On a chilly December day in 1963, when Pratt & Whitney Canada's turbine engine design group celebrated certification of their diminutive new PT6 turboprop, they had no idea they had given birth to one of the world's most successful propulsion concepts.
Fifty years later, with more than 51,000 engines delivered and new ones rolling off the production line at more than 1,000 per year, the PT6 is accepted as a design classic to be admired, imitated and challenged. “It is the only engine in the world that can be found in an aircraft sitting on a mountain peak, ready to take off at 15,000 ft., or just as easily next to a  787 on the ramp of a busy airport,” says P&WC President John Saabas.
In a world where the term “design classic” is frequently abused, the notion that classics cannot be invented but have to evolve is epitomized by the PT6. From humble beginnings with the delivery of the first production PT6A-6s to Beech 50 years ago, the family has branched out to include a bewildering range of turboprops and turboshafts which are flown by over 7,200 operators in more than 180 countries. The trunk of the family tree has grown to include 69 turboprop versions covering the 500-2,000-shp power range, while three other main branches—the PT6T Twinpac, PT6B and PT6C—are formed by a variety of turboshafts covering the 1,000-2,000-shp range.
In all, the PT6 has evolved to include over 135 different build specifications, more than fulfilling its developers' original dream of providing a turbine replacement for light pistons, and sparking the birth of an entire generation of new fixed- and rotary-wing aircraft. “A whole network has been created to support it,” says Saabas, who notes that the engine has accumulated an unprecedented 384 million hours of flight time.
“The PT6 is more than just an engine; it is a culture,” he says. “Physically, the engine has changed very little. The barrel diameter has grown one inch, but the power has gone up four times, the power-to-weight ratio increased by 40%, and it now has 20% better performance in terms of specific fuel consumption. So the PT6 is a living, breathing family of engines.”
The PT6 played a fundamental part in establishing P&WC as a global player in its own right, helping to provide a development springboard for the company's growth through the 1970s, '80s and '90s, into a broad range of markets ranging from military trainers to regional airliners. Spinning off from the PT6 experience, P&WC went on to develop a family of turbofans, as well as larger turboprop and turboshaft engines. The Canadian manufacturer has certificated a remarkable 74 new engines in the past 19 years—including new versions of the PT6, such as the most recently certified PT6A-140 for theCaravan EX.
While its product line has been more recently bolstered by other new engines—including the PW300 and PW800 business-jet turbofans, as well as the PW210 turboshaft family—the PT6 continues to provide a solid foundation for the company as it recovers from the market downturn of the late 2000s. This saw overall engine deliveries fall to a low of 2,617 in 2011, the bulk of which was made up of turboprops, from a high of more than 3,963 engines in 2008. Now, as production continues to recover toward the 3,000-engine mark, around a third of it is sustained by the ever-popular PT6.
As with any perennial best-seller, P&WC now faces the inevitable and challenging issue of where to go next. Having essentially created its own market with the PT6, the engine maker knows it cannot sit still while the competition grows stronger (see page 48). The tempting “if it ain't broke, don't fix it” approach is not an option, and yet P&WC must carefully weigh the pros and cons of too much technology development versus too little as it plots the next move for the jewel in its family crown.
The first questions P&WC engineers have been asking are, “Has the company fundamentally exhausted the baseline PT6 of any further potential improvements? Or is it now a case of diminishing returns for the investment any such upgrade path would incur?” Absolutely 'no' to both, says Saabas.
“Incremental improvements will continue, starting with electronic controls. This improves the interface with modern cockpits,” he says.
However, recognizing that part of P&WC's conundrum with the PT6 succession strategy is meeting the needs across the very breadth of the market it helped develop and serve, Saabas is realistic about the need for a tactical approach. “People want different things. Not everyone wants dual-channel digital control. With the PT6, it is not a case of one size fits all. We're also looking at areas like a time-between-overhaul (TBO) life extension through the use of advanced materials and technologically improved combustion for less distortion.”
“One of the key messages is that we are still investing heavily in technology on the PT6,” says General Aviation Products Vice President Denis Parisien. “For example, for the PT6 A-140 just certified for the Caravan, we've injected technology, increased the size of the compressor and put technology in the hot end and improved specific fuel consumption (sfc) by up to 7%. So our intent is to carry on continuously improving the performance of the engine and make it in as wide as a variety of versions as we get close to 2,000 shp. Our next challenge is do we bring it above 2,000 shp? It's in our technology plan to see.”
