British Aerospace Nimrod AEW3

British Aerospace Nimrod AEW3

Company type	Military aircraft project
Industry	Aerospace, Defence
Predecessor	Hawker Siddeley Nimrod maritime patrol aircraft
Founded	1970s
Headquarters	United Kingdom , United Kingdom
Products	Airborne early warning and control aircraft (planned)

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The British Aerospace Nimrod AEW3 was an envisioned airborne early warning and control (AEW&C) platform intended to deliver airborne radar surveillance capabilities for the Royal Air Force’s defence of United Kingdom airspace. This initiative aimed to adapt the existing Hawker Siddeley Nimrod maritime patrol aircraft airframe by equipping it with a newly designed radar system and avionics suite developed by Marconi Avionics.

The development of the Nimrod AEW encountered significant technical challenges and escalating costs primarily due to the complexity involved in creating advanced radar and computer systems and integrating these into the established Nimrod airframe. Ultimately, the programme was abandoned, and the RAF opted to acquire newly manufactured Boeing E-3 Sentry aircraft to meet its airborne early warning requirements.

Development

File:Avro 696 Shackleton AEW2, UK - Air Force AN1008804.jpg

During the interim phase of the Nimrod AEW development, surplus Shackletons were equipped with radar systems sourced from retired Royal Navy Gannets.

Background

In the mid-1960s, after the introduction of the Grumman E-2 Hawkeye carrier-based AEW aircraft and its related technologies, the British government initiated a search for a radar solution to provide airborne early warning capabilities for the UK. At that period, the only operational AEW platform within British forces was the Fairey Gannet, operated by the Fleet Air Arm aboard Royal Navy carriers. These aircraft utilized the AN/APS-20 radar, a system developed during World War II that was becoming increasingly outdated. Early in the 1960s, efforts began to create a new AEW platform for the Royal Navy to supersede the Gannet, incorporating a novel radar system mounted on a new airframe, the P.139. Although mid-1960s defense budget reductions led to cancellation of the P.139 project, development of a British-designed radar system persisted. Concurrently, it was recognized that the RAF required an AEW aircraft to support national air defense.

Several considerations influenced the government's approach to a new AEW aircraft:

• The initially proposed Frequency Modulated Interrupted Continuous Wave (FMICW) radar for both the P.139 and RAF's new platform could not function effectively near propellers, necessitating a jet-powered aircraft.
• The substantial antenna dimensions needed for adequate scanning range combined with a relatively large mission crew dictated that a sizeable aircraft was essential.

Engineers at Hawker Siddeley Aviation proposed integrating the FMICW radar via a Fore Aft Scanner System into the new Nimrod. This plan was dismissed due to high costs; an alternative suggestion involved converting surplus Andover transports, but this too was rejected because of projected development expenses.

As an interim solution to provide land-based AEW capability, radar units from decommissioned Royal Navy Gannets were installed in surplus Avro Shackleton maritime patrol aircraft, which entered service in 1972. Around this time, research from the United States Air Force (USAF) revealed that pulse-Doppler radar outperformed FMICW technology; this led to abandoning FMICW as a foundation for AEW systems. The USAF’s adoption of pulse-Doppler technology in the developing Boeing E-3 Sentry influenced British reassessment of their AEW strategy, reaffirming that the Nimrod platform met operational requirements.

File:Nimrod AEW3 model.jpg

Scale model of Nimrod AEW produced by manufacturer

The decision was made to acquire an aircraft equipped with pulse-Doppler radar technology, which led to several options being evaluated:

1. Procure the AN/APS-125 pulse-Doppler radar and associated avionics as used on the E-2 Hawkeye and install them in the Nimrod.
2. Combine the AN/APS-125 radar with British-developed avionics.
3. Utilize the rotodome and antenna from the E-2 Hawkeye paired with British-built radar transmitter, receiver, and avionics systems.
4. Develop an entirely British radar and avionics suite employing a Fore Aft Scanner System (FASS) rather than adopting the E-2 radome.

The final option promised to preserve domestic employment and maintain Britain's leadership in radar innovation but carried higher risks compared to purchasing an off-the-shelf system or sharing development among international partners. In 1977, the US offered NATO several E-3 Sentry aircraft—already being delivered to USAF—to provide airborne early warning coverage for European NATO members without sole reliance on American assets. This initiative materialized as the NATO E-3A Component, intended for deployment within the UK. Nonetheless, protracted multinational negotiations ultimately prompted the UK to pursue full indigenous development.

