Bombardier says that the Canadair Challenger 604 is "the fastest-selling Challenger ever". It is also the furthest-flying, and the two are not unconnected. With the 604, the Canadian company finally has exploited the full potential of the Challenger design, and is reaping the reward with an increased share of the large-business-jet market.
The 604 is the first Challenger to exceed 7,400km (4,000nm) range, providing true intercontinental capability for substantially less cost than that of the competing Dassault Falcon 900 and Gulfstream IV. Not that previous, shorter-range, versions have been lacking in popularity, as delivery of the 400th Challenger in late November will illustrate.
The Challenger is at the heart of what has become Bombardier Aerospace, furnishing the basis for the market-making Canadair Regional Jet, the 200th example of which was delivered in October, and providing the jumping-off point for the Global Express long-range business jet, deliveries of which will begin in mid-1998.
Pioneering design
The Challenger design has its origins in the LearStar 600, although all that remains of the original Bill Lear concept is the combination of high-bypass turbofans with a supercritical-section wing - the first such pairing in a transport-category aircraft. When Canadair signed an option on the LearStar 600 design in April 1976, it acquired little more than a concept. The Canadian company made design changes which, although infuriating Lear, have proved crucial to the success of the design and its descendants.
Increasing the fuselage cross-section to 2.69m, from the original 2.23m, was the most important change. Not only did it give the new Challenger the widest cabin among dedicated business-jets, it also allowed Bombardier to stretch the aircraft to produce the 50-seat, four-abreast, Canadair Regional Jet (CRJ). In return, the company has been able to expand its business-jet range by offering the Corporate Jetliner 30-seat shuttle and large-cabin Challenger Special Edition versions of the CRJ.
The Challenger cross-section is also used on the Global Express, providing the largest cabin among purpose-designed corporate aircraft.
Not all of Canadair's early decisions had such happy consequences. Selection of the unproven Avco Lycoming ALF502 turbofan over General Electric's CF34 for the original Challenger 600 led to performance and reliability problems which marred the aircraft's early years. The ALF502's shortcomings contributed to the Challenger 600 falling well short of the design goal of 6,300km range.
Canadair also set an unrealistically aggressive timescale for development of the Challenger, which was already several months behind schedule when it had its first flight in November 1978. Certification then took longer than planned, hit by the loss of the first prototype during controversial flight-testing to prove that the aircraft had good natural-stall characteristics. Certification was finally achieved in late 1980, by which time Canadair had launched development of the improved Challenger 601.
First flown in September 1982, the 601 heralded the switch to higher-thrust, more-efficient, GE CF34s, increased fuel tankage, and drag-reducing winglets. Range was boosted to 6,350km, from the 600's 5,180km. Challenger production switched to the 601-1A after 84 600s had been delivered.
Further improvements followed. The 601-3A had the CF34-3A engine, flat-rated to higher temperatures to improve hot-and-high performance, and introduced Honeywell digital avionics, including an electronic flight-instrument system. Production switched to the 601-3A after just 66 -1As had been delivered, and 134 were built before the Challenger was upgraded again.
Intercontinental range
The 601-3R had as standard a tailcone fuel-tank which had been offered as an option on the -3A, extending range to an intercontinental 6,630km. This variant also introduced the CF34-3A1 turbofan developed for the CRJ. In addition to a slightly increased thrust, this engine incorporated accessibility and reliability improvements proved in regional-airline use. Bombardier delivered 59 Challenger 601-3Rs.
Within a month of delivering the first 601-3R, in August 1993, Bombardier launched development of what might yet prove to be the ultimate version of the Challenger - the 604. Design objectives were to increase range, improve performance, upgrade the cockpit and improve reliability and maintainability, while keeping the cost under $20 million for a completed aircraft, to maintain the Challenger's position as "the best value in its market segment", the company said at the time.
The changes are incremental, but combine to give the 604 a character significantly different to that of earlier Challengers. Most important is the increased range of 7,550km at Mach 0.74, with five passengers, "-which gives operators an alternative to the Falcon 900 and GIV for millions less", says John Lawson, president, sales, for Bombardier Business Aircraft division. The additional range provides a "100% guarantee" of reaching the US East Coast from Europe, he says. At the same time, the Challenger 604 remains a "cost-effective" transcontinental aircraft, able to carry nine passengers from New York to Los Angeles at Mach 0.82, Lawson says.
