Growing pains (and gains) of P&W's Pure Power engine

  • 23-Aug-2016 08:59 EDT
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The promise of new levels of efficiency, significant fuel savings, and ultra-low emissions ensured that the new Pratt & Whitney geared turbofan (GTF) engine would be popular with airframe manufacturers, and this is illustrated by the fact that different GTF versions have been chosen for no less than five new-generation civil programs.

After many years of flat-lining in the supply of new commercial jet engines—with improved models, but with designs dating back to the late 1990s, relying on a continuous path of evolutionary development—a new generation of super-efficient powerplants is entering service, introducing many technologies that will also be applied to future engines.

But the step up in performance, with its potential for greater profitability, has come at a price, with major design and manufacturing issues emerging late in the program cycle, and the added challenge facing airplane manufacturers of having to meet the expectations of anxious and increasingly impatient customers against a backdrop of aging fleets and a surge in demand for new airplanes.

The top-selling jetliners are effectively sold-out for years ahead, stretching deliveries to well beyond 2020. Delays in the delivery of new engines, and further groundings due to in-service problems, are costing some airlines dear and achieving robust solutions is taking longer than envisaged.

As production flows are increased in assembly halls, completed airframes are starting to fill parking ramps as the delivery of engines is awaited. If new engines have to be modified after initial certification and airline delivery, then the delays and extra costs involved in rectification grow with the numbers of airplanes that have already been completed. So, getting the engines to deliver their promised performance with high reliability from the start has always been a goal that engine suppliers aim to achieve.

The highest profile problems, perhaps understandably, have come with the new engine that offers the most innovative technological design—the Pratt & Whitney geared turbofan (GTF), which has developed into the PW1000 Pure Power series. As has been described earlier in Aerospace Engineering, this engine does offer a different design solution, with optimized propulsion efficiency delivered through the different engine stages, resulting in a quiet engine that is cleaner and greener than what has come before.

Using a gearbox to decouple the fan from the low-pressure turbine—the two seated on a common shaft—allows each element to be optimized, with the fan running at a slower rate and the LPT much faster. The promise of new levels of efficiency, significant fuel savings, and ultra-low emissions ensured that the new P&W engine would be popular with airframe manufacturers, and this is illustrated by the fact that different GTF versions have been chosen for no less than five new generation civil programs—the Airbus A320neo, Bombardier CSeries, Embraer E190-E2, UAC MC-21, and Mitsubishi MRJ.

P&W has been highly focused on developing this new engine, which has great potential for scaling up later, and being selected to power the CSeries 130-140 seat regional airliner was an important breakthrough, providing a long-term opportunity to challenge the all-conquering CFM-56 and its replacement LEAP engine in the biggest aero-engine market sector. The CSeries has been a crucial program for Bombardier as it sits at the top end of its commercial jet family and nudges into the global market that has become dominated by the A320 and 737 families.

The combination of a lighter, high-specification airframe and highly efficient engines with low fuel burn should have given this program a head start in a very competitive slot within the commercial sector, but delays in airframe development and then the teething troubles that afflicted the early testing and evaluation of production GTF engines—including an uncontained PW1500G engine fire on a CS-100 during ground tests, caused by the failure of an oil feed tube seal—led to investigations that suggested a longer “cooling off” period was needed before attempting a re-start. The initial uncertainly over this incident was not helpful to Bombardier as CS development costs were soaring and this engine failure happened as Bombardier was into a catch-up testing phase prior to certification.

P&W spared no effort in working to fix the problem created for airlines by having to wait longer between hot shutdowns and restarting. But while the CSeries has finally entered service, the delay in shipping modified production engines has seriously set back the schedule for the urgently awaited PW1100G powering the A320neo model.

In this version of the engine a different problem is connected to temperature fluctuation issues but for the airlines the outcome is the same—start-up times are delayed. (The PW1100G start-up time was longer than on the standard CFM-56- or V2500-powered A320s so clearly a temporary fix became urgent earlier this year as the first PW1100Gs were delivered for service.) This technical problem arose after certification and can occur when the engine is re-started before it has sufficiently cooled after shut down. The temperature variations along the engine’s turbine shaft can lead to misalignment of particular components and this can cause vibration and possible further damage due to rotor tip clearance difficulties and the possibility of components rubbing against each other, wearing away the seal. Customers were advised to allow more time before re-start ups, while the company perfected a technical solution.

German partner on the GTF program with P&W, MTU Aero Engines, is responsible for the high-speed, LPT and four of the high-pressure compressor stages, and is planning to provide a software upgrade later this year to reduce the risk of the earlier problems returning.

Operators like to introduce new aircraft with as few teething problems as possible for obvious reasons, as delays caused by reliability issues early on can lead to reputational damage, as was experienced by Boeing after the delayed introduction of the 787. But while there are incentives allowing for the increased risk of acting as the launch customer, the longer it takes to restore the promised performance, the more nervous key customer will be.

And so it has been with the GTF version of the A320 neo. Original launch customer Qatar Airways refused to accept its delivery, which would have given the airline the lead role of introducing the aircraft into regular passenger service. That task was subsequently taken up by Lufthansa. Although the airline has had to accept and allow for the extended re-start times, P&W has developed a modification based on the application of a new coating for the tips of the rotor. This is being applied to all new engines. Retro-fitting this to existing engines will take up to two years to work through, though thankfully less than 60 early-delivery engines are involved. This task is not simple as it involves the de- and re-assembly of each affected engine, taking around six weeks each time.

The GTF problems attracted the attention of the aerospace and business media as would be expected, but with the cause now established, and modifications being incorporated in all new engines coming off P&W assembly lines in Connecticut and Florida, it is important to see the issue within the wider context of a program that is enormously successful in sales terms, with an annual production output due to rise to 800 engines, and a total backlog of over 7000 GTF engines for all models.

Initial service reliability of the PW1100G has been exceptional, according to P&W’s President Robert Leduc, claiming dispatch reliability is over 99.7% on the first few aircraft in service with Lufthansa, IndiGo, and GoAir. On the subject of low fuel burn, perhaps the biggest selling-feature of the new GTF design has lived up to its game-changing claims, with an improvement over the existing A320 of over 15%. Other benefits include ultra-low noise levels that promise to halve the noise footprint of an already quiet A320 at many airports, and a 60% reduction in CO2 and NOx emissions.

The other commercial airplanes to be powered by the GTF family due to enter service over the next few years will ensure that the design becomes one of the most widely used in the decades to follow. The CSeries, despite extended development delays, has a sales backlog of little short of 300 airplanes, a respectable total at entry into service, and the Embraer 190-E2 family seems destined to repeat the success of the company’s current models. Prospects for the Mitsubishi MRJ and Russian UAC MC-21 are less clear, with both entering a highly competitive sector where rival established manufacturers have a clear lead in terms of overall sales and support infrastructure, but the combined global market for GTF engines is huge and confirms P&W’s wisdom of persisting with such an innovative new product, accepting the risks and then dealing swiftly with the resolution of early issues.

As the deliveries of PW1100Gs build throughout the rest of this year, the lines of new A320neos parked outside the Airbus assembly halls at Toulouse and Hamburg, awaiting engines to fit to their pylons, should soon start to disperse. For both P&W and Airbus, the glitch in engine and airplane deliveries has been most unfortunate, but it also reflects what has become the biggest issue in the commercial aerospace sector—coming to grips with unprecedented market demand while maintaining quality control standards.

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