High gear
By Keith Button|October 2018
Early troubles with Pratt & Whitney’s geared turbofan jet engines have not slowed the aviation industry’s growing enthusiasm for this fuel-saving technology.
At plants in Canada, Germany and the United States, Pratt & Whitney continues to build the first geared turbofans large enough to propel twin-engine single-aisle airliners. So far, the engines are powering at least 220 passenger jets for 26 airlines, and Pratt & Whitney’s peers plan to follow suit with their own geared engines in coming years.
The industry is watching how Pratt & Whitney has handled the mammoth engineering task, tracking whether the new engine’s operational issues are hiccups or a sign of larger problems, and learning from Pratt & Whitney’s hardships and successes with its Pure Power PW1000G series or GTFs, short for geared turbofans.
Geared engines have flown for decades — the British Aerospace 146-passenger plane with four Lycoming ALF 502 geared turbofans started flying in 1982, for example. But Pratt & Whitney is the first engine maker to build them at this scale — capable of generating 15,000 to 33,000 pounds of thrust, compared with up to 7,500 pounds for the Lycoming ALF 502 engines, which are the second-most powerful geared turbofans.
Geared engines’ appeal
What is the attraction of geared engines? In a conventional turbofan, the low-pressure turbine and compressor blades are linked to the engine’s front fan by the same shaft, or spool, which means they must turn at the same revolutions per minute. Turning the compressor and turbine blades too fast would risk damage to the front fan. Geared turbofan designers solve that problem by placing a gearbox between the front fan and the shaft leading to the engine core, where the low-pressure turbine and compressor blades are. Now, the front fan can spin at its optimal slower speed, and designers can put longer blades on it to push a larger volume of air. Meanwhile, the low-pressure compressor and turbine blades can spin faster to boost fuel efficiency.
To illustrate the difference, the front fan on one version of Pratt & Whitney’s engines measures 206 centimeters in diameter compared to 173 centimeters for the largest version of the CFM56, the best-selling airliner turbofan.
Pratt & Whitney’s rollout of its GTFs has been less than smooth, though. Most recently, the company and the FAA have been investigating the cause of excessive in-flight engine vibration that some A320neo pilots have reported. As of Sept. 10, the cause was unknown, the FAA and Pratt & Whitney say. Less than 2 percent of the A320neo GTFs have been affected, says John Thomas, company spokesman.
When the engine was first added to new airplanes in 2016, Pratt & Whitney had to advise airlines to cool the engines for longer periods before engine startup to ensure that compressor blades didn’t rub against the walls of the engine. The problem, also known as “rotor bow,” has affected other types of new engines, but it prompted Qatar Airways to cancel its order of 50 new planes. Akbar Al Baker, the CEO of Qatar Group, told reporters the engines had “huge issues” and “a lot of problems.” Pratt & Whitney says the problem was solved with minor hardware and software fixes. Also in 2016 and 2017, Pratt & Whitney had to redesign and replace a carbon seal in the compressor section of the engines because oil chips — contaminants in lubricating oil — were showing up with the old seal in place. In 2017, Pratt & Whitney had to upgrade combustor liners in the engine, modify software and redesign the combustion chamber because parts of the old combustor liners were running too hot. Also in 2017, the company paid compensation to India airline IndiGo for GTF problems that grounded nine new A320neos, and the airline later reported that it replaced as many as 69 of the engines.
This year, the knife-edge seal on the aft hub of the high-pressure compressor in some versions of the GTF cracked, causing aborted takeoffs or in-flight engine shutdowns for four A320neos in February, Thomas says. The same month, the European Aviation Safety Agency and the FAA banned extended-range flights for some Airbus A320neo airplanes because of the knife-edge seal problems. In March, India’s Directorate-General of Civil Aviation ordered 11 A320neos grounded because the seal design had caused in-flight engine shutdowns and aborted takeoffs. Pratt & Whitney revised the seal design and began building new engines with the revised seals in February, but Airbus missed its promised deliveries for 50 new A320neos because of the problem. In June, the FAA ordered airlines to replace the front hub of the high-pressure compressor in about 190 engines because corrosion could damage the part and cause engine shutdown. Also in June, the FAA ordered visual inspections for possible damage to the fan hub on 14 of the engines because they may not have been installed properly.
Despite all that, the industry so far is standing by the geared-engine concept.
An engine expert who evaluates the performance of airplanes and engines for buyers of new jetliners says that while Pratt & Whitney’s GTF has had more problems than the company would have liked, earlier generations of now-reliable engines had even more issues.
The GE90 is one example. This nongeared engine, which powers widebody Boeing 777s, was hailed by Emirates airline’s chief executive in 2015 as an integral part of the airline’s growth. But in early models, bearings wore too quickly in the engine’s transfer gearbox, which transfers power from the engine to accessories on the plane. In 1998, three years after introducing the new engine to its 777 fleet, British Airways rejected the GE90 in favor of Rolls-Royce engines.
