According to McDonnell Douglas, its Longbow Apache is a lot more than an upgraded attack helicopter.


THE CLAIM BY McDONNELL Douglas that its AH-64D Longbow Apache can be used in an expanded role over its predecessors is lent weight by the "add-ons" which allow it to be anything from a beyond-visual range tactical fighter to a mini-AWACS (airborne warning and control system).

Whatever the claims, there is no doubt that the Longbow Apache is a far cry from the original AH-64A attack helicopter first fielded with the US Army in 1984. In little more than a decade, the helicopter has been heavily modified with advanced weapons, communications and sensors, to support the more demanding needs of the digital battlefield.

Whether by good planning, serendipity, or a mixture of both, the fortunes of the AH-64D are also benefiting from the cost-driven changes in force structures which are taking place throughout the world, particularly in the West. "It's essentially the need to do more with less, and the move to be more flexible and mobile," says Terry Higginbotham, McDonnell Douglas Helicopter System (MDHS) manager of business development for international markets.

The combination of two factors - MDHS's development of a digitally capable multi-role attack helicopter and the affect of the post-Cold War fall-out on procurement - has paid dividends for the company in 1995. Earlier in the year it achieved hard-won export orders for a total of 97 AH-64Ds from the Netherlands and the UK, and in October received the go-ahead from the US Army to begin full-scale remanufacture of A models into Ds.

This work begins early in 1996. Under it, the first of up to 750 US Army AH-64As will be delivered as Longbow Apaches in 1997. Some 227 will be fitted with radars, while the balance (formerly designated AH-64C) will be remanufactured to the new standard to accept the radar, if required.

The evolution of the AH-64D can be traced to the mid-1980s, when the first A models were entering service. MDC began studying several upgrades and, in 1987, announced the AH-64B/G Advanced Apache, which combined improvements for the US Army with a proposed version that would fit the (then) West German anti-tank helicopter requirement.



The Advanced Apache did not materialise, but its legacy of upgrades has survived in some cases, either in original or modified form. The proposed Advanced Apache had a cassette mission-data loading system; a ring laser, gyro-based inertial- navigation system (INS); an automatic target handover system; more powerful, 1,490kW (2,000shp) engines; an air-to-air gun; rearward-looking TV camera; and air-to-air Stinger missiles.

The core of the current Longbow version, however, began to crystallise in 1988 and 1989 when the short-lived multi-stage improvement programme (MSIP) was launched. This wide-ranging upgrade was to include better sensors, fire-control systems and weapons which would improve its all-weather and night-fighting capability. It also specified a digital databus and modified crew displays to handle the increased amount of tactical information. The US Army also set up a competition to supply the radar, out of which emerged the Longbow.

"It all finally got blended out as the Longbow Apache," says Higginbotham. Three major features formed the foundation for the improvement: the mast-mounted Longbow radar; an expanded array of weaponry including up to 16 fire-and-forget anti-tank missiles; and a real-time data modem to transmit digitised battlefield information to other users.

The full-scale Apache Longbow programme was authorised in December 1990 by the US Defense Acquisition Board, which set a 51-month timescale for the effort. The airframe portion of this, however, was later postponed by 19 months to allow the helicopter systems to be developed concurrently with the Lockheed Martin radio frequency (RF) Hellfire II, a new fire-and-forget anti-tank missile.

Although an AH-64A was flown with a dummy mast-mounted radar in March 1991, the first AH-64D prototype did not fly until just over one year later. Also in 1992, a planned near-term upgrade of more than 250 AH-64As to a proposed "B" standard was dropped. This was to have incorporated some "lessons learned" from the 1991 Gulf War and included secure communications, (SINCGARS), target handover capability, and global positioning systems. These features were also rolled into the Longbow.

By 1993, four AH-64D prototypes were being flown, and in May of that year the first radar-frequency (RF) Hellfire II missile was launched. In December, another crucial element of the programme was achieved when the first air-to-ground data communications wereachieved with the improved data modem (IDM) made by Symetrics Industries. By 1994 six prototypes (two without radar) were under test and the first Army pilots were being trained as Longbow Apache instructors.

In October 1994 the build-up to production go-ahead continued when three Longbow Apache prototypes underwent a Force Development Test and Experimentation exercise. This was a forerunner of the last, and most vital, test which took place from January to March this year. All six prototypes took part in the Initial Operational Test and Evaluation (IOT&E) which forms the final "acid test" for any new major US military weapon system before full-scale production can be authorised.

Unusually heavy rains in January and February swept over the test area at China Lake, California, which caused problems, even for the tracked ground "targets" attempting to escape the Apaches. Despite the problems, which forced the testing schedule to slip and the funding to be spent faster than expected, the I&OTE was deemed an outstanding success and finished several weeks early.

