STEWART PENNEY / WOODFORD & WARTON
BAE has spent the past six years updating the Nimrod maritime patrol aircraft to modern standards. But how much of the original airframe remains?
Later this year, BAE Systems will fly the first Nimrod MRA4 at the start of an aggressive flight-test programme which will lead to the handover of the first aircraft to the UK Royal Air Force in August 2004.
The £2.98 billion ($4.6 billion) MRA4 programme, however, has not always run smoothly since it won the Replacement Maritime Patrol Aircraft competition in 1996. BAE's proposal combined a new mission system with rebuilt Nimrod MR2 fuselages, new wings and engines.
The only MR2 components retained are the fuselage pressure shell and empennage. The rest of the structure is new. To accommodate the high-bypass Rolls-Royce BR710 turbofans, which replace low-bypass R-R Speys, the inboard wing had to be redesigned and, as a contract option, the RAF chose to have entire new wings rather than keep the outboard sections.
The rebuild programme did not run smoothly and BAE took back the rebuild work from subcontractors, moving it to the Woodford factory near Manchester. As a result the programme slipped by two years and BAE has been forced to pay the UK government £46 million in damages.
As a further setback, earlier this year the RAF cut its order from 21 to 18, citing the reduced submarine threat and the Nimrod MRA4's improved capability and mission availability as reasons for the move.
Rig lessons
Tom Nicholson, BAE managing director Nimrod, says the RAF was able to reduce the number of aircraft following testing in the MRA4 weapon system integration rig (WSIR) at Warton, north-west England, which is now running flight-standard software ahead of flight testing. Much of the testing has been done by a joint trials team (JTT) comprising RAF specialists with Nimrod experience working with industry specialists, says Nicholson.
The JTT is using the WSIR "to fine-tune the system. It has slowly improved confidence and the maturity of the platform." This is what led to the cut in aircraft numbers. "It's really cutting the through-life support costs," he adds.
Despite the cut in numbers, additional roles for the aircraft have begun to emerge during programme reviews. The MRA4 has been linked with strategic bombing and battle management/communications relay tasks. Its defensive aids subsystem (DASS), comprehensive communications suite and 1760 weapons databus mean the aircraft "could certainly change roles", says Nicholson. Nimrod could be a key platform for land and sea missions within network-centric warfare, he says, adding that, with standard guided weapons, it could be used against "any target on the planet's surface, not just in maritime, not just underwater".
BAE has four Nimrods in final assembly at Woodford. The first three will be used in the test programme, says Nicholson. He says the first flight is due at the end of September, but the critical path indicates late October or early November as a more likely date. Another seven aircraft will have been produced by the March 2005 entry-into-service.
Next year, BAE will be flying prototypes with the full-capability mission system. "I'm pretty sure we'll have 100% functionality. In the following two years we have to prove all the capabilities - the radar, sonobuoy drops, everything," he says.
Complex systems
Because of the complexity of the mission systems and of completing the associated proving process, BAE will begin handing over Nimrods in August 2004 to an initial operational capability (IOC) standard with a "declared capability, but without the supporting paperwork". By March the following year, the scheduled entry-into-service date, the aim is to give the aircraft full operational capability, with full military certification.
Nicholson says the development task is "integration, not interface, as it involves data fusion". The plan is to provide the mission systems operator with a display of "information, not data. It can tell the crew of threats and how to deal with them, but the crew can override the system. It is passing information all the time, there are nigh-on 800 computers on board."
Paul Whiteside, Nimrod mission avionics engineering manager, says the MRA4 specification covers the range of anti-submarine and anti-surface warfare tasks as well as search and rescue missions, which drives the requirements for the sensor suite and what is carried on the aircraft. For future needs, the specification includes 50% processing, memory and storage growth.
The mission system and flightdeck avionics are integrated and provide information for the two flightdeck crew and eight mission crew. The latter consists of two taccos (tactical co-ordinators), two acoustics operators, a communications manager and two dry operators - one for radar, one for electronic support measures (ESM) and a crew member for additional tasks such as launching sonobuoys. The MRA4 has three fewer crew compared to the MR2.
