Friday, June 1, 2007
Technology Focus: Built to Rule Short-Haul Routes
Pratt & Whitney’s PW6000 engine design has focused on reducing maintenance and acquisition costs since day one. From wrench jockeys to numbers crunchers, this powerplant will make their day.
Handling the high-cycle rigors of short-route flights in the single-aisle, 100-passenger commercial aircraft segment is the mission of Pratt & Whitney’s latest engine, the PW6000. Nine years in development, this high-bypass turbofan is designed and built specifically to reduce acquisition and maintenance costs and to significantly extend time on wing. Key features of the 18,000 to 24,000 pound thrust powerplant: simplified architecture, fewer of the most expensive parts, unprecedented levels of pre-testing, and reduced fuel burn, NOx and noise. Bypass ratio is 4.9:1 and overall pressure ratio is 26.6 to 28.7:1. This engine is lean, green and tough on wing.
In 1998, P&W made a commitment with Airbus to use the new engine on its A318 aircraft, and this May, airline launch customer, LAN Chile, entered into service (EIS) its first twin-engine A318 outfitted with PW6000s. Prior to EIS, the accumulation of ground and on-wing testing reached more than 14,000 hours, 28,000 operational cycles and 1,800 actual flight hours. P&W says that’s four times the level of testing for the company’s previous PW4168 and PW484 engines.
Also before EIS, the PW6000 met rigorous validation testing for extended twin-engine operations (ETOPS), and had 180-minute ETOPS capability. Previous engines have achieved ETOPS at or shortly after EIS. Carmen D’Onofrio, P&W’s PW6000 program manager, explained that this ETOPS advantage allowed durability issues to be identified and addressed before first engine delivery and thereby boosted engine reliability with the first delivered aircraft. "Durability and reliability comparable to a mature engine offers airline customers the operational benefits of flying the most efficient route structures," D’Onofrio said. Formal ETOPS certification of the A318 aircraft with the FAA is expected later this year.
Get Good Partners
P&W engineers, in concert with engineers at MTU Aero Engines (MTU in Munich, Germany) have persistently chosen design innovation to reduce cost on the PW6000 and have been willing to change course to stay on this track. MTU is a risk/revenue share partner in the PW6000 with design and production responsibility in the six-stage, high-pressure compressor (HPC), and a preferred supplier of the low-pressure turbine (LPT). "Risk and revenue partners are important in the development of new engines, which require a great deal of technical expertise and significant financial investment with a long-term payback," reflected D’Onofrio.
In the case of the PW6000, R&D took a longer time than initially expected. In 2002, P&W program leadership elected to delay the PW6000’s delivery schedule for 30 months, to mid-2005. "We didn’t want to compromise on our commitment to Airbus and our airline customers in delivering a product that met all their expectations for short-haul performance — bottom line, up to 12 one-hour flights a day," D’Onofrio explained. Replacing a five-stage HPC with MTU’s six-stage unit proved crucial to achieving the final design.
Anton Binder, MTU’s senior vice president of commercial engines, related that the eventual HPC incorporated into the PW6000 "has a very high stage load, extremely erosion-resistant wide chord airfoils, and a very high stall margin reserve. Overall, this HPC design provides six percent performance improvement from the previous five-stage design, and a reduction of weight per engine of around 300 pounds compared to the competition" [primarily the CFM 56-5B engine]. This HPC also features integrally-bladed disks (or blisks) that reduce weight and part count while boosting load tolerance."
Further, he said, "with 15 stages overall, the PW6000 has four stages less than the direct competitor, and 50 percent fewer airfoils. According to the 2005 Engine Yearbook, 60 percent of maintenance material costs are spent on airfoils. These factors, along with state-of-the-art materials to ensure high-temperature durability, result in maximized time on wing." MTU was able to draw upon its work with the HDV12 experimental engine rig "that was directly transferred into engine test hardware for the PW6000 within six months," Binder said.
Showing seminal trust in MTU’s capabilities, P&W will locate final PW6000 assembly to MTU’s facility in Hannover, Germany, which will also serve as the overhaul service shop for the engine. This is a first for P&W, and gives MTU an opportunity to break into the commercial market for compressors after years of experience in compressors in the military arena.
After decades of working together on commercial programs, the P&W/MTU partnership has now been expanded to include MRO services. D’Onofrio reported that a crucial element in LAN Chile’s decision to outfit its Airbus 318s with PW6000 engines resulted from the availability of the fleet management plan. Coordinated by P&W Global Services Partners, this plan directs that all off-wing overhaul activities will be performed by MTU in Hannover. And for installation of the new engines, the proximity of the Hannover shop to the Airbus Toulouse facility in France offers the benefit of minimized transportation costs and time.
Materials and MRO
Part of the material choices to make the PW6000 environmentally "greener" are based on 2000 goals set by parent company, United Technologies Corporation. As of this year, the corporation exceeded its initial goals and recently introduced a new four-year program to further reduce greenhouse gas emissions by 12 percent, water consumption by 10 percent, and non-recyclable waste by 30 percent.
P&W takes a comprehensive approach to environmental responsibility, focusing on manufacturing impact as well as product material and environmental performance. As such, the PW6000 began P&W’s move toward lower fuel use and emissions, lower noise levels and assembly using fewer raw materials, energy and water. Hazardous materials such as cadmium, lead, cyanide and chromates have been eliminated. UTC’s green goals, as reflected in P&W engines that burn cleaner and quieter, can help airlines around the world reduce landing fees and environmental surcharges. The next generation P&W engine, the Geared Turbofan, will deliver a 12 percent reduction in fuel burn and step change reductions in emissions and noise over today’s engines.
