GUY NORRIS / LOS ANGELES
Honeywell's new AS900 regional and business aircraft engine not only places the company in the vanguard of development, but is also set to provide the basis of its long-term design strategy
What do Anheuser Busch, BAE Systems, British European, Garrett Aviation, Home Depot and Qantas have in common? The answer lies in the arid heart of the Gila River Indian Reservation, and in the blue skies over Arizona where the Honeywell AS900 turbofan is in the final stages of testing.
All are among the companies that have helped shape the design of the new business and regional aircraft engine which enters service on the BAESystems Avro RJX and Bombardier Continental in 2002 after several years of test and development. AS900s are still undergoing rigorous evaluations in Honeywell's test cell complex at San Tan Mountain deep in Indian territory to the south of Phoenix, while another is strapped to the side of the company's Boeing 720 flying testbed for a final set of shake-down sorties.
The certification programme's completion marks a watershed for Honeywell as it refines its future engine development strategy in the wake of the abortive merger with General Electric earlier this year. With a flat-rated thrust of between 6,500lb (29kN) on the AS907 for the Continental application, and 7,100lb for the AS977-powered RJX, the AS900 family extends Honeywell's business jet power-range while providing a state-of-the-art successor for the LF502/507 inherited from Textron Lycoming. It also provides the initiative for a family of lower-power siblings dubbed the FX-3, -4 and -5 being developed in tandem to succeed the TFE731 and which, with the AS900, will furnish a seamless solutions for thrust requirements from 3,000lb upwards.
New values
Ironically, the AS900 has its roots in work which AlliedSignal (now Honeywell) conducted in the early 1990s to develop the ultimate members of the TFE731 family, the -20, -40 and -60. "We made a conscious assessment of the marketplace to see what we had to do," says Mike Redenbaugh, Honeywell engines, services and systems propulsion enterprise vice president and general manager. "We recognised a new set of value equations in which the focus was driving up reliability and durability, while reducing cost." As luck would have it, the new values emerging in the business arena were almost wholly applicable to the regional jet market.
AlliedSignal continued with this approach when it began its search for a LF502/507 successor in the late 1990s. "We spent 18 months asking customers what they wanted," says Redenbaugh. A technical advisory committee was established made up of Avro RJ and BAe 146 operators; service centres like Garrett Aviation and Premier Turbines; Bombardier Challenger operators such as Anheuser Busch and Home Depot; and British Aerospace (now BAE Systems). The 14 main members met regularly from January 1999, and helped Honeywell hammer out the major requirements for the new project.
Fundamental issues were also quickly resolved - the biggest of which was the baseline thrust demand. In the late 1990s every new regional airliner design was a twin. Yet BAE Systems "continued to show interest in an RJ family" based on the quad design of the original BAe 146, says Redenbaugh. The solution was an engine that would suit the power, reliability and cost considerations of the business market, while meeting the needs of what was by this time becoming the Avro RJ-X regional jet. "We had to strike a balance to suit the regional custom we'd acquired from Lycoming as well as the needs of the business aviation community which we knew formed the bulk of our market," says Redenbaugh.
Another factor was the sheer cost of developing a higher thrust engine for twin applications in a market already occupied with powerplants ranging from Pratt & Whitney Canada's PW300 to GE's CF34 and the Rolls-Royce AE3007.
The results from its advisory committee were quickly distilled into one basic consideration: cost. "We ended up focusing on a significant reduction in cost per pound of thrust. It was in the order of 25% or below," says Redenbaugh. To get cost down, the engine was tailored with easy on-wing and on-condition maintenance in mind, and a low parts count. It was also outlined from a new integrated propulsion system standpoint with a full nacelle, engine build unit, air turbine starter, thrust reverser and mounts.
To ensure ease of use, the technical group also prompted key improvements to the control system. This provided auto-start, auto-idle with bleed and anti-icing control, engine synchronisation for low noise and automatic performance reserve and thrust reverser control. Safety was also boosted in the baseline design by the incorporation of automatic engine limiting software, and the automatic prevention, detection and recovery from surges and flame-outs. The AS900 was also provided with overspeed protection, dual ARINC 429 databus connections and hardwired signals for several critical sensors.
Making the AS900 easy to maintain and particularly accessible for on-wing maintenance became a key design driver. Airlines and business-jet operators reacted differently to the idea of on-condition and on-wing maintenance, as AS977 chief engineer Terry Pyle recalls. "It certainly generated diverse points of view and on-condition in particular is a mixed bag to them.
"The airlines wanted on-condition and on-wing maintenance, but with the ability to go right to the limit. They also wanted the added flexibility of time-limited dispatch. This added about 125h, which is enough time for them to reposition and plan getting it to an operations base," he says. The result, Pyle adds, is a classic compromise: "We ended up with on-condition but with specific intervals, and developed special tooling for on-wing maintenance."
Design drivers
The advisory group and Honeywell also made up a final list of maintenance requirements to help drive design features. This included 30min engine runs and checks, the removal and replacement of any line replaceable unit (LRU) in 20min or less, and the inspection of chip detectors and replacement of filters, also in 20min. Engine removal and replacement was aimed at 2h, and engine removal, disassembly, re-assembly and re-installation at just 6h.
The results range from the absence of lockwire to a smaller-than-usual line maintenance tool box made up of just 13standard tools "of the sort you'd find in any hardware store," says Redenbaugh. The engine is also riddled with 29 borescope access ports, or "three to five times more than in competitive products," he adds. These allow 360í access for high- resolution videoscopes to enable viewing of all airfoils. The full authority digital engine control also contains self-diagnostic and predictive software as part of the AS900's built-in test capability, which is geared towards trend monitoring and planned maintenance actions.