Lead suppliers have delivered qualified subsystems. "The development is done in the supplier base," says Whiteside, adding that "although we've done some development work", BAE has received equipment and integrated it on to rigs, rather than using the rigs as subsystems development tools. This has significantly reduced the time to the planned first flight and is expected to shorten the period between the maiden flight and IOC, he adds.
Cockpit design
The removal of the flight engineer and en route navigator has driven the cockpit design, says Whiteside. The cockpit uses "a lot of Airbus technology", as well as being a low maintenance design, with high mean time between failures equipment. Other necessities were an acceptable crew workload and the ability to meet military safety requirements, such as low-level flight over the sea. "Automation is used to reduce workload without jeopardising safety," Whiteside adds.
Thales Avionics has supplied the system, which is based on the Airbus A330/A340 cockpit, albeit with seven rather than six large displays. Whiteside says: "Key is the multifunction liquid crystal displays driven by highly integrated subsystems." The system includes the display management computer, which supplies the flight instrumentation data, and the warnings computer. The avionics and mission system are connected by a combination of military 1553 databuses, civil Arinc 429 links and some discrete interfaces, adds Whiteside.
Smiths Aerospace has supplied the flight management system, based on its unit for the Boeing 737, but modified for vertical tactical navigation (superimposing tactical operations on the simple climb/descent vertical flight of an airliner).
Other equipment has come from Honeywell and Northrop Grumman (dissimilar embedded global positioning/inertial navigation [EGI] systems, Ultra Electronics (yokes), Thales Sensors (multimode receiver, triple air data system and radar altimeter) and Avionics Specialities (ground proximity warning system).
Key change
The change to dissimilar EGIs for safety was a "real change, driven through analysis", says Whiteside. The EGIs are coupled to the automatic flight control system for routine navigation and support tactical navigation. "We started key systems [in the rig] early," he says. Hardware integration issues and suppliers' differences of understanding of the specification were overcome with a pre-integration rig built at Smiths in the USA and equipped with the Thales displays. "The suppliers were working together," says Whiteside. "We sorted out a lot of hardwiring problems early on."
Katherine Wykes, BAE's Nimrod human-machine interface (HMI) manager, says that, to reduce the risk of the two-crew cockpit, the company performed a series of workload and interoperability tests using a flightdeck assessment rig (FDAR), which built on a comprehensive analysis using rapid prototyping to develop flightdeck layouts. "There are two main issues in the HMI design, the workload and the integration of the off-the-shelf equipment," she says.
One outcome has been the change to seven displays, says Wykes. "You couldn't use six displays with a two-man crew. We need to present the tactical information. The seventh screen is for the tactical display." BAE also performed workload assessments using 10 pilots to fly 2-3h missions in the FDAR, which allowed the engineers to pinpoint when workload became too intense for the crew to perform the mission adequately, which in turn has led to changes in the HMI.
The MRA4 combines a Boeing-developed tactical command system (TCS), Telephonics communications, CAE magnetic anomaly detector (MAD), BAE North America DASS, Elta ESM, Thales Sensors radar, Northrop Grumman electro-optical surveillance and detection system (EOSDS), Smiths armament control system (ACS), Ultra acoustics systems, data recorders and mission support system, says Whiteside. Apart from the MAD, all the mission systems are new. The MAD was fitted to the MR2, but has been modified with a 1553 databus interface.
The TCS, which interfaces the sensors with other mission systems, is the centre of mission management and where sensor data is correlated. The system is based around the seven reconfigurable workstations used by the mission crew, plus the pilots' tactical display. An eighth workstation can be added as role equipment. The eighth rear crew member on a typical mission does not sit at a console.
The open-architecture system has Unix operating software using off-the-shelf hardware. It is "90% signed off by Boeing". Only capabilities not required for flight testing remain to be cleared, says Whiteside. Six dual-redundant 1553 databuses, fast ethernet and a 1760 weapons databus link the elements together, he adds. Although Boeing performed a significant amount of testing, full integration can only be evaluated on the WSIR, says Whiteside.
Display change
To save weight and improve support, the workstation displays were changed for flat panel LCDs after contract award, says Whiteside. Not all integration was smooth, he acknowledges, citing the linking of the FMS and TCS as one difficulty, although he says all significant software problems have been cleared.
