Hawker Siddeley Nimrod

The Hawker Siddeley Nimrod was a maritime patrol aircraft developed and operated by the United Kingdom. It is an extensive modification of the de Havilland Comet, the world's first jet airliner. It was originally designed by de Havilland's successor firm, Hawker Siddeley; further development and maintenance work was undertaken by Hawker Siddeley's own successor companies, British Aerospace and BAE Systems, respectively.

Designed in response to a requirement issued by the Royal Air Force (RAF) to replace its fleet of aging Avro Shackletons, the Nimrod MR1/MR2 were primarily fixed-wing aerial platforms for anti-submarine warfare (ASW) operations; secondary roles included maritime surveillance and anti-surface warfare. It served from the early 1970s until March 2010. The intended replacement was to be extensively rebuilt Nimrod MR2s, designated as Nimrod MRA4; however due to considerable delays, repeated cost overruns, and financial cutbacks, the development of the MRA4 was abandoned in 2010.

In addition to the three Maritime Reconnaissance variants, two further Nimrod types were developed. The RAF operated a small number of Nimrod R1, an electronic intelligence gathering (ELINT) variant. A dedicated airborne early warning platform, the Nimrod AEW3 was in development from late 1970s to the mid-1980s; however much like the MRA4, considerable development issues were encountered and thus the project was cancelled in 1986 in favour of an off-the-shelf solution in the Boeing E-3 Sentry. All Nimrod variants had been retired by mid-2011.

MR1
On 4 June 1964, the British Government issued Air Staff Requirement 381 to replace the Avro Shackleton. The requirement attracted a great deal of interest from both British and foreign aircraft Manufacturers, including the Lockheed P-3 Orion, the Breguet Atlantic and derivatives of the Hawker Siddeley Trident, BAC One-Eleven and de Havilland Comet. On 2 February 1965, British Prime Minister Harold Wilson announced the intention to order Hawker Siddeley's maritime patrol version of the Comet, the HS.801.

The Nimrod design was based on that of the Comet 4 civil airliner which had reached the end of its commercial life (the first two prototype Nimrods, XV148 & XV147 were built from two final unfinished Comet 4C airframes). The Comet's turbojet engines were replaced by Rolls-Royce Spey turbofans for better fuel efficiency, particularly at the low altitudes required for maritime patrol. Major fuselage changes were made, including an internal weapons bay, an extended nose for radar, a new tail with electronic warfare (ESM) sensors mounted in a bulky fairing, and a MAD (Magnetic anomaly detector) boom. After the first flight in May 1967, the RAF ordered a total of 46 Nimrod MR1s. The first example (XV230) entered service in October 1969. A total of five squadrons using the type were stood up, four were permanently based in the UK and a fifth was initially based in Malta.

R1
Three Nimrod aircraft were adapted for the signals intelligence role, replacing the Comet C2s and Canberras of No. 51 Squadron in May 1974. The R1 was visually distinguished from the MR2 by the lack of a MAD boom. It was fitted with an array of rotating dish aerials in the aircraft's bomb-bay, with further dish aerials in the tail cone and at the front of the wing-mounted fuel tanks. It had a flight crew of four (two pilots, a flight engineer and one navigator) and up to 25 crew operating the SIGINT equipment.

Only since the end of the Cold War has the role of the aircraft been officially acknowledged; they were once described as "radar calibration aircraft". The R1s have not suffered the same rate of fatigue and corrosion as the MR2s. One R1 was lost in a flying accident since the type's introduction; this occurred in May 1995 during a flight test after major servicing, at RAF Kinloss. To replace this aircraft an MR2 was selected for conversion to R1 standard, and entered service in December 1996.

The Nimrod R1 was based at RAF Waddington in Lincolnshire, England, and flown by 51 Sqn. The two remaining Nimrod R1s were originally planned to be retired at the end of March 2011, but operational requirements forced the RAF to deploy one to RAF Akrotiri, Cyprus on 16 March in support of Operation Ellamy. The last flight of the type was on 28 June 2011 from RAF Waddington, in the presence of the Chief of the Air Staff, ACM Sir Stephen Dalton. Aircraft XW664 was transferred to East Midlands Aeropark on 12 July 2011 and will go on public display once classified systems have been removed.