The design also allows for individual fan blades, stators and fuel nozzles to be replaced on-wing, as well as the N1 monopole. The high-pressure (HP) spool can also be manually cranked during inspections from a drive point in the accessory gearbox, which does not require the removal of any LRU. No special vibration equipment is required after fan-blade replacement, and alignment features are built into the inlet. All mainshaft bearings and seals are also accessible on-wing, and the accessory gearbox, HP turbine and combustor, low-pressure turbine (LPT) and fan modules can be removed and replaced on-wing.
Defining an engine
Much of the AS900 architecture is derived from the Technology Validation Test (TVT), a core demonstrator programme that ran as a gas generator for the first time in December 1998. The TVT incorporated Honeywell's first blisk compressor design and tested an advanced combustor similar to the AS900's effusion-cooled unit. It also incorporated a single-stage HP turbine, though it was later ruled out for the AS900.
"The TVT gave the team a foundation to kick off the whole AS900 programme," says Robert Wilson, vice president of AS900 programmes. "Basically we were designing a new centreline [engine] and the TVT helped us to make several difficult compromise decisions." This included the decision to go for a more conventional HP turbine design. "We found fabrication costs were high, and we found some difficulties with the single-stage nozzle. It was clearly not a step we wanted to take," adds Pyle.
The AS900 advisory group viewed the TVT and "one of the results was the trade-off cost of one versus two stages in the HP turbine. Usually less is better, but cost is a powerful consideration." The TVT also influenced the decision not to put a "hot" bearing into the AS900. The engine has a two-sump configuration, with the forward bearing assembly accessed by removing the fan, and the aft sump is reached by removing the thrust reverser, tail-cone and No 5 bearing support.
After 150h of TVT tests, the effort was completed in August 1999. The firstfull AS900 engine was tested ahead of schedule in July 1999, less than a year after the formal launch at the 1998 Farnborough air show.
The 870mm (34.2in-) diameter AS900 fan consists of 22 wide-chord titanium blades mounted in front of a set of 53 full-span composite stators and a four-stage axial plus single-stage centrifugal compressor. The three-stage compressor of the TVT was considered but produced insufficient pressure ratio for the AS900. The added compressor stage was augmented with the extra HP turbine stage, with a subsequent improvement in specific fuel consumption (SFC). All four axial stages are machined blisks, while Honeywell believes the centrigual-flow impeller downstream is best for foreign object damage (FOD) resistance. To further improve FOD immunity, the splitter dividing the bypass and core streams is positioned to allow debrisentering the fan to centrifuge into the bypass duct.
The fan diameter and 4.5:1 bypass ratio at take-off and 4.2 in cruise was driven by the high Mach configuration requirements of the Bombardier aircraft. "We realise that this is a little lower than what we'd want ideally for the BAE aircraft, but we give the RJX an interesting advantage in terms of much improved climb power which, in the regional role, will be very useful," says Wilson. The result is between 6% and 11% improved climb thrust depending on altitude and Mach number, and greater turbine exhaust temperature margins leading to longer life expectations. "So it was a good trade overall," he adds. Sixteen air-blast fuel nozzles, four of which are piloted for improved starting, feed the effusion cooled combustor. Improved starting gives a lower windmill relight speed, allowing Honeywell to eliminate the relight conditions required for LF502/507 users.
Final tests
Hot gases drive through a two-stage HP turbine, the first of which is made up of 38 single- crystal SC180 blades. The second stage consists of 34 blades made from Mar-M 247 nickel alloy. The three-stage LP turbine is also made up of conventional nickel alloy blades, 56 in the first, 62 in the second and 74 in the third stage. The LP and HP turbine disks, together with the compressor system, are designed for a 15,000-cycle lifetime. Aft of the turbine is a 16-lobe, low-noise and reduced-SFC mixer. Wrapped around the engine is a GKN Aerospace-developed nacelle which, on the AS907, integrates a Hurel-Dubois thrust reverser. A US nacelle site is being established by GKN in St Louis, Missouri, to support the AS900 programme. Although initial setbacks in completion of the first flightworthy units delayed first flight of the RJX for four months up to last April, the integrated reverser and nacelle has performed more than 3,500 deployment cycles in tests.
Engine tests as a whole exceed 11,200h, of which around 5,350h are on the "Block 2" standard AS900. This was developed to counter durability and maintainability issues spotted during the initial tests. "These range from optimisation of the fan stator in the fan module, to durability improvements in the combustor to take it up to 7,000h," says Redenbaugh. "We had one that was about half that life so we needed to incorporate cooling enhancements." The change include individually replaceable fan stator vanes, improved HP turbine blade-tip cooling and bleed duct optimisation through FADEC changes to reduce losses. The changes delayed engine certification from September 2001 to early next year for both versions. "But that's what testing is for," says Redenbaugh.
Important final tests for birdstrike resistance and blade-off are being undertaken as Flight International closed for press. Pending completion of these and the final flight tests of the 720, RJX-85 and -100 and Continental aircraft, Honeywell's priority is speedy certification and entry into service throughout next year. After that the search is on for more applications and possible growth to 9,000lb thrust, a full 25% beyond the baseline powerplant. Out on its own once more, it seems Honeywell has everything to gain from the AS900 and nothing to lose.
Source: Flight International