The Thales Sensors Searchwater 2000 mechanical-scan radar is optimised for detecting small objects, such as periscopes, in the open sea or coastal waters, says Whiteside. It also has a pulse-Doppler mode for air-to-air tasks, a weather mode, swath and spot synthetic aperture radar (SAR) modes for imaging, inverse SAR for target profiling as well as search-and-rescue and Royal Navy I-band transponder modes. The radar "gives some fundamental growth potential for overland roles", says Whiteside.
The Ultra acoustics system is based around a CDC system developed from earlier equipment, says Whiteside. It can handle up to 64 sonobuoys, including the full range of analogue and digital, passive and active systems. It is also possible to select which sonobuoys are monitored, so more than 64 can be put into the water. Two 32-channel receivers are fed with signals from antennas on the weapons-bay doors.
The EOSDS is carried in a retractable turret under the forward fuselage. It combines medium-wave and long-wave infrared (IR) sensors with a visible/near IR television sensor, says Whiteside. All three sensors can be used at once, with the images from any two being fused into a single picture.
Increased emphasis on littoral operations has a significant affect on the DASS, says Whiteside, as such operations increase exposure to radio-frequency (RF) and IR threats. "The increased RF pulse density from air, land and sea drives the need for an integrated system," he says. The DASS combines a BAE ALR-56M(V) radar warning receiver (RWR), AAR-57 missile approach warner, a Raytheon ALE-50 towed radar decoy and Thales Vinten Vicon 78 chaff and flares dispensers. The system also includes a Thales Sensors techniques generator.
The company says DASS growth potential will allow the integration of directional IR countermeasures, a laser warner and integrated active electronic countermeasures. Although the Elta EL/L-8300 ESM is dedicated to the mission objectives, rather than feeding RF data to the DASS, it also provides situation awareness to the crew, says Whiteside.
The communications system comprises five V/UHF radios and two HF radios, a teletype modem, Link 11 and Link 16 tactical datalinks and SHF satellite communications. The datalinks can be used independently or simultaneously through a concurrent interface unit, says Whiteside, who adds that BAE is working with the UK Ministry of Defence to consider future datalink requirements because of the rapidly changing communications needs. Link 22 would be a likely addition, he says. The communications system is linked by a fibreoptic intercom network, which includes the two radio management units in the cockpit and each mission crewmember's interface unit.
The SHF satcom will allow secure, anti-jam beyond line-of-sight communications between a maximum of seven aircraft and the maritime air operations centre at Northwood in west London. The antenna will be in the fin-top housing, which will require the system to counter structural flexing. "But we think we have mitigated the risk," says Whiteside
The Smiths ACS is a dual redundant system based on that for the Boeing F/A-18 "with significant changes", says Whiteside. The ACS controls the release of stores from the 12m (40ft)-long weapons bay and four underwing hardpoints. The aircraft will carry a range of maritime stores such as torpedoes, mines and Boeing Harpoon anti-ship missiles, but the 1760 databus will allow integration with all modern Western weapons.
It can also carry air-to-air, anti-radiation and air-to-ground missiles and search-and-rescue equipment. "There is no store lock out and anything can be dropped anytime. There are no centre of gravity issues," says Whiteside. The ACS also manages sonobuoy release - the MRA4 has four rotary units that can only be used when the fuselage is unpressurised, and two rotary launchers that can be used when the aircraft is pressurised. All are located in the rear cabin.
Systems testing
Mission systems testing takes place in the WSIR, which consists of two rigs that can each be divided in two, says WSIR manager Mark Smith. One of the rigs will be delivered to the RAF for in-service software maintenance. WSIR includes rear crew workstations and the flightdeck, and can be used for full mission system testing. Inputs to the TCS are via real (apart from the EOSDS) or simulated equipment. Real sensors can be used by stimulating the receivers. "We just take away the aerial," says Smith.
WSIR trials include workload assessments, he says. Around 75% of the systems capability will be proved by October, which "derisks" much of the system before flight testing, says Smith.
