Estimated reading time 17 minutes, 36 seconds.
Sixty years ago, the sound of a small turboprop aircraft flying overhead was more fantasy than reality.
But all that changed on May 30, 1961, when the first Pratt & Whitney Canada (P&WC) PT6A turboprop engine made its first flight from Toronto’s Downsview Airport in a three-engine RCAF Beech 18 Expeditor 3T, piloted by R.H “Bob” Fowler from de Havilland Aircraft of Canada and John MacNeil from P&WC.
As a company, P&WC believed that the aviation market was ready for a turboprop that could replace piston engines, and hired a team of young engineers to develop a breakthrough design that could meet the performance requirements at a price that aircraft manufacturers and their customers could afford.
Few, if any, in the small group of people that witnessed the first flight of the heavily modified 1943 Beech 18 (RCAF HB109, later CF-ZWY) could fully appreciate how the free turbine would transform aviation on a global scale.
Late last year, P&WC delivered its 50,000th PT6A turboprop engine (to Pilatus) and reported that the reliable engine has accumulated 410 million flight hours and counting – with 25,000 engines still active.
In 1957, the management of Canadian Pratt and Whitney Aircraft (as it was then known) made a strategic decision to enter the small gas turbine engine business.
Founded in Longueuil, Quebec, in 1928 as a sales and service business for Pratt & Whitney Aircraft (P&WA) radial engines used on bush planes, the company expanded into the manufacturing business in the early 1950s by making piston engines (P&WC R-1340 and Wright R-1820) and spare parts for Cold War-era military transports and trainers.
The sale of parts for the obsolete piston parts was profitable, but the Canadian executives (and a handful of Canadian shareholders) clearly recognized the future of aviation was gas turbines.
A small team of gas turbine experts from Canada and overseas were hired to build small gas turbines beneath the power range of its U.S. parent.
The first design, a 3,000-pound thrust turbojet to power Canadair’s new CL-41 Tutor jet trainer, was transferred to the U.S. (as the P&WA JT12).
The loss of its first original engine design to the U.S. shifted P&WC’s attention to a 450-shaft-horsepower gas turbine that could power light aircraft, helicopters, and innovative VTOL aircraft.
The company’s low-cost, low-risk approach to engine development favored a free-turbine design with opposed shafts and reverse flow — which placed the compressor at the rear of the engine, and the power turbine up front, with the air flow from back to front. The aerodynamic and mechanical design was conservative with an eye to later refinements.
In the early stages of the development program, a study was made to determine what aircraft would be the most suitable to carry out a flight test program on an engine in the 450 to 700 horsepower range.
Initially, a DC-3 with supercharged engines was favored, but “this was rejected for difficulties involved in stressing the nose for the turbine installation. The Expeditor required a minimum amount of strengthening in this area and was chosen for the job,” explained test pilot MacNeil in an article he wrote in 1962.
A flying testbed was needed to accurately duplicate the altitudes, temperatures, propeller loads, and icing conditions the PT6 would experience once in service.
Arrangements were made with the Canadian government and the RCAF to acquire a Beech 18 on loan for conversion for flight test purposes. DHC had more experience modifying airframes than P&WC, so they did the conversion which saw the nose tank replaced by a PT6.
The PT6-powered Beech 18 flew three times on March 30. The aircraft was found satisfactory and only 23 pounds of ballast was required to adjust the center of gravity. A wide range of test flying was soon underway, including aircraft handling and performance, specific fuel consumption, propeller handling, propeller drag, air starting, accessory loading, and noise/vibration levels.
While at Downsview, MacNeil also flew with DHC’s Fowler in an experimental DHC-3 Otter with large flaps and a General Electric J85-GE-7 turbojet in the rear fuselage — which had exhaust ducts on the side that could direct the thrust forwards or backwards.
After the Beech 18 was accepted, MacNeil flew the aircraft to St. Jean Airport and made a couple circuits of the Longueuil factory, so employees could see the engine flying for the first time.
The old Fairchild Aircraft of Canada airport was right next door to the engine factory, but the runway was short and not lighted. So the aircraft was based at St. Jean Airport just outside the Montreal Control Area, and the aircraft was maintained by Aircraft Industries of Canada.
During the development program, the aircraft was flown up to 26,000-foot altitudes and through a wide range of temperatures; it was possible to investigate engine handling, performance, specific fuel consumption, propeller handling, propeller drag, governor operation, and air starting. Accessory loading, skin temperatures, noise, and vibration levels were also investigated.
“The testbed is very quiet and smooth when the Wasp Juniors are feathered, and it is most impressive in comparison with the conventional piston engine aircraft,” wrote MacNeil in a September 1961 report.