The R1 is to be replaced by ex-USAF Boeing RC-135W Rivet Joint aircraft, which will be known as the Air Seeker in RAF service. The three Air Seekers will be delivered between 2014 and 2018.

MR2
Starting in 1975, 35 aircraft were upgraded to MR2 standard, being re-delivered from August 1979. The upgrade included extensive modernisation of the aircraft's electronic suite. Changes included the replacement of the obsolete ASV Mk 21 radar used by the Shackleton and Nimrod MR1 with the new EMI Searchwater radar, a new acoustic processor (GEC-Marconi AQS-901) capable of handling more modern sonobouys, a new mission data recorder (Hanbush) and additional ESM pods on the wingtips.

Provision for in-flight refuelling was introduced during the Falklands War (as the MR2P), as well as hardpoints to allow the Nimrod to carry the AIM-9 Sidewinder missile to counter enemy Argentine Air Force maritime surveillance aircraft. In preparation for operations in the Gulf War theatre, several MR2s were fitted with new communications and ECM equipment to deal with anticipated threats; at the time these modified aircraft were given the designation MR2P(GM) (Gulf Mod).

The Nimrod MR2 carried out three main roles – Anti-Submarine Warfare (ASW), Anti-Surface Unit Warfare (ASUW) and Search and Rescue (SAR). Its extended range enabled the crew to monitor maritime areas far to the north of Iceland and up to 4,000 km out into the Western Atlantic. With Air-to-Air Refuelling (AAR), range and endurance was greatly extended. The crew consisted of two pilots and one flight engineer, two navigators (one tactical navigator and a routine navigator), one Air Electronics Officer (AEO), the sonobuoy sensor team of two Weapon System Operators (WSOp ACO) and four Weapon System Operators (WSOp EW) to manage passive and active electronic warfare systems.

The Nimrod MR2 was based at RAF Kinloss in Scotland and flown by 201, 120, 206, and 42(R) Squadrons. The Nimrod MR2 aircraft was withdrawn on 31 March 2010, a year earlier than planned, for financial reasons. The last official flight of a Nimrod MR2 took place on 26 May 2010, with XV229 flying from RAF Kinloss to Kent International Airport to be used as an evacuation training airframe at the nearby MOD Defence Fire Training and Development Centre.

AEW3


In the mid-1970s a modified Nimrod was proposed for the Airborne Early Warning (AEW) mission – again as a replacement for the Lancaster-derived, piston-engined Shackleton AEW.2. Eleven existing Nimrod airframes were to be converted by British Aerospace at the former Avro plant at Woodford to house the GEC Marconi radars in a bulbous nose and tail. The Nimrod AEW3 project was plagued by cost over-runs and problems with the GEC 4080M computer used. Eventually, the MoD recognised that the cost of developing the radar system to achieve the required level of performance was prohibitive and the probability of success very uncertain, and in December 1986 the project was cancelled. The RAF eventually received seven Boeing E-3 Sentry aircraft instead.

MRA4
The Nimrod MRA4 was intended to replace the capability provided by the MR2. It was essentially a new aircraft, with current-generation Rolls-Royce BR710 turbofan engines, a new larger wing, and fully refurbished fuselage. However the project was subject to delays, cost over-runs, and contract re-negotiations; the type had been originally intended to enter service in 2003. The MRA4 was cancelled in 2010 as a result of the Strategic Defence and Security Review at which point it was £789 million over-budget and nine years late; the development airframes were also scrapped. The cancellation of the MRA4 marked an abortive end of the Nimrod's era; the functions it provided were largely abandoned leading to a significant UK capability gap. A few functions were dispersed to other assets, including the use of unmanned aerial vehicles (UAVs) to conduct limited maritime survillence.

Overview
The Nimrod was the first jet-powered maritime patrol aircraft (MPA) to enter service, being powered by the Rolls-Royce Spey turbojet engine. Aircraft in this role have been commonly propelled by piston or turboprop powerplants instead to maximise fuel economy and enable maximum patrol time on station; advantages of the Nimrod's turbojet engines include greater speed and altitude capabilities, it is also more capable to evade detection methods by submarines, whereas propeller-driven aircraft are more detectable underwater to standard acoustic sensors. Inflight, the Nimrods had a flight endurance of ten hours without aerial refuelling; the MR2s were later fitted to receive mid-air refuelling in response to demands in the Falklands War.

