Nimrod

The Nimrod has had a long and distinguished history. It is a unique military aircraft with remarkable longevity, flexibility and capability. Named the 'Mighty Hunter',2 the maritime version has for over 40 years had a preeminent role patrolling the seas around the British Isles, in anti-submarine warfare, anti-surface unit warfare, maritime reconnaissance and search and rescue roles. Following the end of the Cold War, its role and reach has expanded to cover vital intelligence gathering over land, in conflicts such as Afghanistan and Iraq. Throughout its life, the Nimrod has proved a remarkable and adaptable workhorse.

The Nimrod is a relatively rare type, with a production run of only 49 airframes. A total of 43 Nimrod Maritime Reconnaissance Mk 1s (MR1s) were built by Hawker Siddeley and delivered to the RAF: 38 Nimrod MR1s were built at Woodford between 1967 and 1972 and, from a further batch of eight Nimrods, five became MR1s between 1973 and 1975. Of the remaining three aircraft of this batch, two were built as AEW Mk3s and one was built directly to MR2 standard. The remaining three airframes were used as Nimrod Reconnaissance Mk 1s (R1s) and delivered prior to 1973.

The Nimrod was developed from the world's first pressurised jet-propelled commercial airliner to reach production, the De Havilland Comet. The Comet had its maiden flight on 27 July 1949. The first Comet to receive a Certificate of Airworthiness was on 22 January 1952. The first commercial passengers were carried with the British Overseas Aircraft Corporation on 2 May 1952. H.M. The Queen Mother was an early passenger and became the first member of the British Royal Family to fly by jet. Early Comet models suffered metal fatigue and other problems, causing a series of well-publicised accidents, notably at Karachi (March 1953), at Elba (January 1954) and at Naples (April 1954). The Comet was temporarily withdrawn and redesigned (in particular, the rectangular shape of its passenger windows was found to have caused metal fatigue leading to the loss of at least two aircraft and was changed to an oval shape). The final marks of the Comet, the Comet 4 series, proved more successful. Subsequently, however, larger more efficient jet airliners, such as the Boeing 707, became more popular and superseded it in the commercial passenger world.

In July 1963, MOD Air Staff Target (AST) 357 called for a sophisticated, medium-sized, jet-powered, long-range maritime reconnaissance aircraft to replace the piston-engine Avro Shackleton which had had its maiden flight in March 1949, and entered service in 1951, becoming the United Kingdom's principal Maritime Patrol Aircraft (MPA). The Shackleton had witnessed the growth of the Soviet surface vessel and submarine fleet as the Cold War intensified throughout the 1950s and 1960s, increasing the importance of the MPA role. It was realised, however, that the requirements of AST 357 could never be met in the timescale necessary to replace the Shackleton and Air Staff Requirement (ASR) 381 was issued specifying a less complex requirement.

In July 1964, Hawker Siddeley Aviation (formerly De Havilland) made a formal proposal to convert the Comet 4C turbo-jet powered airliner into a military aircraft (designated HS801). The underside of the Comet fuselage was to be substantially reconfigured to fit a large bomb-bay, extra fuel tanks were to be fitted to give greater range and endurance, and the engines were to be changed from Rolls-Royce Avon engines to Spey 250 engines. In February 1965, it was announced in Parliament that the HS801 had been selected to replace the Shackleton.

The Nimrod is of a conventional aluminium alloy, semi-monocoque pressurised fuselage construction. It has a low, cantilevered monoplane wing which has a 20-degree swept-back all metal two-spar structure. It is approximately 129 ft long, 30 ft high, with a wingspan of 115 ft and wing area of 2,121 sq ft. It weighs 96,000 lbs without fuel, and 184,000 lbs fully loaded with fuel and stores. It has a maximum speed of 360 kts, a service ceiling of 42,000 ft and a maximum range of about 3,800 nautical miles without AAR. It has a typical maximum flight time of eight hours on internal fuel and maximum endurance of about ten hours; this was achieved by routinely shutting down two engines for fuel economy. Flight times can be further extended to 20 hours with multiple AAR.

The aircraft is powered by four Rolls-Royce RB 168-20 Spey 250 engines embedded in pairs in the root of each wing. The Spey 250 is a two-spool, low ratio, by-pass turbo-fan engine with a tubo-annular combustion system, developing 12,160 lb of thrust. An Auxiliary Power Unit (APU) is used to supply air for engine starts on the ground. The air supplied by the APU is ducted to a Cross-Feed duct running across the bomb bay between the port and starboard engines.