The GE90 “was an absolute pig, when introduced,” with “huge problems,” says the engine expert, who asked not to be identified discussing clients. For any new engine, he says, “the normal state is that the engine has issues, and then some lucky guys who’ve done their homework have fewer issues. … It will normally take several years of in-service experience before engines weed out their initial problems.”
For potential jetliner buyers, the main appeal of the GTFs is their improved fuel efficiency. “They love the fuel consumption, and they hate the problems,” the engine expert says. Pratt & Whitney consistently boasts that the engines reduce fuel burn by 16 percent compared to similarly sized engines, along with reducing nitrogen oxide emissions 50 percent and producing 75 percent less noise. Per flight hour, that translates to about 380 liters of fuel saved and 1 metric ton of carbon emissions avoided.
The GTFs are extremely efficient for passenger jets that fly many short hauls of one to two hours or up to 800 kilometers. For three- to four-hour, 1,600-kilometer flights, the engines provide less of an advantage. That’s because their principal fuel efficiency advantages come during the climb, when the engine is generating near-maximum thrust before shifting to the cruise portion of the flight. The advantage tapers off on long flights, because geared engines have larger diameters and create more drag, and the gearboxes add weight.
In February, Greg Hayes, CEO of Pratt & Whitney’s parent company United Technologies, characterized the GTF’s problems as “teething issues” and told industry analysts that the knife-edge seal issue was a “short-term snafu.” He also said that most of the teething issues would be cleared up by the time the engine passed 1 million hours of accumulated flight time — a milestone the GTF cleared in August.
Pratt & Whitney’s consistent response to the early engine problems has been “to identify, mitigate and resolve” them “while minimizing impact to our customers’ operations,” Thomas says.
Sales of the GTF are brisk, in spite of the roll-out issues. As of September, the GTF was powering a mix of Airbus A320neos, Airbus A220s and Embraer E190-E2s, all twin-engine, narrow-body, single-aisle airliners. About 2,000 GTF orders were placed in the previous 12 months by 80 customers, including airlines and airplane leasing companies, Thomas says. In July, the GTF for the two-engine A220 had earned FAA certification for single-engine flying within 180 minutes of airports. The engine builder is also caught up on its GTF deliveries to Airbus and is on track to meet its 2018 delivery commitments, the spokesman says.
Squeezing out efficiencies
Despite the early difficulties with the geared turbofan engines, manufacturers of large passenger jets have geared designs in the works, says Ron van Manen, a Dutch aerospace expert and program manager for the European Union’s Clean Sky 2 engine research and development initiative based in Brussels.
As designers revamp their turbofan designs to squeeze out greater and greater efficiencies, they run up against a limit that they can’t overcome without either moving to geared designs or entirely rethinking airframe designs, van Manen says. Uncoupling the front fan and the engine core’s turbine and compressor blades creates greater bypass ratios — the ratio of the volume of air pushed around the exterior of the core to the volume of air sucked through the core. The higher the bypass ratio, the more fuel efficient the engine is, because the fan generates thrust more efficiently than the core.
The GTF produces a bypass ratio of up to 12:1, compared to up to 6:1 for the CFM56, the engine it replaces. A new-engine competitor to the GTF, the nongeared LEAP engine, produces a bypass ratio of up to 11:1. LEAP is built by CFM International, the 50-50 partnership of Safran and GE that also produces the CFM56.
“There seems to be a certain level of consensus among the large-engine integrators that the next big step in bypass ratio is going to involve a gear,” van Manen says. “While that presents some challenges, it does seem to be on everyone’s road map now rather than being one guy’s leap of faith.”
Rolls-Royce is testing components of its planned UltraFan geared engine, including a 66-megawatt gearbox — almost three times the size of the largest GTF gearbox — that it plans to offer to airplane builders in 2025 with a bypass ratio of 15:1. Safran is designing its Ultra High Propulsive Efficiency engine with a gearbox, which it plans to ground test starting in 2021.
Powerful gearboxes
Rolls-Royce fired up an engine core at full power for the UltraFan for the first time in July in Derby, United Kingdom, and is testing what it calls the world’s most powerful gearbox in Dahlewitz, Germany, outside Berlin. Rolls-Royce says UltraFan will run 25 percent more efficiently than its nongeared Trent engine, which powers long-range widebody passenger jets like the Boeing 777 and 787 and the Airbus A330, A340, A350 and A380. One of Rolls-Royce’s targets for the new engine is Boeing’s planned New Midsize Airplane, also known as the NMA or 797, which Boeing is tentatively targeting to begin service in 2025. Pratt & Whitney reported that its gearbox, which ranges from 12 to 24 megawatts in size for current GTFs, could be scaled up for “significantly higher megawatts” for future airplanes.