Development challenges

File:Royal Aircraft Establishment Comet Freer.jpg

A Comet 4 equipped with a nose radome for preliminary aerodynamic flight evaluations

The AEW project's complexity exceeded the capabilities of British industry to manage independently. A significant obstacle in project oversight arose from the designation of British Aerospace (BAe) and GEC Marconi as co-leaders of the programme. This arrangement often resulted in the two firms deflecting responsibility onto each other when technical difficulties emerged, rather than collaborating to find solutions. While BAe successfully met its contractual obligations by delivering aircraft on schedule (with the initial delivery set for 1982 and completion by 1984), GEC struggled to resolve the avionics development challenges.

In 1977, an RAF Comet 4 was adapted with a nose radome for flight testing and underwent a series of trials. The encouraging outcomes led to an order for three prototype Nimrods constructed from surplus MR1 airframes. The first prototype was unveiled in March 1980 and took its maiden flight in July, focusing on evaluating flight performance, while the second was intended to test the Mission Systems Avionics (MSA) suite.

File:British Aerospace Nimrod AEW3, UK - Air Force AN0999455.jpg

Nimrod AEW.3 at RAF Finningley in 1985

Despite ongoing difficulties, the programme proceeded with an order for eight production aircraft, also derived from spare MR1 airframes. The first production model flew in March 1982. Concurrently with efforts to overcome technical setbacks, the aircraft were delivered to the RAF's No 8 Squadron in 1984 to initiate crew training. However, the unresolved technical issues prevented deployment of the Nimrod AEW during the Falklands War. To partially fill this capability gap, several Nimrod MR.2 aircraft were rapidly converted for airborne surveillance duties supporting the task force.

Aircraft

Selecting the Nimrod platform ultimately proved unsuitable due to its insufficient size to house the extensive radar, electronic systems, power generation, and cooling apparatus required for such a sophisticated system. Measuring just over 38.5 m (126 ft), the Nimrod was nearly 8 m (26 ft) shorter than the Boeing 707 airframe used as the foundation for the E-3 Sentry. Despite having approximately half the maximum takeoff weight of its American counterpart, it was expected to support comparable crew numbers and equipment necessary for equivalent operational roles. The Nimrod accommodated six operator consoles—four dedicated to radar functions, one for electronic support measures (ESM), and one for communications—fewer than the nine stations installed on the E-3A. The larger size of the Sentry allowed flexibility to increase operator capacity.

The Sentry’s rotodome-mounted radar benefited from direct airflow cooling via integrated cooling doors, while its transmitter employed a separate liquid cooling system; avionics within the fuselage were adequately cooled by a conventional air cycle environmental control system. In contrast, Nimrod utilized a "heat sink" cooling approach that dissipated heat through its fuel system, requiring fuel tanks to remain at least half full to maintain effective cooling when operating at full power.

Avionics

File:Nimrod-XZ286-181.jpg

Synchronizing the radar scanners positioned on the Nimrod's nose and tail presented significant challenges

The core of the MSA was a GEC 4080M computer tasked with handling inputs from two radar scanners, the ESM system, IFF, and inertial navigation equipment. However, combining all these components into a unified system overwhelmed the underpowered computer, which had a maximum data storage of only 2.4 MB. By the time the project was terminated, the mission system’s mean time between failures was approximately two hours, while uploading mission data via tape required about two and a half hours.

The limited operational effectiveness primarily stemmed from the Cossor IFF interrogator, which enhanced the radar’s capabilities: when IFF transponders were active, aircraft tracking was successful. However, if IFF was deactivated, radar tracks would quickly disappear. This resulted in reliable tracking of civilian and ‘friendly’ military aircraft but failed to consistently detect Warsaw Pact aircraft lacking compatible IFF systems—the very targets the project aimed to identify.

The mission system’s electronic racks were grounded to different points on the airframe, causing variations in earth potential that introduced intermittent and erratic track data, further burdening the computer. Additionally, the radar’s sophisticated design posed difficulties: the FASS technique intended to provide complete 360° coverage required the nose scanner to sweep from left to right, immediately handing off signals to the tail scanner sweeping right to left. Achieving synchronization between these two scanners proved problematic, resulting in suboptimal all-around surveillance performance. The radar system also divided incoming raw signals into upper and lower beams, each further separated into in-phase and quadrature-phase channels. Each of these four channels contained identical components (such as spectrum analysers), theoretically allowing interchangeability among them.