Speaking of emissions, another long-term collaborative P&W partner/supplier is Mitsubishi Heavy Industries (MHI), based in Tokyo. For the PW6000, MHI provided the Talon II combustor/diffuser module. According to D’Onofrio, "this module ensured our ability to meet existing CAEP/6 emissions requirements, and will also meet emissions requirements for many years to come."
Material choices also affect an engine’s total cost. "With low maintenance cost being a fundamental objective of the PW6000’s design," D’Onofrio stated, "our engineers generally relied upon proven materials from previous engine designs. In certain cases, however, the objective for low maintenance cost resulted in the selection of more advanced materials. In the high-pressure turbine (HPT) airfoils, for example, we used the latest thermal barrier coating technology. This meets the durability objectives and will maximize on-wing time. Both parameters reduce maintenance requirements and cost of ownership."
A major MRO headache can occur if debris gets into an engine’s core. Wide-chord turbine fan blades on the PW6000 have high root stagger and low aspect ratio, which helps centrifuge dirt and debris to the fan bypass duct rather than to the engine’s core. The fan blades have a tip diameter of 56.5 inches; flange-to-flange length of the engine is 108 inches, and dry engine weight is 4,950 pounds without the nacelle. Long-duct, carbon fiber/epoxy nacelle provides maximum surface area for noise attenuation material on the inner surfaces.
The L Words
Delving into more of the nuts and bolts of this engine’s design to reduce maintenance inevitably leads to its line replacement units (LRUs) and life limited parts (LLPs). Chris Kmetz, P&W’s PW6000 chief engineer, reported "75 percent of all LRUs can be replaced within 15 minutes, and 90 percent can be removed and replaced within 30 minutes. Furthermore, the simplified externals architecture of the PW6000 ensures that all LRUs can be removed and replaced without disturbing other components in the vicinity of the given LRU."
Kmetz added, "assuring LRUs are mounted one deep on the engine definitely improves maintainability. One of the most challenging LRU integration tasks involved packaging the accessory gearbox. This assembly encompasses a large electrical generator, hydraulic pump and the accessories necessary to support the engine."
Making the assembly highly maintainable in a compact package required creative, innovative solutions. Cored passages were incorporated in the gearbox to transfer air and oil from one side of the gearbox to the other. This eliminated 12 external pipes. A manifold was also added to transfer fuel between the fuel metering unit and fuel pump, which reduced the number of external pipes connected to these components to just one. "These integration improvements significantly reduce the time and cost of performing LRU troubleshooting and maintenance," said Kmetz.
Arrow Gear Company manufactured bevel set gearing in the accessory gearbox, which can experience a high degree of deflection during operation. After 75 hours run time on a PW6000 test engine, visual inspection of the gears showed contact patterns on both the run side and start sides as predicted.
Another LRU adjustment comes in the replacement of O-rings by reusable face seals for all major engine modules — not only for better oil sealing performance, but also because the face seals can be visually inspected. O-rings tend to be viewed as "a blind installation." Beside benefits in the oil retention characteristics of the PW6000, this change reduces the need for rework and the possibility of maintenance-induced schedule disruptions due to oil leaks.
P&W has constructed all LLPs to meet a lifetime of 25,000 cycles. This is achieved primarily through material selection and design of fewer parts with higher durability. The PW6000 has 16 LLPS, including: fan hub, LPC drum, HPC first through third integrally bladed rotors, HPC fourth, fifth and rear disks plus hub, HPT disk one and front mini-disk as well as front and rear side plates, LPT disks two through four, LPT rotor seals, and low shaft, and a low pressure shaft that connects the LPC to the LPT.
Examples of design aspects affecting LLPs cost/maintenance goals:
A 2.5 air bleed positioned behind the fourth stage rotor assists in removing debris out through the bleed, prior to the air entering the core of the engine.
Variable vanes, TCC actuators and 2.5 bleed actuators designed without rigging specifically for the A318 in high-cycle operation also contribute to ease of maintenance. These components can be removed and replaced without cycling each system to validate proper range of motion. By helping prevent delays and cancellations, maintenance time is again reduced.
An Integrated Success
Based on success in previous engines — to the tune of millions of dollars — P&W has incorporated advanced diagnostic sensors and monitoring equipment into its new engine, to help detect and prevent engine operational issues.
P&W’s D’Onofrio concluded that all the partnership, design, manufacturing, monitoring and MRO service efforts put into the PW6000 "ultimately benefit the airline customer in this particular market, and have enhanced our technology readiness to ensure that future engine programs meet all customer requirements within the intended schedule." For MTU, Binder commented, "P&W’s vote of confidence strengthens our system integration capability, and substantially enhances our business strategy to expand our MRO business."
Both P&W and MTU relayed to AM that successful partnership arrangements improve program affordability, speed the engine development cycle, and streamline the transition to full production. By design, LAN Chile should reap the benefits of this partnership for at least 15,000 operational in-service cycles before an overhaul on the PW6000 is required.
PW6000 Development Milestones
There’s no short route to development of a new engine, and critical milestones must be met. Among those achieved with the PW6000:
*1998: Program commitment with Airbus Industries for PW6000 engines on the A318.
*2000: Completion of 1,000 hours of engine endurance testing.
*2002: First test flight of PW6000 on Airbus A318.
*2003: Program entry of MTU’s high-pressure compressor (HPC).
*2004 (February): Completion of 1,800 cycles and 480 hours of HPC endurance testing.
*2004 (November): FAA FAR 33 status earned.
*2005: Engine validation by European Aviation Safety Agency (EASA).
*2006: First production PW6000 delivered to LAN Chile.
*2007 (February): System checks during four-hour test flight of first production Airbus A318 outfitted with PW6000s.
*2007 (May): First LAN Chile entry-into-service flight of A318 powered by PW6000s.