Parisien asserts that “we can't stop injecting new technology,” and he highlights the parallel focus on reducing pilot workload and improving overall engine performance. The approach has been taken in the past, such as when P&WC continued to improve the JT15D while concurrently introducing its effective replacement, the PW500 turbofan. “We have a tradition to lead the next technology step,” he says. “That's why we have a big research and development effort underway, so when the market requirement is there, the technology is available.” From the fuel-burn-reduction perspective, “the approach is to look at how far we can push it with technology,” Parisien adds.
When it comes to achieving step-changes in performance—particularly in areas like fuel consumption—manufacturers have often resorted to wholesale changes in architecture to meet the ambitious targets expected of the next generation. But doing so could impact the fundamental simplicity (and therefore reliability) of the PT6's hallmark design. So this factor is being taken into consideration when it comes to next steps, says General Aviation general manager Nick Kanellias.
“The thing I hear most about is the reliability of the PT6. Reliability is the biggest factor and it is where we differentiate ourselves. That's where the people who designed the PT6 really got it right. The reason it has gone from 500 shp to 2,000 shp is because of the modular design. It just made it easier for us to offer a variety of options, as well as characteristic advantages such as reverse flow and foreign object damage (FOD) prevention,” he adds.
A related consideration is avoiding the temptation to make a new, or upgraded, PT6 too sophisticated. “We can't take the risk of taking away anything the customers rely on,” says Kanellias “It is paramount that any technology has to be consistent with what people have come to know. It also has to be something the customer can afford.” The same limitations would apply to whether any future PT6 upgrades could be offered for potential retrofit, he says.
However, deep in the bowels of P&WC's General Aviation organization, a future PT6 replacement effort has been underway for at least two years. Characterized by Saabas as a technology program, the initiative first emerged in public in late 2011. “We're also looking at a technology demonstration as well, particularly in areas that could be a step change,” Saabas says. “We've done demonstrations of some of the basic concepts and we will maybe marry those together. The goal is a 10-15% improvement in fuel burn. The timing of that is also a balance between what we have and where the market needs are going.”
Intriguingly P&WC's initial launch target for the new engine is the low power bracket where the PT6 originally began. “I think we'll start off at the smaller end. It's a different demographic with avgas,” says Saabas, referring to the widespread use of this type of fuel in general aviation. “We will try it out there, without calling it a PT6, and if it is successful, we'll migrate it across. We replaced ourselves on the JT15D with the PW500 and we can do it again with the PT6,” he adds.
If successful, the new engine is expected to be quickly extended up the power range. “We know we have to bring a new game to the table,” says Maria Della Posta, P&WC senior vice president of sales and marketing. “There are also changing market drivers, with new growth from emerging counties and the next generation of pilots. The current market is in need of more power—and when we introduce a new component with more advanced materials that offers more, we make it available to the others. We are bringing to the market things that are useful and practical.”
P&WC remains coy about design details. “There are just some basically smart ideas that have to be tested,” says Saabas, who adds that the engine maker is assuming the “3P” approach to development. “Production, preparedness and processes—we do that with any new product we introduce. But the key thing to the PT6 design and the market was its modularity and the ability to access the hot section on wing [without needing to remove the entire engine]. Those are the kind of features you want to maintain,” he says.
One potential option believed to be under study is a PT6-scale advanced derivative partially based on architectural elements of the recently developed PW210S. Like the PT6, this is a simple engine, with only five major rotating components. These include a two-stage centrifugal compressor driven by a single-stage turbine with a reverse-flow combustor and a free, two-stage power turbine. The power turbine drives the output shaft through a front-mounted combined reduction and accessory gearbox. The PW210 cycle also has more performance and additional temperature margin, making it fundamentally more capable. Equipped with a digital engine control system, it produces 10-20% more power for the same fuel consumption as current-generation engines and could make the logical jumping-off point for the next generation.
The company's recent experience with the development of two new engines, the PW600 for the very light jet market and thewith its bigger sister company, has given it invaluable insight into designing advanced production systems.
“Clearly, we'd use the PW600 process, just like we did with the new [PW1000G] facility at Mirabel, [Quebec,] where we apply them at a different scale,” says Saabas. The process is adapted from that used by German car maker Audi to build the R8 sports car. “The Audi R8 is a similar weight to the geared turbofan, so the PW1500G [version] conveyer system is the R8 system. We didn't have to build and develop anything new, and we didn't have to shake it down,” he says.
Tap the icon in the digital edition of AW&ST for more on the history of the PT6 and some of the aircraft it powers, including a time line and photos of some unusual applications, or go to AviationWeek.com/pt6