However, testing by the Joint Trials Unit (JTU) revealed that each device functioned correctly only when placed in its original channel; relocating it to any of the other three channels prevented proper operation. Consequently, JTU test flights carried numerous spare electronic components so that if a failure occurred, they could attempt different combinations of parts to restore functionality. This approach would have been impractical for operational service. The underlying cause remained unresolved, though JTU suspected that insufficiently tight tolerances in signal transmission timing across channels meant that when processed data from each channel was recombined, correlation was impossible because each channel delivered information at slightly different time intervals.

Cancellation

When the initial Nimrod production models were being delivered to the RAF, the MoD initiated a comprehensive reassessment of the AEW programme. This review led to a competitive tender process starting in 1986 to provide AEW aircraft for the RAF, which included various contenders such as the E-2C Hawkeye, E-3 Sentry, P-3AEW&C Orion, a concept from Airship Industries, and the Nimrod itself. Ultimately, Best and Final Offers were requested from GEC Marconi with their Nimrod proposal and from Boeing offering the Sentry. In December 1986, the MoD selected the Sentry, resulting in the termination of the Nimrod AEW programme. Despite challenges faced by the project, India showed continued interest in acquiring the Nimrod AEW3 even after its cancellation by the UK government.

File:A U.S. Navy E-2C Hawkeye aircraft assigned to Airborne Early Warning Squadron (VAW) 117 flies over the California coast near Ventura, Calif. 121120-N-HX866-006.jpg

File:P-3 AEW US Customs and Border Protection in flight 2009.jpg

File:An overhead view of an E-3A Sentry aircraft of the 963rd Airborne Warning and Control Squadron (963rd AWACS) enroute to Eglin Air Force Base, Fla., during the multi-service Exercise Solid Shield '87 DF-ST-91-10834.jpg

Alternatives to Nimrod considered included the E-2 Hawkeye, P-3 Orion AEW&C, and E-3 Sentry

File:An E-3D Sentry of 23 Squadron RAF Waddington MOD 45140454.jpg

Following its evaluation, Boeing's E-3 Sentry was selected by the MoD over Nimrod as part of the AEW programme review.

The Nimrod initiative had accumulated costs around £1 billion by its cancellation, a stark contrast to manufacturer estimates in 1977 which projected total expenses between £200–300 million. The surplus airframes were ultimately stored and repurposed as spare parts for both the Nimrod R1 and MR2 fleets. Meanwhile, the aging Shackleton aircraft—introduced in 1971 as an interim AEW solution pending Nimrod’s deployment—remained operational until replaced by the Sentry in 1991. The collapse of the Nimrod AEW project significantly influenced Prime Minister Margaret Thatcher's decision to encourage greater competition within the UK defence sector.

After terminating the Nimrod AEW programme, BAe explored potential alternative uses for the now-obsolete airframes. Studies were initiated into converting Nimrods into missile-equipped strike platforms by removing AEW-specific modifications such as FASS scanners and associated fuel and cooling systems located in the weapons bay. The plan envisioned fitting a nose-mounted Searchwater radar, then installed on Nimrod MR.2s, while modifying the weapons bay to carry up to six Sea Eagle anti-ship missiles. Nevertheless, this concept remained at study stage only, with all airframes eventually dismantled throughout the 1990s.

Operators

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• Royal Air Force
  • Nimrod AEW Joint Trials Unit

Aircraft on display

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Cockpit section of XV259 exhibited at Solway Aviation Museum

No fully preserved Nimrod AEW3 aircraft exist today, though three cockpits or fuselage sections remain intact.

• XV259 - Solway Aviation Museum, Carlisle Airport, Cumbria, England – cockpit is the only surviving component
• XV263 - Brough, Yorkshire, England – fuselage utilized as a fatigue testing rig for the Nimrod MRA4 wing
• XZ287 - Stafford camp, Staffordshire, England – fuselage remains as the sole surviving portion

Specifications (Nimrod AEW3)

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Nimrod AEW3 displayed at the Farnborough Airshow, 1980

References

Bibliography

• Bellany, Ian and Tim Huxley, editors. New Conventional Weapons and Western Defence. Routledge, 1987. ISBN 0-7146-3310-0.
• Braybrook, Roy. Battle for the Falklands: Air Forces. London: Osprey Publishing, 1982. ISBN 0-85045-493-X.
• Chin, Warren. British Weapons Acquisition Policy and The Futility of Reform. Ashgate Publishing, 2004. ISBN 0-7546-3121-4.
• Conner, Steve. "Nimrod Reaches to the End of the Runway." New Scientist, vol. 111, no. 1519, 31 July 1986, pp. 33–36.
• Smith, Chris. India's Ad Hoc Arsenal: Direction Or Drift in Defence Policy? Oxford University Press, 1994. ISBN 0-19-829168-X.