Changes introduced on the 604 include increased fuel tankage, more powerful and fuel-efficient CF34-3B engines, increased maximum take-off weight and new integrated digital avionics. The landing gear is new and several systems have been improved, based on operating experience gained with the CRJ. Lawson says that the 604, at $18.45 million "green", costs only about 2% more than did the 601-3R.
Most of the 604's additional range results from increased fuel capacity. Two saddle tanks have been installed in the rear fuselage, and the capacities of the tailcone and forward-fuselage tanks increased, to boost the maximum fuel load to 9,070kg, from 8,120kg in the 601-3R. The 500litre rubber-bladder saddle tanks, installed in previously empty space in the aft equipment bay, outside the engine rotorburst zone, are connected and considered as one. The tail tank was enlarged by rerouting a vent line, while smaller avionics boxes enabled the forward-fuselage tank to be stretched.
The fuel system looks complicated, with no fewer than seven tanks, including the main left and right wing tanks, but Bombardier insists that its operation is simple and reliable. The additional fuel moves the centre of gravity (CG) further aft compared to that of the 601, reducing trim drag, but makes fuel-system operation more critical. All fuel-flow scheduling is automatic, with the transfer rate from the tail tank computer controlled, to minimise CG movement. Solid-state fuel-level sensors and brushless AC-electrical pumps are used to increase reliability. Single-point pressure refuelling capability is provided.
Engine enhanced
GE's CF34-3B provides 7% more thrust capability by being flat-rated at 41kN (8,730lb) to a higher temperature than that of the earlier -3A (to ISA+15íC, which is 38íC at sea level compared to 21íC). Specific fuel consumption is reduced by 3%.
Thrust capability has been increased by redesigning the first low-pressure-compressor stage as an integrally bladed disk, or blisk, which increases airflow compared with that of the earlier disk with mechanically attached blades. Fuel efficiency has been improved by using new materials in the high-pressure-turbine nozzle and shroud, to better match their thermal response and so provide tighter clearances, and by improvements to the interstage seal, transition section and low-pressure turbine shrouds.
Bombardier says that the CF34-3B is a cheaper engine to operate because of the shift to "task-oriented", or on-condition, maintenance, which replaces fixed-interval inspections and overhauls with borescope inspections and trend monitoring. There is no costly hot-section inspection at 3,000h, but instead a roughly $1,000 borescope inspection at 1,600h.
On-condition maintenance was introduced with the CF34-3A1 powering the Challenger 601-3R and CRJ, along with accessibility improvements to reduce the time taken to exchange line-replaceable units. There is no lock wire on the engines, for example. These improvements are carried over to the -3B. Based on CRJ experience, the company calculates that 604 operators should average 9,000h before the first unscheduled engine shop-visit.
The increased fuel capacity and thrust capability of the Challenger 604 results in an increase in maximum take-off weight (MTOW) to 21,600kg, up from the original Challenger 600's 18,600kg. Maximum landing weight has been increased over that of the earlier 601-3R, to 17,200kg. This enables a 604 with five passengers and two crew to be flown 3,575km, landed, then flown a further 3,785km without refuelling.
Among the enhancements announced by Bombardier since beginning Challenger 604 deliveries in late 1995 is the ability to operate the aircraft at an optional lower MTOW of 17,240kg to reduce navigation and landing costs in Europe. The company estimates that this will allow an operator to fly the 604 on intra-European missions at costs comparable to those of Dassault's smaller Falcon 2000, while retaining the Challenger's higher-MTOW capability for longer-range missions.
Increased weights necessitated a redesign of the landing gear, and Bombardier took advantage of this to introduce an all-new gear and rolling assembly to tackle an acknowledged "weak area" of earlier Challengers. The twin-wheel main gear is enlarged, with bigger, lower-pressure, tyres to reduce pavement loading and avoid the premature tread-cutting experienced with the 601.
The wing-to-fuselage and underbelly fairings were redesigned to accommodate the larger gear while minimising the drag increase.
The brakes have 60% more mass and 100% more energy absorption, through the use of an improved carbon material. Grooved discs improve water shedding and reduce taxi noise, the company says. A higher-frequency anti-skid system, adopted from the CRJ, offers increased reliability and a longer inspection interval.
