The BAE SYSTEMS Nimrod maritime patrol aircraft is derived from the De Havilland Comet, the world's first jet airliner. It is unique in being the only dedicated land-based, jet-powered maritime patrol aircraft in service today. (Other jet-powered maritime patrol aircraft such as the S-3 Viking are primarily deployed from aircraft carriers, or simply civil types with added electronics.) The era of turboprop maritime patrol aircraft may be drawing to a close, as the US Navy's P-3 Orion fleet is replaced by the 737 MMA over the next decade.

The Nimrod serves the RAF in two variants: the R1 variant in a reconnaissance and electronic intelligence gathering capacity (officially, these were once coyly described as "radar calibration aircraft"), and the MR2 variant in the maritime reconnaissance role. The R1 is recognisable by the fact it doesn't have a MAD boom like the MR2.

MR1

Nimrod development began in 1964 as a project to replace the elderly Avro Shackleton. Like many other successful maritime patrol aircraft, it was based on a civil airliner which had reached the end of its market life - in this case, the Comet 4. The first two RAF aircraft were unfinished Comet 4 airliners. The Comet's turbojet engines were replaced with Rolls-Royce Spey turbofans (for better fuel efficiency, particularly at the low altitudes required for maritime patrol), and major changes made to the fuselage, including an internal weapons bay, an extended nose for radar, a new tail with ESM sensors mounted in a bulky fairing, and a MAD boom. After a first flight in May 1967 the RAF ordered 46 Nimrod MR1s, and the first example entered service in October 1969. Five squadrons were eventually equipped with the MR1.

R1

Three Nimrod aircraft were adapted to the SIGINT role, replacing the Comet C2s and Canberras of No. 51 Squadron in May 1974. Only since the end of the Cold War has the role of the aircraft been officially acknowledged. The R1s have not suffered the same rate of fatigue and corrosion of the MR2s and will continue in service long after the MR2 is replaced by the MRA4.

The Nimrod R1 is based at RAF Waddington in Lincolnshire, England and flown by 51 Sqn.

MR2

Starting in 1975, 32 aircraft were upgraded to MR2 standard, involving modernisation of the electronic suite and (as the MR2P) provision for inflight refueling and additional ESM pods on the wingtips. The inflight refueling capability was introduced during the Falklands War, as well as hardpoints to allow the Nimrod to carry the AIM-9 Sidewinder missile. Eventually all MR2s gained refueling probes and the "P" designation was dropped.

The Nimrod MR2 is based at RAF Kinloss in Scotland and flown by 201, 120 and 42(R) Sqns.

AEW3

In the mid-1970s, the Nimrod's duties were expanded to include AEW - again, as a replacement for the Lancaster-derived, piston-engined Shackleton which, astonishingly, was still in service in that role. The aircraft were modified by BAe at the former Avro plant at Woodford, Manchester to house the GEC Marconi radars in a bulbous nose and tail (see picture). From the start of the first flight trials in 1982 the Nimrod AEW3 project was plagued by cost over-runs and electronic difficulties. Eventually, the UK MOD realised 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 Sentries (AWACS) instead, with proven radar performance and electronic enhancements to the original USAF systems to address UK-specific requirements. Of the eleven RAF aircraft that were selected for conversion to AEW3 standard, none returned to the maritime reconnaissiance role: all were eventually reduced for spares to support the maritime Nimrod fleet.

MRA4

Towards the end of the 20th century, BAe began planning to build a Nimrod replacement. The RAF considered bids from Lockheed with its P-3 Orion, Loral Corp. with rebuilt ex-US Navy Orions, and Dassault with the Atlantique 3 , but in July 1997 awarded the contract to BAe with the substantially rebuilt Nimrod MRA4. The MRA4 is essentially a new aircraft, with current-generation BMW Rolls-Royce BR710 turbofan engines, a new, larger wing, and fully refurbished fuselage. The rebuilt aircraft borrow heavily from Airbus technology, the wings are designed and manufactured by BAE Systems (the UK Airbus partner) and the glass cockpit is derived from that of the Airbus A340.

Development has taken longer than anticipated but the first of 18 MRA4s are expected to enter service shortly. The contract was initially for the supply of 21 rebuilt Nimrods but due to technical problems the project was brought to a halt. Following public recriminations between the Ministry of Defence and BAE the contract was renegotiated, for the revised number of 18 aircraft. Officially this is put down to the fact that increased capability and availability of the MRA4 will require fewer aircraft, but it has been suggested that this is in effect compensation to BAE, who had to absorb the cost increases of the project. Announcing plans for the future of the British military on July 21 2004 Defence Secretary Geoff Hoon detailed plans to reduce the upgrade programme to cover only 16 aircraft and suggested that an eventual fleet of 12 may suffice.

For the RAF, the jet-powered Nimrod offers greater speed and range than any turboprop, and enhanced ability to detect a threat without itself being detected because of the reduced noise and lower radar signature of the Nimrod's buried engines. New Bombardier Sentinel R1 (ASTOR) aircraft due for delivery from mid 2004 may take on some duties performed by the R1 variant described above.

BAE SYSTEMS offered a new build version of the Nimrod for the US Navy's Multimission Maritime Aircraft (MMA) competition. The competition was launched in 2000; BAE withdrew in October 2002, recognising the political reality that the failure to find a US-based production partner made the bid unrealistic. Boeing's 737 won the competition on June 14, 2004.

The first flight of a Nimrod MRA4 (the aircraft designated PA1) occurred on Thursday 26 August 2004.

Units Using the Nimrod

Royal Air Force

• No. 42(R) Squadron - Nimrod OCU
• No. 120 Squadron
• No. 201 Squadron
• No. 206 Squadron