Initial results show that in 14 out of 16 test scenarios, the survivability of the Longbow Apache was increased by six times over the A models. Some 30 AH-64As were "destroyed" during the tests compared with just five AH-64Ds. Additionally, the tests also showed that the fire-control-radar (FCR)-equipped helicopters destroyed more than four-and-a-half times the number of enemy targets (including tanks, armoured personnel carriers and air- defence targets) than did the AH-64As . The D version was said to be "...many, many times more lethal" than the A version, according to the US Army Apache programme manager, Col Robert Atwel.

"The essence of the A model is its multi-role potential which is derived from its payload. The essence of the D model is to transport and extend that A-model mission flexibility to the digital battlefield. The A model did things that no attack helicopter had done before to the battlefield. The D will do the same thing to the digital battlefield," says Higginbotham.


Sensors and weapons

The key to the resurrection of the Apache, and the most obvious visible change from the AH-64A model, is the Longbow FCR, mounted prominently on top of the rotor mast. The Longbow is produced by a Lockheed Martin/ Westinghouse joint venture and automatically detects, locates, classifies, and prioritises air and ground targets.

The millimetre-wave radar acts as the eyes and ears of the crew, while the body of the helicopter hovers out of sight behind terrain, thus making it "the US Army's first BVR tactical fighter", says Higginbotham. Complementing the FCR in line-of-sight operations is the Lockheed Martin TADS/PNVS (target-acquisition designation sight/pilot night-vision sensor), which is standard on all Apaches. Up to 256 targets can be shown on the radar's tactical-situation display after each 360¡ sweep. Airborne targets can be detected during sweeps in air-to-ground mode, though the FCR has an air-to-air mode for flying targets only.

Once a target is detected, processors in the Longbow system determine its precise location, speed and vector. Targets are classified as being tracked or wheeled, air defence, helicopter, or fixed-wing, before being prioritised. The most dangerous or priority targets in the array are displayed and indicated to the crew as the system automatically transmits the data via the IDM to other similarly IDM-equipped Apaches or users.

The system aboard the FCR-equipped helicopter also divides the battle area up and assigns priority fire zones to the best-positioned AH-64Ds. It then transmits the fire zones to the attack team and initiates the battle by launching its first set of RF Hellfire missiles. The IOT&E proved that the entire sequence, from unmasking the radar for a single data collection sweep to firing the missiles, could be achieved in less than 30s. Some early development problems were caused when clutter confused the radar and prevented proper classification of stationary targets, but these have since been resolved.

Target information is sent via the IDM in a digital data burst at 16,000bits/s. The modem also receives data via the aircraft's secure VHF/FM radio. It is planned to eventually transmit real-time video imagery over the IDM which also connects with ground-based command posts and the E-8 Joint STARS airborne command post.


Hellfire II guiding hand

The Longbow acts as the guiding hand to the Hellfire II anti-tank missile, the next major feature of the AH-64D. Unlike the laser-guided version of the Lockheed Martin-produced Hellfire II (or AGM-114K), the radio frequency (RF) version is an autonomous fire-and-forget missile. The seeker in each 1.6m long RF Hellfire II is fitted with the same TRW-made Ka-band power amplifier modules used in the radar. The missile can lock-on before launch, or can be launched on radar-supplied coordinates and then lock-on in flight.

In place of the Hellfires, the helicopter can also carry up to 76 70mm (2.75in) folding-fin aerial rockets, or a combination of both. The A model's MDC M230 Chain Gun 30 mm automatic cannon is also retained in its original position between the mainwheel undercarriage legs. Provision is being made for two extra hardpoints at the tips of the wings which will carry four Stinger, four Matra Mistral, Short Starstreak or two AIM-9 Sidewinder air-to-air missiles. The wingtip port and starboard position lights have been moved to the sides of the fuselage to make room for the change.

Survivability is improved with the addition of a radar-frequency interferometer (RFI), located beneath the Longbow antenna on the rotor shaft. The Loral (formerly IBM) Federal Systems APR-48 airborne electronic support measures system provides warning of hostile radar energy and compares the emitter characteristics with data stored in its threat library to classify and prioritise the threat.


Digital differences

Despite the externally obvious radar and Hellfire II missiles, the majority of new features that distinguish the D from the A are hidden beneath the Apache's skin. To cope with the expanded digital-data requirements, MDC completely redesigned the systems architecture and used the MIL STD-1553B databus for the helicopter's central nervous system.

The AH-64D has redundant 1750A processors for weapons and systems and has two processors for the new multi-function displays (MFDs) in the front and rear cockpits. The revised architecture produced a fully integrated system which means that some dedicated parts of the AH-64A architecture, such as the fire- control computer, could be taken out as their roles are now taken over by the weapons-systems processors.

