The Grumman A-6 Intruder is an American twinjet all-weather attack aircraft developed and manufactured by American aircraft company Grumman Aerospace and operated by the U.S. Navy and U.S. Marine Corps. It was designed in response to a 1957 requirement issued by the Bureau of Aeronautics for an all-weather attack aircraft for Navy long-range interdiction missions and with short takeoff and landing (STOL) capability for Marine close air support. It was to replace the piston-engined Douglas A-1 Skyraider. The requirement allowed one or two engines, either turbojet or turboprop. The winning proposal from Grumman used two Pratt & Whitney J52 turbojet engines. The Intruder was the first Navy aircraft with an integrated airframe and weapons system. Operated by a crew of two in a side-by-side seating configuration, the workload was divided between the pilot and weapons officer (bombardier/navigator (BN)). In addition to conventional munitions, it could also carry nuclear weapons, which would be delivered using toss bombing techniques. On 19 April 1960, the first prototype made its maiden flight. The A-6 was in service with the United States Navy and Marine Corps between 1963 and 1997, during which time multiple variants were prototyped and produced. Two of the more successful variants developed were the EA-6B Prowler, a specialized electronic warfare derivative, and the KA-6D tanker version. It was deployed during various overseas conflicts, including the Vietnam War and the Gulf War. The A-6 was intended to be superseded by the McDonnell Douglas A-12 Avenger II, but this program was ultimately canceled due to cost overruns. Thus, when the A-6E was scheduled for retirement, its precision strike mission was initially taken over by the Grumman F-14 Tomcat equipped with a LANTIRN pod. Development Background As a result of the fair-weather limitation of the propeller-driven Skyraider in the Korean War and the advent of turbine engines, the United States Navy issued preliminary requirements in 1955 for an all-weather carrier-based attack aircraft. The U.S. Navy published an operational requirement document for it in October 1956. It released a request for proposals (RFP) in February 1957. This request called for a 'close air support attack bomber capable of hitting the enemy at any time'. Aviation authors Bill Gunston and Peter Gilchrist observe that this specification was shaped by the service's Korean War experiences, during which air support had been frequently unavailable unless fair weather conditions were present. In response to the RFP, a total of eleven design proposals were submitted by eight different companies, including Bell, Boeing, Douglas, Grumman, Lockheed, Martin, North American, and Vought. Grumman's submission was internally designated as the Type G-128. Following evaluation of the bids, the U.S. Navy announced the selection of Grumman on 2 January 1958. The company was awarded a contract for the development of their submission, which had been re-designated A2F-1, in February 1958. YA2F-1 showing the original tilting tailpipes Grumman's design team was led by Robert Nafis and Lawrence Mead, Jr. Mead later played a lead role in the design of the Lunar Excursion Module and the Grumman F-14 Tomcat. The team was spread between two sites, the company's manufacturing plant as Bethpage and the testing facilities at Naval Weapons Industrial Reserve Plant, Calverton. During September 1959, the design was approved by the Mock-Up Review Board. The A2F-1 design incorporated several cutting-edge features for the era. In the early 1960s, it was novel for a fighter-sized aircraft to have sophisticated avionics that used multiple computers. This design experience was taken into consideration by NASA in their November 1962 decision to choose Grumman over other companies like General Dynamics-Convair (the F-111 had computerized avionics capabilities comparable to the A-6, but did not fly until 1964) to build the Lunar Excursion Module, which was a small-sized spacecraft with two onboard computers. Test program The first prototype YA2F-1, lacking radar and the navigational and attack avionics, made its first flight on 19 April 1960, with the second prototype flying on 28 July 1960. The test program required to develop the aircraft took a long time. The very advanced navigation and attack equipment required a lot of development and changes had to be made to correct aerodynamic deficiencies and remove unwanted features. Extending the air brakes, which were mounted on the rear fuselage, changed the downwash at the horizontal tailplane which overloaded its actuator so the tailplane was moved rearwards by 16 inches (41 cm). Later evaluation of the aircraft showed that the airbrakes were not effective enough in controlling the speed of the aircraft and they were moved to the wing-tips. Early production aircraft were fitted with both the fuselage and wingtip air brakes, although the fuselage-mounted ones were soon disabled, and were removed from later aircraft. The trailing edge of each wing-tip split to form a much more effective speed-brake which projected above and below the wing when extended. The rudder needed a wider chord at its base to give greater exposed area to assist spin recovery. A major difference between the first six production aircraft