ALTHOUGH THE SOVIET threat has receded, the need for a submarine hunter remains. Other nations that could pose a threat have built, or are building, formidable weapons of war, including modern submarines and surface vessels. This makes the maritime patrol aircraft (MPA) as valuable as ever and it remains an effective strategic military asset. For years Nimrod has been the only jet-powered, land-based MPA operating anywhere in the world, and it also has the ability to reach an area of operation quickly.

The Ministry of Defence (MoD) Staff Requirement (Air) 420 called for a replacement for Nimrod MR.2 and looked at available options including the EADS Atlantique 3, Lockheed Martin Orion 2000, Loran/Marshall Valkyrie and the BAe Nimrod 2000, now MRA.4. The last had a clear advantage in being jet-powered rather than turboprop, making it quieter on patrol and difficult for a submarine to detect.

Interest in this ability attracted other countries, particularly the US, to the prospect of buying Nimrod but BAe Systems withdrew the design from the US competition for the multi-mission aircraft (MMA) programme. Malcolm Adams, Communications Manager at Woodford said: "We always said we would get involved in MMA only with a US partner and in spite of many approaches to US industry this has not proved possible. Our focus and full concentration is on delivering the full capability of the MRA.4 to the MoD".

The MRA.4 uses MR.2 airframes so a new Nimrod MPA would have to be new-build and putting the aircraft back into production. Although possible, the order book would have to be substantial to make economic sense. Any new- build aircraft for the US would have to be constructed in that country. In July 1996 a £2.5bn contract was awarded to BAe to convert 21 (modified to 18) Nimrod MR.2 airframes to Nimrod 2000 standard. The programme began with four airframes being flown to FRA/Serco at Bournemouth. This did not work out and the airframes were moved to BAe Woodford.

The MRA.4 programme represents one of the world's most complex airborne systems integration tasks and involves over 4,000 customer (MoD) requirements, over six million lines of software code, over 90 antennas and sensors, extensive use of commercial off-the-shelf units for military application, over 1,500 major bought-out items and over 1,000 mission avionic line replacement items. It is a new aircraft; more than 80 per cent is newly manufactured and fitted. The greatly adapted Nimrod fuselage will give a further 25 years' service.

Main external changes are the mainplanes, centre-box, bomb- bay/weapons and undercarriage. New Airbus-designed wings use Airbus technology and build techniques. They have a 23 per cent larger surface area to house four Rolls-Royce Deutschland BR710s in the wing roots that will give 25 per cent more thrust, better reliability and lower fuel consumption. An extra 30 per cent fuel capacity will enable an endurance of 14 hours. Gary Dalton, Build Manager for MRA.4 said, "We have an all-glass cockpit with two crew, not three as with MR.2, that is based on Eurofighter and Airbus. There is also a new navigation and flight management system fitted, and a ground-proximity warning system".

Also being fitted to the airframes are new hydraulic, environmental control, crew oxygen and automated fuel systems, together with a completely new wiring loom, upgraded to take the new systems, generators and computers. The use of Airbus technology offers the MoD reduced cost of ownership and the benefits of civil airliner reliability. The need to sustain operational effectiveness throughout its service life has been built into the programme so that operational capability can be sustained or enhanced in the most cost-effective manner. As Malcolm Adams says, "The weapons system was designed from the outset with 25 years' development in mind. There is provision for three additional work stations to support developed, new or special-fit systems. The aircraft will accommodate mass-growth without structural modification".

Nimrod MR.2 has the largest weapons bay in any current MPA. This has been redesigned for the MRA.4 to make it able to accept future weapons procured to MIL STD 1760 Class II standard. The aircraft has been fitted with wiring and hardpoints for a range of contingency stores that provide the capability for carriage and release of future weapons.

These hardpoints will be able to carry ALARM, ASRAAM, AIM 9, SLAMMER, ARM, Harpoon missiles or Stingray torpedoes. This means the aircraft has 30 per cent increased weight but the BR 710s are more than equal to the task. To carry the extra weight the entire MRA.4 undercarriage is new, stronger with greatly improved braking.

Nimrod MRA.4's fully integrated Mission System is based around the Boeing-developed tactical command system (TCS). Information gathered by the TCS will be on seven work stations giving a powerful, flexible, computer-controlled data distribution and display system. BAe Systems claim no single fault will cause a mission abort. About 5.4 billion lines of computer codes are incorporated into MRA.4. The MR.2 magnetic anomaly detection (MAD) system is retained, albeit modified with an integral digital database to work with the new mission system.

Logistics support

Overall the aircraft requires less than half the maintenance of compatible MRA platforms. It is designed with maintenance personnel in mind, including use of external data panels for checking consumables. The on-board maintenance management systems and the ground support system make rapid diagnostic and isolation of faults possible. MRA.4 also is designed for minimum maintenance; an annual A check and a six-yearly C check are highlighted. Ground operations can be managed via portable, NATO-standard mission and ground- support systems and refuelling, via a high-pressure single point, takes just 30 minutes, greatly reducing turnaround time. It is expected that an MRA.4 can deploy worldwide for ten days without re-supply.

The new technology gives a better availability rate, faster turnaround and so enables a greater number of sorties to be flown. Its role will remain similar to that of Nimrod but the ability of the MR.2 will be greatly enhanced by MRA.4 and BAe Systems say it could be used to assist non-military authorities; anti-drug smuggling, anti-gun running, counter-terrorism and in fisheries protection. After the initial delay progress has been rapid. The four pre-production aircraft are at Woodford where MRA.4 XV247 PA.1 was rolled out on its own undercarriage on August 16, 2002. The other aircraft are on schedule.

At RAF Kinloss modifications are underway to buildings and taxiways to accommodate the MRA.4. It is a major investment by the MoD and to illustrate changes being made to the Nimrod MR.2 new serial numbers (ZJ514 to ZJ534) have been allocated, although only 18 of the serials will be used. The Defence Procurement Agency (DPA) and the RAF visit Woodford regularly to monitor their new Nimrod. When flying begins a BAe Systems flight team will work with pilots from RAF Kinloss to train crews prior to service entry in 2005.