Tom McMichael, engineering manager-airframe, says the MRA4 aerodynamic development policy was "from the outset not to look for specific improvements, but to capitalise on our knowledge [of the Nimrod]". He adds that "with the exception" of the 3.7m inboard section containing the buried engine installation, the wing "is the same de Havilland sections" as used in earlier Nimrods and the Comet airliner, from which it was developed. From outboard rib 7, where the manufacturing break is, the wing aerodynamics are the same, says McMichael.
The biggest aerodynamic changes are to the inboard wing to accommodate the four 15,500lb-thrust (69kN) BR710s. While these engines consume 30% less fuel than the MR2's Speys and produce 25% more thrust, the digitally controlled BR710 has a diameter almost 50% greater. The sweep of the earlier intake design has been removed and the intakes have been moved forward of the wing, with their structure cantilevered from leading edge, says McMichael.
To take the greater air flows required, the front-spar has been deepened and a "clever diffuser design" within the intake has improved the pressure recovery without the air separating from the intake walls. A small flap on the lower side of the intakes has been removed, with the area of the two-piece main flaps increased to compensate, says McMichael.
The ailerons are unchanged, but the actuators have been moved from the fuselage to be adjacent to the control surface, which removes control system slackness. The new wingtip pods are also shorter than the MR2 variety, but are also equipped with a rugby-ball shaped fairing mounted on a short pylon. Although these were not designed to improve the wingtip aerodynamics, a computational fluid dynamics (CFD) study shows the pylons do have winglet-like characteristics, says McMichael.
Changes to the wing structure include integrally machined stringers and machined spars, says Tom Cook, chief structural engineer. Because the engines will be removed upwards rather than rearwards, as on the MR2, the inboard skins have doors in them. The wing-fuselage structural join is carried over from the MR2 to reduce requalification of the structure, but is strengthened and the MRA4 will be cleared for 3g manoeuvring, which the MR2 is not, says Cook.
Corrosion protection
Wing materials have been selected to give better corrosion protection, and new paint and anodising will also improve the protection, adds Cook.
Changes to the fuselage include the addition of a ram-air turbine (RAT) in a teardrop fairing ahead of the port wing leading edge. On the underside, weapons bay doorclosure has been implemented as the four acoustic systems' aerials are mounted on the doors. Improved door closure also enhances the environmental controlsystem (ECS) performance, says McMichael.
Because the wing root is deeper, the weapons bay internal aerodynamic flows could have been adversely affected, says McMichael, but windtunnel tests indicated that "the principal characteristics are retrained. The bay is not too deep and shallow cavities are a known quantity". The deeper wing root allows an auxiliary power unit to be mounted in the starboard wing trailing edge.
To counter the yaw effects of the EOSDS turret when deployed, 1.5m composite finlets have been added above and below the horizontal tail stabiliser. These also generally increase directional stability, says McMichael. He adds that the finlets have increased tow-in as well as the additional height compared with the MR2's finlets. Cook says the tailplane skins have been locally strengthened to take the finlet loads.
The MR2's two ECS intakes, mounted on the rear fuselage adjacent to the fin, have been replaced by a single larger intake at the base of the fin, which feeds bigger air conditioning packs. "We've changed the dorsal fin to recover the lost area," says McMichael.
Although the fuselage has been retained, it has been altered, with simple changes such as moving some of the windows, while others reflect the increase in maximum take-off weight from 86,260kg (190,000lb) to 104,420kg. Nicholson says: "It's only about 5% of the components that are retained - the empennage and the fuselage shell." Cook adds that there is little corrosion in the retained structure, "the corrosion was in the wings". He says when the MRA4 design was initiated, areas of known corrosion on the MR2, such as around the direct vision window and the rear entrance door, were redesigned to remove the problems, "so they shouldn't recur".
Cook says the new pressure floor has been put in the aircraft "just to take out the damage" sustained during the first 30 years of the Nimrod's service life.
Other modifications include a change to the radome material, although aerodynamically there is no difference, and a stronger undercarriage to handle the increased weights, while more powerful carbon brakes will impose greater loads on the nose gear attachment. The aircraft's electrical, fuel, hydraulic and oxygen systems are also new.
Source: Flight International