Overwater reliability
Several systems have been improved because the longer-range 604 is expected to spend more time over water than did previous Challengers. These include changes to the hydraulic system to increase reliability. Because of its aft-loaded wing aerofoil, the Challenger has hydraulically boosted flight controls, requiring three independent hydraulic systems for redundancy. Two of the 207bar (3,000lb/in2) systems are each powered by an engine-driven and an AC-electric pump, while the third uses two AC-electrical pumps.
All flight-control surfaces, except the flaps and horizontal stabiliser, are hydraulically powered with no manual reversion capability. There are two actuators on each aileron and elevator, and three on the rudder. Two spoiler panels are provided per side on the wing; each outboard flight spoiler has two actuators while each inboard ground spoiler has one on/off actuator. The variable-incidence horizontal stabiliser and two-section, double-slotted, wing flaps are electrically driven and electronically controlled.
To avoid the possibility of a deep stall, a potential problem with all T-tailed aircraft, the Challenger has a stall-protection system. This consists of a dual-channel computer controlling two stick shakers and providing independent signals to one stick pusher. As required for Canadian certification, the aircraft also has good natural stall characteristics.
All this is unchanged in the 604, where the emphasis has been on updating the cockpit. The latest Challenger has Rockwell-Collins' Pro Line 4 integrated digital avionics, replacing the 601-3R's Honeywell SPZ-8000 system. The change is dramatically apparent, the cockpit being dominated by the six 185mm-square cathode-ray-tube displays ranged across the instrument panel.
The outer pair on each side make up the electronic flight-instrument system, consisting of each pilot's primary-flight and multi-function displays. The centre pair are the engine-indication and crew-alerting system - the first such system in a Challenger.
The Pro Line 4 system is based around two integrated avionics processors, each housing two flight-control computers providing autopilot, flight-director and yaw-damper functions. This arrangement provides fail-passive autopilot redundancy.
The 604's standard avionics suite includes dual Litton laser-gyro inertial-reference systems and dual Collins digital air-data computers. Dual Collins global-positioning- system (GPS) receivers are optional and, eventually, will be approved for approaches. Dual Collins flight-management systems (FMS) are available, and are able to use GPS for navigation. FMS functions include performance management.
Bombardier has announced that it will offer a head-up display on the Challenger 604, and has begun installing provisions in aircraft now on the production line. The company has selected the Flight Dynamics Headup Guidance System (HGS) already certificated in the CRJ, but with additional functionality allowing the display to be used throughout the flight, from take-off to landing. Additional functions will include head-up display of angle-of-attack margin, windshear-escape guidance, and cues generated by the ground-proximity warning and traffic-alert and collision-avoidance systems.
Certification of the HGS in the 604 is scheduled for late 1999. Flight Dynamics is part-owned by Collins and has previously integrated its HGS with the Pro Line 4 system in the CRJ. The company expects a high proportion of 604 owners to buy or retrofit the head-up display.
Bombardier's major selling point for the Challenger remains its widebody cabin, which has been improved in the 604. New, passive, isolation reduces cabin noise below 50dB, compared with a typical 52-53dB, the company says. The insulation consists of two layers of polymide polymer foam and a layer of glassfibre , and provides a weight saving of 20kg over that of the 601.
Optional noise control
Active noise and vibration control is available as an option, to reduce the transmission of low-frequency engine fan and core tones into the cabin. The Ultra Electronics system, already used in Bombardier's de Havilland Dash 8Q regional-turboprop, consists of 20 microphones connected to a microprocessor which drives active-tuned vibration absorbers mounted on the airframe.
While cabin comfort is a key factor in business-jet sales, price and performance are increasingly important. By extending its range beyond 7,400km, Bombardier has made the Challenger 604 a more effective rival to the Falcon 900 and Gulfstream IV, and the performance gap has been widened between the aircraft and the cheaper Falcon 2000, which was "eating into" the Challenger's market, the company admits.
Additional range, albeit at the cost of some payload, has also made the 604 a more suitable interim aircraft to offer customers ordering the 12,400km-range Global Express, Lawson says. This has helped boost demand for both aircraft, he adds, and has contributed to Bombardier's decision to step up production of the Challenger from its traditional two a month to some 36 in 1998, Lawson reveals.
From a project which almost bankrupted Canadair, the Challenger has matured into the leader of a line of commercial aircraft which has boosted Bombardier to a position among the world's leading manufacturers, just ten years after it entered the aerospace industry.
Source: Flight International
