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Light on horizon


When Raytheon's new super-mid sized business jet enters service next year, it will have been eight years in the coming. Has it been worth the wait?

Around a year from now, Raytheon's Hawker Horizon super mid-sized business jet will be poised for service entry. Launched into a virtually untapped market as far back as November 1996, the big question is will the new jet be worth the wait? Raytheon's Horizon team, not surprisingly, believes the answer is an unequivocal yes.

Not only will the handsome Horizon bolster the growing Hawker line-up, but it comes bristling with advanced yet proven features to tackle a potential market niche of around 1,000 aircraft, valued at more than $17.6 billion over the next decade.

Although there are several competitors in the super mid-size market, not least Bombardier's Challenger 300 and Gulfstream's G200, Raytheon remains committed to the project, which has jumped hurdle after hurdle on the path to certification. To Raytheon product development and engineering vice-president David Riemer, the real focus is on preparation for a reliable and robust aircraft at service-entry, rather than a late gallop towards certification - particularly since the market itself seems in no rush to recover.

A veteran of Raytheon's T-6 Texan trainer programme, Riemer brings a quasi-military approach to the Horizon development work. This dovetails well with the rigorous operational requirements developed for the new aircraft, many of them generated by discussions with fractional operators like NetJets which, along with other operators, played a significant role in defining the Horizon. "The fractionals brought a new demand to the business jet market. They want high availability and good dispatch reliability. It is a world that we on the T-6 programme came from, and the fractionals brought it to the commercial market," says Riemer.

Certification, originally targeted for 2001, is now expected around mid-2004. "After we're done with certification, we will operate the aircraft out of our Little Rock [Arkansas] base like a military OT&E [operational test and evaluation). This will work out all the kinks, not only in the aircraft but also in the supplier system. If no improvements are needed we will deliver the first two aircraft around November and December 2004 when the whole support system will be ready," he says.

Fuelling enthusiasm

Riemer believes this approach will boost confidence in the Horizon, and fuel renewed enthusiasm for the project, which has suffered more than its fair share of knocks. Readily acknowledging the setbacks, Riemer puts the slippage down to several causes, not least Raytheon's heavy development commitment, with engineering resources stretched thin on T-6, Premier I and Horizon. "That takes a huge number of resources, and I don't believe the Horizon got as many as it needed in a single year," Riemer says.

The Horizon is also the most sophisticated and fully integrated aircraft yet built by Raytheon, and is therefore no simple undertaking. Its gestation has also been slowed by a host of issues ranging from materials problems on early Pratt & Whitney Canada PW308 engines to extensive aft fuselage system redesigns necessitated by revised rotor-burst certification standards. The tough economic conditions have played their part too, causing NetJets to cancel its order for the Horizon last year.

Although a serious setback to the orderbook, now standing at the mid-30s, the fractional loss could be a good thing in the long run, says Riemer, who sees stronger sales to individual operators over the next few years as one of its consequences. "In some ways it's better this way. If the fractionals had sucked up the first few years of production, then we'd have missed the opportunity to serve the wider commercial market. My point is the fractionals will come, but it's equally important to penetrate the commercial market and increase exposure to the aircraft."

So what product will these first operators be getting? Externally and internally Raytheon appears to have instilled a distinctive Hawker-like feel to the Horizon that, in some cases, can be traced back to the Hawker Siddeley HS125, developed in the 1950s. Ranging from the look of the flightdeck windows and the underslung wing, to the ram's-horn control yoke and crisp control forces, it has produced a rugged aircraft combining Hawker heritage with the technological benefits of a clean-sheet design.

Priced in the $15-$20 million range, the Horizon is sized for US transcontinental missions of 5,735km (3,100nm) or more with six passengers. The result is a comprehensive intercontinental range capability that allows the Horizon with four passengers and 365kg (800lb) of baggage to fly direct from New York to London, Dakar to S‹o Paulo, or Kuala Lumpur to Tokyo.

Based on evidence from recent flight tests, Raytheon believes the aircraft will have a range of almost 5,830km with six passengers flying at Mach 0.82 with a maximum fuel load of 6,500kg. "This is the same range as the Challenger 300 at Mach 0.75 with only four passengers," says Horizon business unit director Dwayne Johnston.


As with all business jet designs, Raytheon's focus is on cabin size and comfort. Capitalising on its hard-won composite experience, the company has again turned to carbon fibre/epoxy honeycomb for the fuselage structure.

Unlike the shorter two-section Premier I fuselage, the larger Horizon fuselage is made up of three sections - a forward, centre and aft shell. The forward andaft shell splices, connecting the centreshell with the rest of the fuselage, are located over metal frames at the wing front- and spar positions.

Titanium attachments connect to the frames at the splice line, while the bonded splice itself is reinforced with fasteners. "We only have metal where we have to have metal fasteners, that is at the wing-fitting attachment points," adds Johnston.