To help the crew's situational awareness of the digital battlefield, and the new wealth of data supplied by the radar and IDM, large new displays are fitted in both cockpits. The AlliedSignal Aerospace colour active-matrix liquid-crystal displays measure 150 x 150mm, and replace earlier monochrome cathode- ray-tube displays used in the prototypes. The colour display processors which drive the MFDs are capable of being modified to accommodate a future digital-map upgrade. In all, revising the cockpit layout has reduced the number of switches from 1,200 in the A, to around 200 in the D.

Hamilton Standard was able to take advantage of the digitised Apache architecture by developing a lightweight flight-management computer. The 32-bit processor-based system provides integrated stability augmentation, air-data processing and stability control. The system works by adjusting the tailplane incidence to streamline it with the downwash from the main rotor during hover. It also adjusts automatically to hold the best fuselage attitude during all phases of flight.

Air data are supplied by an omni-directional sensor on the rotor which replaces the distinctive spike mounted atop the rotor hub on the A model. The revised flight-management system is contained in one box, rather than four on the A, saving around 12kg in weight.

The production Longbow Apache will also be equipped with the US forces tri-service embedded global-positioning and inertial-navigation system (EGI). Two of the prototypes being operated from Fort Hunter Liggett, California, as part of the IOT&E were fitted with the EGI, which is mounted on the nose directly ahead of the front cockpit.

The larger volume of avionics on the Longbow Apache is accommodated in enlarged panniers along both sides of the fuselage. The enhanced forward avionics bays (EFABs) will be made from composites for production aircraft to ease manufacturing and cut weight, although they were made of aluminium on the prototypes. The bays are extended aft under the stub wings as well as outward and upward. The starboard EFAB is cut down to ease crew access to both cockpits.

The forward part of the EFAB still contains the same basic TADS/PNVS avionics suite as used in the original A version, but it is extended to contain the newer avionics and the compressor which provides air conditioning. A vapour-cycle cooling system is used to dissipate heat from the avionics in the EFABs. The environmental control system uses an environmentally friendly form of freon known as R134A.

Other than by local strengthening, the basic structure has been little changed from the original Hughes Model 77 design. The Apache airframe is structurally capable of up to 9,990kg, which is the design weight in ferry configuration, so the typical maximum mission gross weight of around 7,860kg (escort mission at 2,000ft, 21¡C temperature, 38 rockets and 1,200 rounds of ammunition) gives plenty of margin.

Thanks to MDC's traditional use of the static mast concept, no strengthening was required to support the additional 136kg weight of the Longbow radar and its associated avionics boxes. Torque loads pass from the rotor through the transmission, but all lift and other rotor loads, including the weight of the radar, are transferred down to the keel beams via rigid frames.

The radar itself is mounted on a baseplate which sits on top of a de-rotation unit. Wire connections from the radar, RFI and other sensors in the rotor unit, are collected into a harness which passes through a torque tube attached to the static mast.

The larger -701C version of the General Electric T700 turboshaft is used as the standard powerplant for the heavier AH-64D and has been built into all A versions from the 604th aircraft onwards. The -701C is rated at around 1,410kW maximum continuous, compared with 1,265kW for the basic engine. The -701C can generate 1,447kW in one-engine inoperative emergencies. The engines also drive a larger 70kVA peak generators to cope with the Longbow Apache's higher electrical requirements.


The Future

MDC sees further sales opportunities for "400-500 attack helicopters out there", according to Higginbotham. The total remaining market is estimated to be up to 1,600 helicopters in this category, of which half are in the USA. To date, a total of 1,034 Apaches have been ordered, of which 821 are for the US Army. Of the grand total, 937 are AH-64As with the balance made up of the new Longbow version.

Total international AH-64A sales have reached 116, of which the largest single user is Israel with 42 (18 ordered, plus 24 ex-US), Egypt with 36, followed by the United Arab Emirates with 30, Greece 20 and Saudi Arabia with 12. Future sales prospects include South Korea, Malaysia and Singapore in Asia, Kuwait in the Middle East and Sweden and Spain in Europe.

Both A and Ds are on offer, though the more expensive D is an improvement in terms of "...survivability, capability and reliability" even without the radar. "The Dutch believe that, because they've bought 30 of them," says Higginbotham. MDC sees no "technical reason" why foreign sales of the radar-equipped D version should be restricted, an issue that has reared in the case of Malaysia. "The radar issue should be examined in the context of the other available radar technologies," he says.

"In the 1980s we got used to hearing the 'three twos': too expensive, too capable, too large. Recent events have shown that's wrong. We used to talk about the D being an evolutionary step in the Apache's life and it's not. It's now being described by the US Army as a revolutionary step, and they're right," he concludes.

Source: Flight International