The Amazing PT6A Story

The famous slogan says - 'Trust in God and Pratt & Whitney'. And it is largely thanks to the ubiquitous PT6A that this statement is still true.

- GUY LEITCH

THE INCREDIBLE RELIABILITY of the PT6A gave birth to a whole new class of aircraft: the single-engine turboprops approved for air charter ops at night and in IMC. The Pilatus PC-12 and Cessna Caravan have overwhelmingly demonstrated how reliable the PT-6 is.

The Story of the PT6A

The genesis of the PT6A begins after World War 2 when it was realised the radial engines were fast becoming obsolete in the face of the jet engine revolution. However general aviation still needed propellor-driven planes with engines between 400 and 1000 hp.

In 1957, recognising the need for radial engine replacement, Pratt & Whitney Canada (PWC) assembled a team of Canadian and British engineers to replace the large radials. At the time the British were leading the field with their Rolls Royce Dart.

Detailed design on what was called the PT6A-6 (because it was the sixth engine development) started in January 1959. The team’s goals were clear: reliability; long maintenance intervals; low noise; high power; compact size; safety and utility.

After much contemplation the team came up with a truly original idea – the reverse flow free turbine turboprop. Just how extraordinary and radical a design this was, is hard to imagine now.

The free turbine uses separate power turbine and gas generator shafts. This was chosen as it minimises engineout drag while preserving fast response to power changes and enabling easier starts in extreme cold. The free-turbine design also permits on-wing maintenance and easy access to the PT6A hot section.