At the start of a patrol mission all four engines would normally be running, but as the aircraft's weight is reduced by the consumption of onboard fuel up to two engines may be intentionally shut down, allowing the remaining engines to be operated in a more efficient manner. Instead of relying on ram air to restart an inactive engine, compressor air could be crossfed from a live engine to a starter turbine; the crossfeed duct was later discovered to be a potential fire hazard. Similarly, the two hydraulic systems onboard were designed to be powered by the two inner engines that would always be running. Electrical generation was designed to far exceed the consumption of existing equipment in order to accommodate additional systems installed over the Nimrod's operational service.

The standard Nimrod fleet carried out three basic operational roles during their RAF service: Anti-Submarine Warfare duties typically involved survelliance over an allocated area of the North Atlantic to detect the presence of Soviet submarines in that area and to track their movements. In the event of war, reconnaissance information gathered during these patrols would be shared with other allied aircraft to enable coordinated strikes at both submarines and surface targets. Search and Rescue (SAR) missions were another important duty of the RAF's Nimrod fleet, operating under the Air Rescue Coordination Center at RAF Kinloss, and were a common sight in both military and civil maritime incidents. Throughout the Nimrod's operational life, a minimum of one aircraft was being held in a state of readiness to respond to SAR demands at all times.

Avionics
The Nimrod featured a large crew of up to 25 personnel, although a typical crew numbers roughly 12 members, most of which operate the various onboard sensor suites and specialist detection equipment. A significant proportion of the onboard sensor equipment was housed outside the pressure shell inside the Nimrod's distinctive pannier lower fuselage. Sensor systems included radar, sonar, and the magnetic anomaly detector; a 'sniffer' could detect exhaust fumes from diesel submarines as well. The Nimrod and its detection capabilities was an important component of Britain's military defence during the height of the Cold War.

The Nimrod's navigational functions were computerised, and were managed from a central tactical compartment housed in the forward cabin; various aircraft functions such as weapons control and information from sensors such as the large forward doppler radar were displayed and controlled at the tactical station. The flight systems and autopilot could be directly controlled by navigator's stations in the tactical compartment, giving the navigator nearly complete aircraft control. The navigational systems comprised digital, analogue, and electro-mechanical elements; the computers were directly integrated with most of the Nimrod's guidance systems such as the air data computer, astrocompass, inertial guidance and doppler radar, navigation information could also be manually input by the operators.

Upon its introduction to service, the Nimrod was hailed as possessing advanced electronic equipment such as onboard digital computers; the increased capability of these electronic systems allowed the RAF's fleet of 46 Nimrod aircraft to provide equal coverage to that of the larger fleet of retiring Avro Shackletons. The design philosophy of these computerised systems was that of a 'man-machine partnership'; while onboard computers performed much of the data sift and analysis processes, decisions and actions on the basis of that data remained in the operator's hands. To support the Nimrod's anticipated long lifespan, onboard computers were designed to be capable of integrating with various new components, systems, and sensors that could be added in future upgrades. After a mission, gathered information could be extracted for review purposes and for further analysis.

Armaments and equipment
The Nimrod features a sizable bomb bay in which, in addition to armaments such as torpedos and missiles, could be housed a wide variety of specialist equipment for many purposes, such as up to 150 sonobuoys for ASW purposes or multiple air-deployed dinghies and droppable survival packs such as Lindholme Gear for SAR missions; additional fuel tanks and cargo could also be carried in the bomb bay during ferrying flights. Other armaments equippable in the bomb bay include mines, bombs, and nuclear depth charges; later munitions included the Sting Ray torpedo and Harpoon missile for increased capabilities.

The Nimrod could also be fitted with two detachable pylons mounted underneath the wings to be used with missiles such as the Martel; two specialised pylons were later added to enable the equipping of Sidewinder missiles, used for self-defence purposes from hostile aircraft. A powerful remote-controlled searchlight was installed underneath the starboard wing for SAR operations. For reconnaissance missions, a pair of downward-facing cameras suited to low and high-altitude photography were also equipped on the Nimrod; in later years a newer electro-optical camera system was installed for greater imaging quality.

