BAE Systems Nimrod MRA4

The BAE Systems Nimrod MRA4 was a planned maritime patrol and attack aircraft intended to replace the Hawker Siddeley Nimrod MR2. The rebuilt aircraft would have extended the operating life of the Nimrod fleet by several decades and significantly improved the aircraft by installing more efficient Rolls-Royce BR700 turbofan jet engines to almost double the flight range. The conversion of the flight deck to a digital glass cockpit would have simplified control operations and reduced crew requirements. New detection systems were to be installed, as well as additional weapons for anti-submarine warfare.

However, the project was subject to significant delays due to cost overruns and contract re-negotiations. This was partly due to difficulties combining refurbished Nimrod MR2 fuselages, which had not been built to a common standard, with newly built wings. The numbers of aircraft to be procured fell from twenty-one to nine over a course of years, while costs continued to climb.

The MRA4 was ultimately cancelled in 2010 as a result of the Strategic Defence and Security Review (SDSR), at which point it was £789 million over-budget and over nine years late. No direct replacement was under development at that stage, with the roles intended for the MRA4 filled by existing assets such as the Type 23 Frigate and the Merlin helicopter. However the UK announced its intention to order nine P-8 Poseidon ASW aircraft as part of the 2015 SDSR at a cost of £3 billion; these aircraft were delivered between October 2019 and January 2022 and operate from RAF Lossiemouth in Scotland.

In 1990, the UK abandoned options for avionics, sensors and tactical system upgrades of the MR2. In 1993, the RAF issued a request for information for a Replacement Maritime Patrol Aircraft (RMPA) procurement programme, also known as Staff Requirement (Air) 420, which was intended to replace their fleet of Nimrod MR2 aircraft. In response to the requirement, several different companies soon submitted their bids to meet the outlined demands. British Aerospace (BAe), which had studied prospective options to replace the Nimrod MR2 since 1986, including a variant of the Airbus A310 and other civil conversion projects, ultimately settled on a proposal that involved an extensive rebuild of existing Nimrod MR2s marketed under the name Nimrod 2000. As well as operational performance, the key criteria were cost and a short procurement timescale; these "ruled out a new design or adaption of an existing civil aircraft."

BAe evaluated several different engines to power the Nimrod 2000, such as the Rolls-Royce Tay turbofan, the Pratt & Whitney JT8D-200 turbofan, and the General Electric CF34 turbofan, before settling upon the Rolls-Royce BR710 engine. In 1995, it was claimed that the Nimrod MR2 airframes were of a sufficient condition in that they could readily serve through the intended 25-year service life and that "retained airframe items are low risk". According to BAe, the Nimrod's airframe represented "probably the best understood airframe in the RAF inventory".

Amongst various other bids submitted were Lockheed Corporation, which marketed its P-3 Orion as the Orion 2000, Loral Corporation proposed to rebuild ex-US Navy Orions, and Dassault which sought to develop the improved Atlantique 3. On 12 January 1996, Dassault announced that they had withdrawn their Atlantique 3 following discussions with the MOD; Whitehall sources stated that Dassault had found "...no value in pursuing its tender", and that the RAF had an expressed preference for a four-engine aircraft.

In March 1996, BAe promoted the option of establishing a production line to manufacture new-build Nimrod aircraft as an alternative to the refurbishment of existing airframes, which was reportedly motivated by service concerns regarding attrition rates of the existing aircraft. It was also announced that preliminary talks had been conducted with the South African Air Force on the topic of the Nimrod 2000. In June 1996, BAe announced that it had partnered with McDonnell Douglas to market new-build Nimrod 2000s to the export market; it has been claimed that this move was in response to scepticism from the MoD over the prospects of export sales for the type. BAe argued that the Nimrod 2000's selection would give it entry to the global maritime patrol aircraft market.

As part of Lockheed's submission, the firm offered a formal guarantee of a 20 per cent workshare on all future export sales of its Orion 2000 while simultaneously lobbying for the US Navy to also procure the type. Loral stated that it believed that its bid to refurbish stored P-3s was the most cost-effective submission; however, there was public controversy over whether Loral possessed the necessary technical information on the airframe, claims which Loral refuted. In early 1996, Loral's defense assets were acquired by Lockheed, leaving just the latter and British Aerospace in the competition by mid-1996. In May 1996, it was reported that bidders were considering self-funding their own early development costs in order to compensate for alleged funding shortfalls within the MoD's procurement budget; it was claimed that these funding shortfalls were due to differences between the original specification, which had envisioned as an off-the-shelf procurement without much in terms of development, and the increasingly extensive nature that the programme had progressively taken on.