The assembled fuselage is also comprised of three main sections: an unpressurised radome section; a pressurised flight deck and cabin section; and an unpressurised tail section. Just aft of the splice line with the centre shell section, the tail section is distinctly contoured around the engine mounts to provide area ruling for high transonic cruise conditions. Devoid of internal ribs and stiffeners, the internal cross-section of the all-composite constant section is 1.97m (77.5in), while the maximum height from floor level is 1.83m. Total volume is 21.6m3 (760ft3), including the lavatory and a large baggage area at the rear. Standard seating is for eight passengers, with future interior options including a side-mounted divan. Maximum seating is for up to 13, including crews and observers. The baggage area can be accessed during flight at altitudes of up to 40,000ft (12,200m) through a door in the secondary pressure bulkhead. This, like all major fuselage structures, is also composite and comprises the aft lavatory wall.

Field performance

The Horizon's wing, manufactured by team-mate Fuji Heavy Industries, is moderately swept at 28.4° (measured at a quarter chord), and sized to house adequate fuel for the transcontinental range requirement, while providing both high-speed cruise and good field performance capability. Each wing shipset arrivespre-drilled and ready for assembly in two separate parts.

The centreline rib forming the join between the two wing sections divides two independent integral wing tanks. Ribs inside the wing structure form anti-surge compartments for the tanks, which can collectively house up to 8,080 litres (2,135 USgal) or 6,490kg. Pressure refuelling of empty tanks can be achieved in less than 12min from a single-point receptacle in the starboard wing root fairing.

Once assembled into a single structure, the wing is contoured to fit snugly in classic 125 style beneath the fuselage to which it is attached at five points.

These include a horizontal load attachment on the front spar, a spigot-type fitting on the rear spar to transfer drag loads, and two vertical load attachments on each spar. One of these also doubles as a horizontal attachment, completing the compliment of five points.

Arching up to 0.7m at the tip at take-off, the supercritical wing easily meets the guaranteed take-off field length of 1,600m, with around a 105m margin. Raytheon is still working towards its landing distance guarantee of around 715m, and is currently about 30m over. The wing has no leading-edge devices, but has inboard and outboard double-slotted flaps with vanes. Maximum flap angle is 35°, 5° less than planned, after flight tests at flaps 40° showed pitch control issues due to flow interferences with the T-tail.

Stall speeds in still-air conditions and at a weight of 16,800kg are 127kt (235km/h) constant air speed (KCAS) at flaps 0° and 103 KCAS at flaps 35°. A power drive unit (PDU) mounted on the wing's rear spar drives the flaps. Flex drive shafts connect the PDU to conventional ballscrew flap actuators.

Vortex generators

Flight control surfaces consist of ailerons as well as outboard and mid-board roll control spoilers. A set of 12 vortex generators is mounted outboard on each wing to maintain attached flow at high Mach number, and to prevent potential aileron flutter. The ailerons have tabs to lighten control loads and to augment the hydraulically powered, fly-by-wire spoilers.

Each wing has three spoiler panels, the inboard of which deploys on the ground only for braking, while the outboard pair can be used on the ground and for roll control. From aircraft number 20 the flaps, ailerons and spoilers, as well as fixed trailing-edge wing panels, will be made from composites.

Standing almost 6m above ground, the T-tail consists of a fully trimmable horizontal stabiliser and a fly-by-wire, hydraulically operated rudder. The relatively broad chord vertical stabiliser is made up of three aluminium spars with aluminium ribs, encased in a composite skin. The overfin fairing, also composite, is large enough to contain the increasingly popular satellite communications antenna. The horizontal stabilisers are constructed similarly, with the upper and lower skin assemblies made up of one piece root-to-tip composites, while the entire unit pivots on a four-lug fitting. The leading edges are fitted with an electro-magnetic expulsive de-icing system which sheds ice by transmitting vibrational pulses through the structure.

Elevator control, like the primary flight control system, is manually operated through a conventional cable and pulley design. Close to the top of the fin, the cables connect to the aft elevator system quadrant, which enhances pitch control through upsprings. The elevator surface is mass balanced and attached to the stabiliser with three hinges, while geared servo tabs, driven by pushrods, are attached to each elevator.

Overhanging the aft elevator quadrant is the tail pitch trim control system, which is positioned by an electromechanical linear actuator. The stabiliser can travel at two speeds, depending on aircraft speed. At high Mach numbers, a Mach trim function automatically provides programmed stabiliser trim offsets based on Mach number.

Automatic control

The rudder control system includes dual power control units (PCU), electronic control units (ECU), pressure control modules, rudder surface position differential transformers (which provide control surface feedback to the control surfaces), as well as feel and trim units, the latter providing rudder trim. The ECU is also provides yaw damping, and takes signals from the automatic flight control system, superimposing them on the pilot's command and instructing the PCU to eliminate any dutch roll.

Power for the Horizon is provided by two 6,900lb-thrust (30.7kN) thrust Pratt & Whitney Canada PW308A turbofans and a Honeywell 36-150 auxiliary power unit (APU). The PW308A was chosen over the (then) AlliedSignal AS907 because the latter "was totally new, and we wanted to go for a derivative for reasons of guaranteed reliability", says Horizon product manager Steve Mead. Designed for an initial hot section inspection interval of 3,000h and a 6,000h time between overhauls, airframe design life, by comparison, is an estimated 20,000h, says engineering director Paul Jonas.