Various new ECMs and electronic support systems were retrofitted onto the Nimrod fleet in response to new challenges and to increase the type's defensive capabilities; additional equipment also provided more effective means of identification and communication. A number of modifications were introduced during the 1991 Gulf War, a small number of MR2s were fitted with improved Link 11 datalinks, new defensive ECM equipment including the first operational use of a towed radar decoy, and a forward looking infrared turret under the starboard wing.

Operational history
At first the crews, who were transferred to the Nimrod from the piston-engine Avro Shackletons, were not enthusiastic with the craft, mainly because its sensor suite was only marginally superior to the Shackleton's. In fact most sensors were the same, although the aircraft had a new digital data fusion computer. The Nimrod gave sterling service during the "Cod Wars" between Iceland and the UK over fishing rights. During the Falklands war (Operation Corporate), several Nimrods combed the sea for enemy submarines. The Nimrods took part in Operation Granby (the Gulf War 1990/1991), the NATO operations against Serbia in 1999, Operation Telic (the Iraq war in 2003 and beyond), the campaign in Afghanistan, and over Libya in 2011. They also were a routine component of British search and rescue (SAR) operations in the North Sea.

Falklands War
Nimrods were first deployed to Wideawake airfield on Ascension Island on 5 April 1982, the type at first being used to fly local patrols around Ascension to guard against potential Argentine attacks, and to escort the British Task Force as it sailed south towards the Falkands, with Nimrods also being used to provide search and rescue as well as communications relay support of the Operation Black Buck bombing raids by Avro Vulcans. As the Task Force neared what would become the combat theatre and the threat from Argentine submarines rose, the more capable Nimrod MR2s took on operations initially performed by older Nimrod MR1s. Aviation author Chris Chant has claimed that the Nimrod R1 also conducted electronic intelligence missions operating from Punta Arenas in neutral Chile.

The addition of air-to-air refuelling probes allowed operations to be carried out in the vicinity of the Falklands, while the aircraft's armament was supplemented by the addition of 1,000 lb (450 kg) general purpose bombs, BL755 cluster bombs and AIM-9 Sidewinder air-to-air missiles. The use of air-to-air refuelling allowed extremely long reconnaissance missions to be mounted, one example being a 19 hour 15 minute patrol conducted on 15 May 1982, which passing within 60 miles (97 km) of the Argentine coast to confirm that Argentine surface vessels were not at sea. Another long-range flight was carried out by an MR2 on the night of 20/21 May, covering a total of 8,453 miles (13,609 km), the longest distance flight carried out during the Falklands War. In all, Nimrods flew 111 missions from Ascension in support of British operations during the Falklands War.

Gulf War
A detachment of three Nimrod MR2s was deployed to Seeb in Oman in August 1990 as a result of the Iraqi invasion of Kuwait, carrying out patrols over the Gulf of Oman and Persian Gulf. Due to the level of threats present in the Gulf theatre, operational Nimrods were quickly retrofitted with a Marconi towed active decoy. Once hostilities commenced, the Nimrod detachment, by now increased to five aircraft, concentrated on night patrols, with daylight patrols carried out by US Navy Lockheed P-3 Orions. Nimrods were used to guide Westland Lynx helicopters and Grumman A-6 Intruder attack aircraft against Iraqi patrol vessels, being credited with assisting in sinking or damaging 16 Iraqi vessels. Britain elected to maintain an RAF presence with the Nimrod and other assets following the Gulf War.



Afghanistan and Iraq War
Nimrods were again deployed to the Middle East as part of the British contribution to the US-led invasion of Afghanistan; missions in this theatre involved the Nimrods performing lengthy overland flights for intelligence-gathering purposes. On 2 September 2006, 12 RAF personnel were killed when a Nimrod MR2 was destroyed in a midair explosion following an onboard fire over Afghanistan, it was the single greatest loss of British life since the Falklands War. The outbreak of the Iraq War in March 2003 saw the RAF's Nimrods being used for operations over Iraq, using the aircraft's sensors to detect hostile forces and to direct attacks by friendly coalition forces.

Search and rescue
While the Nimrod MR1/MR2 was in service, one aircraft from each of the squadrons on rotation was available for search and rescue operations at one-hour standby. The standby aircraft carried two sets of Lindholme Gear in the weapons bay. Usually one other Nimrod airborne on a training mission would also carry a set of Lindholme Gear. As well as using the aircraft sensors to find aircraft or ships in trouble, it was used to find survivors in the water, with a capability to search areas of up to 20000 sqmi. The main role would normally be to act as on-scene rescue coordinator to control ships, fixed-wing aircraft, and helicopters in the search area.