and subsequent aircraft were the jet nozzles; close-air support by the Marine Corps required STOL performance to operate from forward airstrips. Jet deflection using tilting tailpipes was proposed. The performance benefits from varying the angle were not worthwhile, whether operating from short strips or carriers, and they were fixed at a 7 degree downward angle. Further development During February 1963, the A-6 was introduced to service with the US Navy; at this point, the type was, according to Gunston and Gilchrist, "the first genuinely all-weather attack bomber in history". However, early operating experiences found the aircraft to be imposing very high maintenance demands, particularly in the Asian theatre of operations, and serviceability figures were also low. In response, the Naval Avionics Lab launched a substantial and lengthy program to improve both the reliability and performance of the A-6's avionics suite. The successful performance of the A-6 in operations following these improvements ended proposals to produce follow-on models that featured downgraded avionics. Various specialized variants of the A-6 were developed, often in response to urgent military requirements raised during the Vietnam War. The A-6C, a dedicated interdictor, was one such model, as was the KA-6D, a buddy store-equipped aerial refuelling tanker. Perhaps the most complex variant was the EA-6B Prowler, a specialized electronic warfare derivative. The last variant to be produced was the A-6E, first introduced in 1972; it features extensive avionics improvements, including the new APQ-148 multimode radar, along with minor airframe refinements. The last A-6E was delivered in 1992. During the 1980s, a further model, designated A-6F, was being planned. Intended to feature the General Electric F404 turbofan engine, as well as various avionics and airframe improvements, this variant was cancelled under the presumption that the in-development McDonnell Douglas A-12 Avenger II would be entering production before long. Instead, a life-extension program involving the re-winging of existing A-6E aircraft was undertaken; initially a metal wing had been used before a graphite-epoxy composite wing was developed during the late 1980s. Other improvements were introduced to the fleet around this time, including GPS receivers, new computers and radar sets, more efficient J-52-409 engines, as well as increased compatibility with various additional missiles. Design An A-6E landing on the aircraft carrier USS America (CV-66), showing the split airbrakes on the tips of its left wing The Grumman A-6 Intruder is a two-seat twin-engined monoplane, equipped to

light conditions. The cockpit used an unusual double pane windscreen and side-by-side seating arrangement in which the pilot sat in the left seat, while the bombardier/navigator (BN) sat to the right and slightly below to give the pilot an adequate view on that side. In addition to a radar display for the BN, a unique instrumentation feature for the pilot was a cathode ray tube screen that was known as the Vertical Display Indicator (VDI). This display provided a synthetic representation of the world in front of the aircraft, along with steering cues provided by the BN, enabling head-down navigation and attack at night and in all weather conditions. The A-6's wing was relatively efficient at subsonic speeds, particularly when compared to supersonic fighters such as the McDonnell Douglas F-4 Phantom II, which are also limited to subsonic speeds when carrying a payload of bombs. The wing was also designed to provide a favorable level of maneuverability even while carrying a sizable bomb load. A very similar wing would be put on pivots on Grumman's later supersonic swing-wing Grumman F-14 Tomcat, as well as similar landing gear. For its day, the Intruder had sophisticated avionics, with a high degree of integration. To aid in identifying and isolating equipment malfunctions, the aircraft was provided with automatic diagnostic systems, some of the earliest computer-based analytic equipment developed for aircraft. These were known as Basic Automated Checkout Equipment, or BACE (pronounced "base"). There were two levels, known as "Line BACE" to identify specific malfunctioning systems in the aircraft, while in the hangar or on the flight line; and "Shop BACE", to exercise and analyze individual malfunctioning systems in the maintenance shop. This equipment was manufactured by Litton Industries. Together, the BACE systems greatly reduced the Maintenance Man-Hours per Flight Hour, a key index of the cost and effort needed to keep military aircraft operating. The Intruder was equipped to carry nuclear weapons (B43, B57, B61) which would have been delivered using semi-automated toss bombing. Operational history Entering service and Vietnam War An A-6E Intruder aircraft assigned to USS Dwight D. Eisenhower. S-3A Viking, A-6E Intruder, and an EA-6B Prowler aircraft are parked on the flight deck of aircraft carrier USS John F. Kennedy during a storm. The Intruder received a new standardized US DOD designation of A-6A in the Autumn of 1962, and entered squadron service in February 1963. The A-6 became both the U.S. Navy's and U.S. Marine Corps's principal medium and all-weather/night attack aircraft from the mid-1960s through the 