In common with most new business jet engines, the PW308A has a dual-channel full-authority digital engine control which regulates low-rotor (fan/N1) speed in response to thrust lever angle. The N1 value is determined by an electronic engine control (EEC) with two independent modules, either of which can fully control the engine. The EEC controls thrust management, compressor surge, and the high- and low-pressure compressor rotor overspeed protection. An engine diagnostics unit (EDU) records data for trend monitoring, as well as keeping tabs on exceedances for fault diagnosis. P&WC is responsible for integrating the entire propulsion system, which includes a Nordam-built nacelle and target-type thrust reverser.

The APU is mounted in the aft fuselage, with clamshell doors for easy access, and drives an AC generator to produce three-phase 115/200VAC rated at 15/16.7kVA for air and ground operations. The APU provides main engine starter assist to 26,000ft, and is approved for operations to 35,000ft. On the ground, the APU also provides bleed air for the environmental control system. In the air, a cabin pressure control system (CPCS) pressurises the aircraft from 0-0.7bar (0-9.64lb/in2), achieving a cabin altitude of 6,000ft at a cruising altitude of 45,000ft.

Reduced workload

The CPCS is hands-off to reduce pilot workload, and automatically controls the environment, using data from the flight management systems via the Honeywell modular avionics unit, housed within the Primus Epic system. "It's one of the things you can do with a fully integrated system. It has been a pain to develop, but it's been worth it," says Jones.

While some electrical power is tapped off the APU-driven generator, the primary electrical power generation system consists of two engine-mounted variable frequency AC generators each rated at 25/30kVA. The main source of 28V DC power is a pair of transformer rectifier units, while extra DC power comes from two 28V sealed lead-acid batteries. Emergency electrical power is supplied by a 4kW hydraulic motor-driven generator, which can be driven by either the left or right hydraulic system.

The two independent 207 bar hydraulic systems are mechanically connected by a power transfer unit to sustain operations after the loss of one engine-driven pump. The accumulators supplement engine-pump capacity and provide emergency braking power and landing gear free-fall assistance. The hydraulic system also provides an additional back-up to the rudder system, and consists of a continuous-duty electric motor-driven pump, reservoir, filtration system and two selector valves.

Monitoring all the systems, and representing the first such application of this level of integration in the Horizon class of business aircraft, is Honeywell's Primus Epic avionics system. Dominating the cockpit, with five 200 x 255mm (8 x 10in) flat panel displays, the pilot interfaces with and manages the aircraft subsystems through a side mounted cursor control device (CCD).

Similar to the device developed for the Boeing 777, the touchpad is used to control pull-down menus, menu-guided inputs and on-screen soft-keys. The CCD has three buttons, the left of which positions the cursor within the primary flight display (PFD), the centre moves the cursor to the multifunction display (MFD), and the right assigns the CCD to the engine indicating and crew alerting system (EICAS). A knob on the CCD is used to direct radio tuning on the PFD, to manipulate the map mode, radar tilt range, and other variables on the MFD, and to scroll down messages on the crew advisory system on the EICAS. As well as the usual attitude director and horizontal situation indicators, the PFD also displays resolution advisories from the ACSS-supplied TCAS 2000, weather radar and enhanced ground-proximity warning system.

Synoptic pull-down displays on the MFD include the environmental control, electrical, flight-control, hydraulics, bleed air, fuel and maintenance systems, with synoptic pages developed for the Horizon providing extra information. To validate and develop the complex interactivity of the Horizon, Raytheon created its ISDF integrated test facility, or iron bird. Housed in a building close to the flight test hangars in Wichita, Kansas, the ISDF has accumulated around 4,000h during tests, and is expected to contribute towards certification. "It has been essential," says Jonas, who adds that, apart from fuel and engines "everything is pretty much laid out to exact scale in most cases." The cockpit and related systems are connected through a partition to the rest of the iron bird which occupies an adjacent bay of the building.

Making frequent use of the iron-bird cockpit to augment flight test work, Horizon lead test pilot Randy Rosebrock says the Primus Epic layout "is intuitive, even though it looks complex at first. By reducing my workload it allows me to concentrate on operating the aircraft, and that's going to make things safer."


Display features developed for the Horizon include power trend/command curve lines on the EICAS. "You basically set them where you want them, and they are set-and-leave power levers," Rosebrock says.

In terms of handling he says the Horizon is "sweet to fly. The stick forces, force gradients and general handling are very Hawker-like. The aircraft is also powerful, and when it's light, the Horizon is a rocket - it's very impressive - crusty old Hawker pilots will like the yoke," he says.

At the time of Flight International's visit to Wichita, Horizon flight testing was around half complete. The bulk of time has been accumulated by the flying qualities and basic test aircraft, RC-1 which first flew in August 2001. RC-2, the systems certification test airframe, has also been building time since first flight on 10 May 2002, while the principal avionics testbed aircraft RC-3, joined the test fleet on 31 July last year. Two of the fleet are expected to be displayed at next week's National Business Aviation Association convention in Orlando, where Raytheon is optimistic their presence will stimulate renewed interest in the latest Hawker.