Because of the search and rescue role, Nimrod aircraft often appeared in the media in connection with major rescue incidents. In August 1979, several Nimrods were involved in locating yachting competitors during the disaster-stricken 1979 Fastnet race and coordinated with helicopters in searches for survivors from lost vessels. In March 1980, the Alexander L. Kielland was a Norwegian semi-submersible drilling rig that capsized whilst working in the Ekofisk oil field killing 123 people; six different Nimrods searched for survivors and took turns to provide rescue co-ordination, involving the control of 80 surface ships and 20 British and Norwegian helicopters. In an example of the search capabilities, in September 1977 when an attempted crossing of the North Atlantic in a Zodiac inflatable dinghy went wrong, a Nimrod found the collapsed dinghy and directed a ship to it.

Offshore Tapestry
Tapestry is a codeword for the activities by ships and aircraft that protect the United Kingdom's Sovereign Sea Areas, including the protection of fishing rights and oil and gas extraction. Following the establishment of a 200 nmi Exclusive Economic Zone (EEZ) at the beginning of 1977 the Nimrod fleet was given the task of patrolling the 270000 sqmi area. The aircraft would locate, identify, and photograph vessels operating in the EEZ. The whole area was normally covered every week, with each vessel being photographed. The aircraft would also check and communicate with all oil and gas platforms. In 1978 a Nimrod arrested an illegal fishing vessel from the air in the Western Approaches and made the vessel proceed to Milford Haven for further investigation. During the Icelandic Cod Wars of 1972 and 1975–1976 the Nimrod aircraft operated with Royal Navy surface vessels protecting British fishing fleets.

Operators

 * Royal Air Force
 * Royal Air Force

Survivors

 * MR2
 * XV226 – Bruntingthorpe Aerodrome
 * XV231 – Manchester Airport aviation viewing park
 * XV229 – Manston Airport, Kent, used as MOD evacuation trainer
 * XV232 – Coventry airport
 * XV244 – Stored at RAF Kinloss for preservation.
 * XV250 – Yorkshire Air Museum
 * XV255 – City of Norwich Aviation Museum


 * R1
 * XV249 – RAF Museum Cosford
 * XW664 – East Midlands Airport

Accidents and incidents
Five Nimrods have been lost in accidents:


 * On 17 November 1980, a Nimrod MR2 XV256 crashed near RAF Kinloss after three engines failed following multiple birdstrikes. Both pilots were killed but the remaining crew survived.


 * On 3 June 1984, a Nimrod MR2 XV257 stationed at RAF St Mawgan suffered extensive damage when a reconnaissance flare ignited in the bomb bay during flight. The aircraft successfully returned to base but was subsequently written-off due to fire damage. There were no casualties.


 * On 16 May 1995, XW666, a Nimrod R1 from RAF Waddington, ditched in the Moray Firth 4.5 mi from Lossiemouth after an engine caught fire during a post-servicing test flight from RAF Kinloss. The Ministry of Defence (MoD) inquiry identified a number of technical issues as the cause. There were no casualties.


 * On 2 September 1995, a Nimrod MR2 XV239 crashed into Lake Ontario while participating in the Canadian International Air Show, killing the seven crew members.


 * On 2 September 2006, a Nimrod MR2 XV230 crashed near Kandahar in Afghanistan, killing 12 airmen, one marine and one soldier – the largest single day loss of UK personnel since the Falklands War. This was the first Nimrod to enter operational service, originally as a MR1 but upgraded to MR2 standard in the 1980s. On 23 February 2007, the Ministry of Defence grounded all MR2 aircraft while fuel pumps were inspected. The MoD stressed that this was not necessarily related to the crash in Afghanistan.


 * On 5 November 2007, XV235 was involved in a midair incident over Afghanistan when the crew noticed a fuel leak during air-to-air refuelling. After transmitting a mayday call, the crew landed the aircraft successfully. The incident came only a month before the issue of the report of a Board of Enquiry into the 2 September 2006 fatal accident to XV230 in (likely) similar circumstances. The RAF subsequently suspended air-to-air refuelling operations for this type.