at the aft end of the wing-body fairing, and a tailhook is mounted underneath the fuselage. A fairing beneath the rudder often houses ECM equipment or a drag chute. Later F-16 models feature a long dorsal fairing along the fuselage's "spine", housing additional equipment or fuel.[43][69] Aerodynamic studies in the 1960s demonstrated that the "vortex lift" phenomenon could be harnessed by highly swept wing configurations to reach higher angles of attack, using leading edge vortex flow off a slender lifting surface. As the F-16 was being optimized for high combat agility, GD's designers chose a slender cropped-delta wing with a leading-edge sweep of 40° and a straight trailing edge. To improve maneuverability, a variable-camber wing with a NACA 64A-204 airfoil was selected; the camber is adjusted by leading-edge and trailing edge flaperons linked to a digital flight control system regulating the flight envelope. The F-16 has a moderate wing loading, reduced by fuselage lift. The vortex lift effect is increased by leading-edge extensions, known as strakes. Strakes act as additional short-span, triangular wings running from the wing root (the junction with fuselage) to a point further forward on the fuselage. Blended into the fuselage and along the wing root, the strake generates a high-speed vortex that remains attached to the top of the wing as the angle of attack increases, generating additional lift and allowing greater angles of attack without stalling. Strakes allow a smaller, lower-aspect-ratio wing, which increases roll rates and directional stability while decreasing weight. Deeper wing roots also increase structural strength and internal fuel volume.