For geared turbofans, the upper limit of efficiency will probably be bypass ratios of 15:1 or 16:1, says Jean-François Brouckaert, a Clean Sky 2 project officer. Beyond that point, the fan diameter creates too much drag and the size of the gear box makes the engine too heavy. After geared turbofans, the next leap in efficiency for aircraft propulsion would come from open-rotor turbine engines and distributed propulsion concepts.
“There’s a high level of consensus that this gearbox driving a bigger fan at a slower speed is kind of the only solution that makes sense, and then after that, there are going to be competing steps, but they’re going to look very different,” van Manen says.
What are the challenges of designing and building a geared turbofan? “Everything,” Brouckaert says. “A lot of things are changing in the engine because of the evolution of the gearbox.”
At the front of Pratt & Whitney’s GTF engine, the longer fan blades were redesigned to control how much they untwist or bend as they push a greater volume of air with a slower spin rate. Normally engines of that size require titanium fans, but designers of the GTF created hybrid blades that are unique to the engine: hollow blades, to reduce weight, made of an aluminum alloy with leading edges of titanium.
Because the gearbox sits between the front fan and the shaft that spins the compressor and turbine blades, its added weight presents a challenge for designers. The weight changes where the bearings are placed to support the spinning shafts of the engine and how the front-heavy engine is balanced and mounted under the airplane wing.
“Integration is not only about finding the space to put the gearbox, with the relevant compactness and so on, but it has also a huge impact on the engine dynamics because you are adding an amount of weight on the shaft line,” Brouckaert says.
Another challenge is the heat created by the gear friction. The input shaft spins the center gear, or “sun” gear, which turns five “planetary” gears arranged around it, and those gears turn the outer geared ring that spins the fan. Even with the engine operating at 99 percent efficiency, a 24-megawatt gearbox like that of the largest GTF generates 240 kilowatts of heat, or the equivalent of as many as 24 home heating furnaces, and the heat must be dissipated with lubrication oil circulating through air-cooled heat exchangers, Brouckaert says.
The GTF fan spins at about one-third the speed of the shaft that spins the low-pressure compressor and low-pressure turbine blades in the engine, or 3,200 revolutions per minute versus 9,000 RPM. In a conventional turbofan engine, the pressure of the air that flows into the core is stepped up by the spinning blades of the low-, intermediate- and high-pressure compressor stages. The air then combusts and pushes through the high-, intermediate- and low-pressure stages of the turbine, spinning the blades that rotate shafts turning the fan and compressor sections. In the GTF, the low-pressure compressor and low-pressure turbine blades are spinning faster than those in the core of a nongeared engine, which boosts the engine’s overall fuel efficiency. Engineers designed the GTF core to generate the same power as conventional turbofans, but with fewer rows of compressor and turbine blades.
Separate from adding the gearbox, GTF designers improved the engine core’s efficiency by designing it to run about 5 percent hotter than the previous-generation turbofans. The hotter an engine’s core is through its turbine, the more power it can generate. With the larger fan, GTF engineers designed a thinner cowling, or exterior covering of the engine, to minimize the distance it hangs below the wing, which cancels some of the noise reduction benefits of the slower-spinning fan. The cowling is also shorter to improve aerodynamics, reduce weight and improve the fit under the wing.
While the source of the latest engine vibration issues is unknown, so far, Pratt & Whitney’s other GTF roll-out problems have come from the engine core. For example, the knife-edge seal problem is in the compressor section.
“The area where the GTF made the big advance, the gearbox for that size of engine, hasn’t given them problems,” the airline and aircraft engine adviser says. “They spent a lot of energy on making sure it didn’t. They took that step, did their homework on that piece of tech, and didn’t give them any problems.”
Thomas, the Pratt & Whitney spokesman, confirms that there have been no issues with the gearbox — what the company labels the “fan drive gear system.” He says the company is pleased that the “fundamental architecture of this new engine” — the gearbox — “has performed extremely well.”
Pratt & Whitney probably would have welcomed more time to test the engine to better weed out the initial problems, but the company was on a tight schedule to build engines in time for the Airbus A320 rollout, the adviser says. And now the GTF’s initial issues will be worked out in the field.
But the positive for Pratt & Whitney — the performance of its gearbox — could be a lesson for Rolls-Royce and Safran when they introduce their engines, the adviser says: If you invest $10 billion and 20 years of development, you too can launch a geared turbofan without problems with the gearbox.
“If you want to do a geared turbofan, you’d better do your homework on the gearbox.”
Related Topics
Aircraft Propulsion
“There seems to be a certain level of consensus among the large-engine integrators that the next big step in bypass ratio is going to involve a gear.”
Ron van Manen of the European Union’s Clean Sky 2 program