1990s and as an aerial tanker either in the dedicated KA-6D version or by use of a buddy store (D-704). Whereas the A-6 fulfilled the USN and USMC all-weather ground-attack/strike mission role, this mission in the USAF was served by the Republic F-105 Thunderchief and later the F-111, the latter which also saw its earlier F-111A variants converted to a radar jammer as the EF-111 Raven, analogous to the USN and USMC EA-6B Prowler. A-6 Intruders first saw action during the Vietnam War, where the craft were used extensively against targets in Vietnam. The aircraft's long range and heavy payload (18,000 pounds or 8,200 kilograms) coupled with its ability to fly in all weather made it invaluable during the war. However, its typical mission profile of flying low to deliver its payload made it especially vulnerable to anti-aircraft fire, and in the eight years the Intruder was used during the Vietnam War, the U.S. Navy and U.S. Marine Corps lost a total of 84 A-6 aircraft of various series. Twenty U.S. Navy aircraft carriers rotated through the waters of Southeast Asia, providing air strikes, from the early 1960s through the early 1970s. Although capable of embarking aboard aircraft carriers, most U.S. Marine Corps A-6 Intruders were shore based in South Vietnam at Chu Lai and Da Nang and in Nam Phong, Thailand. Lebanon and later action A-6 Intruders were later used in support of other operations, such as the Multinational Force in Lebanon in 1983. Intruders also saw action in April 1986 operating from the aircraft carriers USS America and Coral Sea during the bombing of Libya (Operation El Dorado Canyon). A-6s flew more than 4,700 combat sorties, providing close air support, destroying enemy air defenses, attacking Iraqi naval units, and hitting strategic targets. They were also the U.S. Navy's primary strike platform for delivering laser-guided bombs. The U.S. Navy operated them from the aircraft carriers USS Saratoga, USS John F. Kennedy, USS Midway, USS Ranger, USS America and USS Theodore Roosevelt, while U.S. Marine Corps A-6s operated ashore, primarily from Shaikh Isa Air Base in Bahrain. Three A-6s were shot down in combat by SAMs and AAA.Following the Gulf War, Intruders were used to patrol the no-fly zone in Iraq and provided air support for U.S. Marines during Operation Restore Hope in Somalia. The last A-6E Intruder left U.S. Marine Corps service on 28 April 1993. Navy A-6s saw further duty over Bosnia in 1994. On 4 June 1996, during RIMPAC a US Navy A-6E performing the unusual target towing task to train Japanese Navy air defense crews was mistakenly engaged and shot down by the Japanese destroyer JS Yūgiri with its Phalanx CIWS gun. Both pilots ejected and were recovered. Retirement Despite the production of new airframes in the 164XXX Bureau Number (BuNo) series just before and after the Gulf War, augmented by a rewinging program of older airframes, the A-6E and KA-6D were quickly phased out of service in the mid-1990s in a U.S. Navy cost-cutting move driven by the Office of the Secretary of Defense to reduce the number of different type/model/series (T/M/S) of aircraft in carrier air wings and U.S. Marine aircraft groups. The A-6 was intended to be replaced by the McDonnell Douglas A-12 Avenger II, but that program was canceled due to cost overruns.[31] The Intruder remained in service for a few more years before being retired in favor of the LANTIRN-equipped F-14D Tomcat, which was in turn replaced by the F/A-18E/F Super Hornet in the U.S. Navy and the twin-seat F/A-18D Hornet in the U.S. Marine Corps. During the 2010s, the Unmanned Carrier-Launched Airborne Surveillance and Strike program was at one point intended to produce an unmanned aerial vehicle (UAV) successor to the Intruder's long-distance strike role, but the initiative has since changed priorities towards the tanker mission instead. The last Intruders were retired on 28 February 1997. Many in the US defense establishment in general, and Naval Aviation in particular, questioned the wisdom of a shift to a shorter range carrier-based strike force, as represented by the Hornet and Super Hornet, compared to the older generation aircraft such as the Intruder and Tomcat. However, the availability of USAF Boeing KC-135 Stratotanker and McDonnell Douglas KC-10 Extender tankers modified to accommodate USN, USMC and NATO tactical aircraft in all recent conflicts was considered by certain senior decision makers in the Department of Defense to put a lesser premium on organic aerial refueling capability in the U.S. Navy's carrier air wings and self-contained range among carrier-based strike aircraft. Although the Intruder could not match the F-14's or the F/A-18's speed or air-combat capability, the A-6's range and load-carrying ability are still unmatched by newer aircraft in the fleet. At the time of retirement, several retired A-6 airframes were awaiting rewinging at the Northrop Grumman facility at St. Augustine Airport, Florida; these were later sunk off the coast of St. Johns County, Florida to form a fish haven named "Intruder Reef". Surviving aircraft fitted with the new wings, and later production aircraft not earmarked for museum or non-flying static display were stored at the AMARG storage center at Davis-Monthan Air Force Base, Arizona. Variants The eight prototypes