The replacement of the Comet's Avon powerplants with the Spey 250 engine necessitated the introduction of an air starter system. To provide this, APU was installed in the tail area and a duct used to take hot air from the APU to the engine starter turbine. The endurance of the Nimrod could be increased by shutting down up to two engines. As and when it was necessary to re-start engines during flight, hot, pressurised air could be routed through the Cross-Feed duct from engines on one side of the aircraft to re-start the engines on the other side. Therefore, there were occasions, both on the ground and airborne, when the Cross-Feed duct would contain very hot air and consequently it was insulated with a fiberglass covering contained within a stainless steel outer layer. This insulation was primarily designed to prevent heat damage to the surrounding structure.

In order to further extend the endurance of the MR1, additional fuel tanks, No. 5 and No. 6, were added below the fuselage cabin floor, in what were previously baggage compartments, and two further tanks, No. 7 tank port and starboard, were attached on either side of the fuselage within the wing root area. The Cross-Feed duct passed in front of the new No. 7 tank, running through an area known as the No. 7 Tank Dry Bay on the port and starboard side.

The MR1 was designed and certified to MOD Specification No. MR 254 D&P dated 1965. MR 254 D&P accepted that the basic Comet 4C aircraft design was certificated to British Civil Airworthiness Requirements (BCAR) 1956 Edition. The MR2 was designed and certified to MOD Specification No MR286 D&P dated 6 May 1975. Specification No MR286 D&P accepted the same general certification base as MR 254 D&P. The change in design from a Comet 4C to a Nimrod MR1 was required to comply with AvP 970 1965 Edition (Re-issue).

The number of sorties and hours flown by the Nimrod types has been relatively low compared with civilian commercial aircraft. This is not unusual for military aircraft. BAE Systems in BAE Report MBU-DEF-R-NIM-SDC-076 Review of Nimrod In-Service Accident History dated September 2004 noted that the Nimrod MR 2 and R 1 fleet collectively had accumulated approximately 400,000 flying hours. As of late 2009, the aircraft in the fleet which had accumulated the most flying hours have flown more than 18,000 flying hours.

The Nimrod fleet was adapted for Air-to-Air Refueling [AAR] and deployed in the Falklands War (1982), where it provided much needed support to the deploying British fleet and ensured that the UK's forward deployment base at Ascension Island remained secure. The fall of the Berlin Wall in 1989 and the subsequent end of the decades long Cold War witnessed the demise of the vast Soviet naval arsenal against which the Nimrod fleet had been ranged. However, new, equally challenging tasks soon presented themselves. Nimrods were deployed in the Persian Gulf War (First Gulf War) of 1990, when, among other things, they provided targeting information against Iraqi naval units, and participated in the UN blockade of Iraq (1990-1991). The aircraft also found gainful employment patrolling the waters of the Adriatic during the conflicts that followed the collapse of Yugoslavia.

Advances in computer technology meant that it became increasingly possible to co-ordinate the activities and products of military information gatherers. The importance of ensuring that commanders had access to all the information that they required led to concepts such as Network Enabled Capability, in which multiple gatherers of information feed an interlinked network, supplying data to multiple recipients. Whether the gatherer is an individual on the ground, a tank, an aircraft or a satellite does not matter. Thus developed the concept of Intelligence, Surveillance, Target-Acquisition and Reconnaissance (ISTAR). Clearly the Nimrod (in both MR and R roles) is a major ISTAR asset. The size and flexibility of the Nimrod airframe and its crews allowed Nimrod to assume new intelligence roles as and when required. The aircraft's ability to loiter for long periods was utilised, in combination with a new optical sensor, in operations over Afghanistan and Iraq. The fact that the aircraft was now operating over land, rather than in its traditional maritime environment, was not significant as it continued to fly within its cleared operational parameters.

Nimrods have been extremely successful in this new role. They have proved pivotal to the successful completion of many missions and enabled risks to allied ground forces to be reduced significantly. Although the Nimrod is no longer cleared to undertake AAR, limiting the time that it can provide support to ground forces, it has continued to make a valuable contribution to current operations in theatre. The combination of traditional and innovative roles means that the contribution that the Nimrod and its crews makes to UK defence has, if anything, grown over recent years, despite the demise of the Soviet fleet.

The modification of the Comet design to create the Nimrod produced a unique and outstanding aircraft which for many years had a rightful claim to be the world's premier MPA. Throughout its 40-year life, the Nimrod has very successfully and safely fulfilled a wide range of complex roles, including monitoring Soviet maritime activity and, more recently, in support of land conflicts. The Nimrod was a British success story.