The story of the Beechcraft King Air is also the story of the Pratt & Whitney Canada PT6, the engine that has powered the aircraft since its first flight in 1964.
For now more than half a century, this uniquely ingenious pairing has proven successful for both parties, but especially so for the PT6.
Now it its 51st year of flight operations, the PT6 family has amassed all the turbine-driven records for longevity and sheer numbers, with 52,000 copies of 90 models in service, 130 applications certificated, and the fleet approaching 400 million flight hours.
These impressive figures speak to the diversity of a remarkably flexible turbine architecture. The power range has evolved by nearly 400% over five decades, spanning a range of 500shp to 1,950shp. The applications, meanwhile, cover fixed-wing, helicopters, a tiltrotor and auxiliary power units.
It all came from the Canadian subsidiary of P&W. Its invention sprung during a desperate period of transition from its roots as a licensed maker of Wasps, Hornets and other piston-driven aircraft engines to what was then an uncertain market for light turbine engines.
A small team of design engineers – immortalised in company lore as the “Dirty Dozen” – came up with the key ideas that formed the PT6 in the late-1950s.
The workmanlike reputation of the PT6 belies how unconventional and daring it seemed as it entered testing in the early 1960s.
As a reverse-flow, free turbine engine, the PT6 departs from traditional gas turbines in two ways. First, the air inlet is located at the rear of the engine nacelle in relation to the nose of the aircraft, and the air flows from the back of the engine to exhaust ports in the front. Second, the power turbine is not mechanically linked to the shaft that drives the air through the compressor and combustor, but sits behind a single-stage compressor turbine and drives the propeller in a fixed-wing application like a pin-wheel.
The Dirty Dozen may have had various reasons for the arriving at such an architecture, but the result proved to be practical and highly adaptable.
“Both ideas matched together and created a real breakthrough in design for this category of engine,” says Denis Parisien, vice-president of general aviation for Pratt & Whitney Canada.
The reverse flow meant that the air inlet could be protected from ingesting debris. The free turbine made the engine modular and more efficient.
Over the decades, the core architecture has been preserved, although the components have been improved. P&WC moved to a first staged integrally bladed rotor in the 1980s and a single-crystal turbine blade material in the early 1990s, borrowing technology pioneered on P&W's larger commercial turbofan engines.
The P&WC engine has been so dominant in its power class that only now is it facing competition for new, fixed-wing applications. General Electric acquired the Czech-designed Walter M601, which is now renamed the H80. Though progress has been slow, the H80 has been approved for applications once reserved for the PT6, including a re-engining programme for the King Air C90 by Nextant Aerospace.
But the PT6 continues to adapt more than a half century after it was introduced. P&WC introduced the PT6A-140 in 2012, providing about 7% better specific fuel consumption. The next step for improvement may be adding affordable engine electronic controls on smaller versions of the PT6.
“We will always have projects at Pratt Canada to push those boundaries,” says Parisien. “The capability of the PT6 today is still very relevant for many years to come, in spite of the fact that we always work on the new technology to improve it.”
Parisien, however, also notes that the Connecticut-based parent company is deploying several new technologies in the PurePower PW1000G turbofan engine – and not just a reduction gear, which is a familiar component already to the PT6. But the key issue for introducing new technology on the PT6 is keeping the engine affordable and practical.