The PT6 engine marks its 50th anniversary this year, and this provides us with an opportunity to recognize and applaud the decisions made by the original engine’s engineering team. The team made decisions that detractors thought were counterintuitive at a time when the piston engine ruled the small-aircraft aviation world. But they were, in fact, brilliant choices that directly contributed to the enduring success of the now legendary engine.

The first complete PT6 engine runs in the test cell in February 1960.

The first complete PT6 engine runs in the test cell in February 1960. P&WC Archives (Records and Information Management).

“The team’s first decision – to start with a 500 shaft horsepower (shp) engine – was a smart one,” says Luc Landry, Senior Manager, Business Development, General Aviation Products, “because it’s often much easier to create a small engine, prove it and then grow it.”

Allan Newland, who was chief designer on the PT6 engine team, with the first engine. It is enmeshed in instrumentation wiring.

Allan Newland, who was chief designer on the PT6 engine team, with the first engine. It is enmeshed in instrumentation wiring. P&WC Archives (Records and Information Management).

The traditional approach at the time was to use a single shaft layout connecting both the gas generator and the power turbine sections of the engine. This approach tended to limit engine performance in the operating envelope, increased operability restrictions and decreased flexibility in both operation and maintenance. The engineers decided that all of these aspects needed to be addressed in their new engine design. 

Following the PT6 engine’s 50 hour pre-flight rating test, engine components are being examined by Elvie Smith, LCMDR J.Barnes (USN), W/C W.R. Cole (RCAF) and C. Stim (US Army).

Following the PT6 engine’s 50 hour pre-flight rating test, engine components are being examined by Elvie Smith, LCMDR J.Barnes (USN), W/C W.R. Cole (RCAF) and C. Stim (US Army). P&WC Archives (Records and Information Management).

The team chose a free turbine layout whereby the power source, the gas generator turbine, is not connected directly to the shaft that turns the propeller.

Because the turbine is not directly connected to the shaft, starting the engine is much simpler. The starter needs only to ignite the gas generator, which then sets the rest of the engine in motion. If the power turbine and the shaft were connected, the starter would have to effectively start the entire engine, produce much more torque therefore requiring a much larger starting system. “Think of it as the equivalent of starting a car engine when it’s in neutral compared to starting it when it’s in drive,” says Landry.

The free turbine and the selection of the fuel-distribution system also allow the use of a simple hydro-mechanical engine control system. The engine control system does exactly what the term suggests: It controls the engine, making sure that it doesn’t over-speed, ensuring that it gets enough fuel to complete the task that’s being asked of it, etc. The configuration also allows for a selection of a wide range of propeller speeds during operation; this is an important feature that allows the propeller to absorb full takeoff power, give maximum thrust during the takeoff run and climb and improve overall engine and aircraft performance over a wide range of operating conditions, something that the original engineers intended. The basic architecture of the PT6 and its gearbox also provide a wide choice of propeller speeds for aircraft requiring either large or small propellers. Some aircraft require small propellers to provide clearance from the tarmac; the propellers must turn quickly to work efficiently. Large propellers must turn more slowly to work as intended.

Finally, the free turbine is analogous to a car transmission’s fluid coupling. “The original engineers knew this to be an important feature at the time,” says Landry. A free turbine helicopter engine needs a somewhat less complex simple sprag clutch arrangement to decouple the main rotor when compared to a fixed turbine engine.

The selected architecture also allows for a number of other benefits, including the use of a simple propeller, which is also known as a single-acting propeller because it is configured with a spring on one side and a hydraulic piston on the other.

“Had the engine required a double-acting propeller – which has a hydraulic piston on both sides – it would have added complexity to the propeller control system,” says Landry. “When you reduce complexity, you are not only reducing cost; single-acting propeller systems are known for their reliability. Every PT6 engine we’ve ever made uses a single-acting propeller.”

The original engineers also chose an opposed shaft layout, which is simpler than the traditional alternative: a concentric shaft layout. An opposed shaft layout has the two shafts pointing in opposite directions, separate from each other. The concentric layout places one shaft inside the other, adding to the complexity of design and potentially causing other concerns. As Landry says, simpler is better.

The layout selected also means that a screen can be used to cover the air inlet leading into the air chamber (known as the plenum) of the compressor. This screen prevents foreign object damage (FOD).

Given its design, the engine essentially can be split into two sections. “Because the power and gas generator sections are separate, a technician can open up the engine and access all the hot section parts for inspection or repair while the aircraft is in the field and the engine is on the wing,” says Landry. “That’s tremendously convenient when the alternative is to remove the engine from the wing and ship it to a shop for a hot section inspection.”

The complete PT6 engine compressor (three axial and one centrifugal stage, plus the compresor turbine assembly).

The complete PT6 engine compressor (three axial and one centrifugal stage, plus the compresor turbine assembly). P&WC Archives (Records and Information Management).

Finally, it’s by design that the PT6 engine’s power and specific fuel consumption have greatly improved over the years. Due to enhancements made to certain elements of the engine, including the compressor, the turbine and the combustion chamber, today’s PT6 engine is up to four times more powerful than the engine of 1963. It has a 40 percent better power-to-weight ratio and up to 20 percent better specific fuel consumption than the original. “We have been able to use advanced design techniques to make the parts more aerodynamically efficient and employ higher capability materials in their manufacture thereby enhancing engine performance without substantially increasing its size,” concludes Landry. “What the original engineers didn’t know was that the engine they were building would become legendary for its reliability. Today, the PT6’s basic in-flight shutdown (IFSD) is one event per one million hours of flight, compared to an industry standard of 10 events per one million hours.”

To read about other improvements made to the PT6 engine over the years, please click here.

COVER PHOTO: PT6A-65AG cutaway engine.


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