By Carroll McCormick
In 1963, the Beech Aircraft Company installed the very first production model of Pratt & Whitney Canada’s (P&WC) new PT6 gas turbine engines in its proof-of-concept Beech 87.
By Carroll McCormick
In 1963, the Beech Aircraft Company installed the very first production model of Pratt & Whitney Canada’s (P&WC) new PT6 gas turbine engines in its proof-of-concept Beech 87. Fifty years later, the PT6 engine has found more than 130 different applications in commercial, military, general aviation, business, helicopter, agricultural and utility aircraft.
|In 2010, Pilatus delivered its 1,000th PC-12, powered with a 1,200 SHP PT6 engine. PHOTO: P&WC
With more than 390 million hours of flight time logged, it is no wonder P&WC likes to say the PT6 engine opened up the world. To date, P&WC has manufactured 42,000 PT6A engines, the “A” designating turboprop engines for fixed-wing aircraft, and another 10,000 PT6B, PT6C and PT6T engines, the “B,” “C” and “T” designating turboshaft engines for helicopters.
Fully half of P&WC’s annual engine assembly production is devoted to the PT6 engine: About 100,000 square feet of production area in Lethbridge, Alta., Longueuil, Que. and Bridgeport, W.V. are devoted to assembling more than 1,000 PT6 engines a year, with a surge capacity capable of boosting that by several hundred more units a year.
When P&WC decided in 1957 to build a turboprop engine for a target market then powered by piston engines, company engineers made a seminal decision: in a departure from other gas turbine engines of the time, P&WC would design the new gas turbine with an opposed shaft, in which the compression and power sections are placed end to end and are uncoupled; that is, the compression and power sections can turn at different speeds.
In the conventional, non-opposed shaft design, the shaft for the power end is inside the compression shaft, requiring more complexity to, for instance, meet varying power requirements during flight.
From this single design feature flow several key advantages of the PT6 engine: lighter components, since the engine does not have to turn the propeller or helicopter blades during startup; a higher power-to-weight ratio; a simpler and modular design that, among other things, enables a wide choice of propeller speeds; easier maintenance; and greater reliability.
“At the time of the PT6 development, single-engine turboprops were not allowed to operate for commercial instrument flight rule flights because they did not meet the reliability threshold,” says Denis Parisien, vice-president of general aviation at P&WC.
In-flight shutdowns (IFSD) is one measure of an engine’s reliability. As the decades flew by and flight hours climbed, the PT6 engine established an IFSD rate that has consistently been only one-third of today’s turbine industry standard of 10 events per million hours.
Ease of maintenance translates into aircraft technicians in all corners of the world being able to complete all essential line maintenance tasks. “You can do most of the maintenance in the field. This was completely different from the other engines of that time. The opposed shaft allows you to remove the power section on-wing. This allows easy access for turbine inspections and maintenance. This is a big advantage,” explains Nick Kanellias, general manager of sales and marketing at P&WC.
Keystone Air Service Ltd., in St. Andrews, Man. operates a shop for its Beech 99 and three Beechcraft King Air planes which are equipped with PT6 turboprops. “They are user-friendly engines. Usually engineers design engines for longevity and structural integrity, but the PT6 designers also kept in mind that maintenance personnel have to work on it. The hot section maintenance is done on the wing. It reduces downtime,” says Dan Bourgouin, director of maintenance at Keystone Aircraft Maintenance Ltd.
The time between overhauls (TBO) of the engine is far longer than that of the piston engines the PT6 was designed to compete against – another plus for operators. “The first TBO is 3,600 hours, but some fleets go to well over 8,000 hours to TBO, depending on the trend monitoring, etcetera,” Parisien says.
The first PT6 engine had 500 shaft horsepower (SHP), but that was just the warm up. P&WC now offers 90 certified variations ranging from 500 SHP to 2,000 SHP. “Because of the design it is easier to mix and match components, such as different gear boxes and accessories for various aircraft applications,” Parisien explains.
Even as the range of SHP and the variations have grown, the engine size has changed very little. “Because of the opposed shaft the engine got longer and more powerful without making the engine larger in diameter, or changing the aircraft configuration,” Kanellias says.
For example, PT6 engines in the “small” class – 500 to 900 SHP – range in size from 21 to 25 inches high, 21.5 inches wide and from 61.5 to 64 inches long. Those in the “large” class – 700 to 1,700 SHP – are 22 inches high, 19.5 inches wide and from 69 to 75.5 inches long. This helps explain how Beechcraft can use several different PT6 engine models – eight in all – in its King Air, and can accommodate requests to install other PT6 engine models as aftermarket modifications.
Thanks to advances such as better aerodynamics, a more efficient compressor and higher-temperature alloys, P&WC has increased the engine’s power by 400 per cent and its power-to-weight ratio by 40 per cent. Here are just a few of the improvements:
In 1973, the PT6A-41 received a two-stage power turbine, increasing the engine’s power and fuel efficiency.
In 1984, a new technology on the PT6A-65, called first-stage integral bladed rotor, reduced the number of engine parts and increased efficiency.
In 1993, P&WC introduced single-crystal blade technology to the PT6A-67A, allowing the engine to run at higher temperatures and deliver more power for the same-size engine.
The mid-1990s to the early 2000s saw many other improvements that contributed to lower fuel consumption and more component durability and power.
Many manufacturers have chosen the PT6 engine, including Air Tractor, Cessna, Daher-Socata, Embraer, Piaggio, Pilatus, Piper, Thrush and Viking. It has appeared in more Beechcraft King Air planes than any other type, however. In continuous production since 1963, the King Air had accumulated 45 million hours of flight by the time Hawker Beechcraft delivered its 7,000th PT6A-powered King Air last year.
Beech chose that first production unit of the PT6 because it was the most fuel efficient, had a more robust design and offered the best value at the time, according to Shawn Vick, executive vice-president of sales and marketing at Beechcraft. Since then the aircraft manufacturer has installed some 17,350 PT6 engines on its various single and multi-engine aircraft. “It has been a good choice due to its ease of operation and maintainability, performance in severe and harsh conditions, fuel efficiency and tremendous power for its light weight,” Vick explains.
The turboshaft versions, including the PT6T Twin-Pac, power helicopters such as the Sikorsky, AgustaWestland, Eurocopter and Bell.
The PT6A engine has fans outside aviation circles too: an industrial version, called the ST6, has been installed in Lotus racing cars, speedboats and even snowplows. CN operated a PT6A-powered passenger train between Montreal and Toronto from 1973 to 1982 that regularly cruised at almost 120 miles per hour.
Fifty years on, P&WC continues to build on the engine’s original design. The company’s latest version of the PT6A, the 867-SHP PT6A-140, powers Cessna’s 208B Grand Caravan EX, which the Federal Aviation Administration certified only this year. As P&WC puts it, “The dependability and versatility of the PT6A engine family continues to earn the highest respect from pilots worldwide.” Respect and dependability indeed – the mark of a reliable stalwart that has changed the aviation world.