An ordinary Beech 18 cruised at 150 mph and had a ceiling of 10,000 ft. The testbed was flown at more than 200 mph and up to 30,000 ft (with oxygen masks for crew).
Tests & Tours
In June 1962, the PT6 testbed was demonstrated to several aircraft manufacturers in the U.S., and logged 45 hours without a single unserviceability.
Since the PT6 was a free turbine design, aircraft manufacturers were concerned about having sufficient drag for a reasonably steep descent and landing. To their satisfaction, the approach drag was equivalent to that of a piston engine in the same horsepower category; hence, manufacturers were convinced that conventional descent and landing techniques would be applicable.
MacNeil carried out a series of “quite interesting” in-flight propeller reversing tests in 1963. “The aircraft is rather unstable in this configuration, and suffers from elevator buffet,” he said. “As reverse power is applied, the buffeting increases. . . . At this point [220 shp] a decision was made to stop further application of reverse power because elevator buffet was becoming too severe to manage, and the possibilities of flutter was a concern.”
In 1963, MacNeil took the testbed to Knob Lake (now Schefferville) in northern Quebec for winter trials. He departed Montreal with a temperature of 38 F (3 C) and landed 6.5 hours later at -21 F (-29 C).
The cold weather tests were cut short for another urgent tour of the U.S. in 1963, when aircraft manufacturers were gearing up to compete for a new U.S. military counter-insurgency (COIN) aircraft.
MacNeil and P&WC flight test engineer John Hunt criss-crossed the U.S. doing demonstrations for 16 different airframe manufacturers.
de Havilland Canada
The first fixed-wing customers for the PT6A were de Havilland Canada and Beech Aircraft.
The relationship between P&WC and DHC has always been close, since P&W started supplying R-985 Wasp Junior engines for the Beaver in 1947.
The first engine installation was a pair of PT6A-6 turboprops on the experimental Otter (still also fitted with a J85), which first flew on May 7, 1963 — and flew for two years. This was followed by the DHC-2 Mk III Turbo Beaver, which first flew on Dec. 30, 1963. The Ontario Provincial Air Service became the launch customer with an order for 17 Turbo Beavers.
P&WC was already providing spare parts for the R-985-powered Beech 18 when the two companies agreed to combine products in a PT6 test program in 1961.
It was therefore a great surprise when Beech unveiled a mockup of a pressurized turboprop at the National Business Aviation Association (NBAA) convention in September 1961, powered by a pair of Turbomeca Bastan turboprop engines made in France. Beech was marketing the French MS.760 Paris executive jet in North America at the time, which seemed to give Turbomeca a head start. But nothing came of the plans.
The first PT6 engines to fly in a Beech aircraft were given to the Wichita company for free, and used to convert a Model 65 Queen Air (military L-23F / NU-8F) from a pair of 340-horsepower Lycoming piston engines to PT6As.
Funded by the U.S. Army, the NU-8F flew in May 1963. This provided Beech with a stepping stone to launch the pressurized King Air 90 in August 1963, which flew for the first time on Jan. 20, 1964 – the 27th anniversary of the first flight of the Beech 18.
P&WC was first in line to be a King Air 90 customer and bought aircraft No. 3, which was registered CF-UAC and delivered in July 1964.
Through these early years, chief test pilot MacNeil and test pilot Ross Lennox flew the Beech 18 and Sikorsky CHSS-2 Sea King helicopters that P&WC was assembling in Longueuil for the Royal Canadian Navy.
The King Air’s “first to market” success helped establish Beech as the PT6’s largest single customer. Beech progressively introduced new models of the King Air as P&WC engineers were able to increase the power produced by the PT6.
During the 1960s, the PT6A established a solid base in the business aviation (King Air), utility (DHC-2 Mk III Turbo Beaver and Pilatus PC-6 Turbo Porter), and commuter airline markets (DHC-6 Twin Otter, Beech 99, Embraer EMB-110 Bandeirante).
The PT6 quickly started to demonstrate its reliability and time between overhauls began to increase, which helped bring operating costs down for commuter airlines.
Launch of the PT6 Twin Pac (two PT6 power sections with a combining gearbox) in the late 1960s firmly established the PT6 in the emerging medium twin helicopter market (Bell 212/UH-1N, Sikorsky S-58T, AH-1J).
The P&WC Beech 18, CF-ZWY-X, went to Ecole Nationale d’Aeronautique (ENA) at St. Hubert Airport in June 1981 after logging 719 flights in 1,068 flying hours.
In the last 40 years, many other business, commuter, utility, and training aircraft, as well as helicopters and even unmanned aerial vehicles, have come to life under PT6 power as the engine continually evolves.
The highest power PT6 today is the PT6C-67A, which draws on P&WC’s turboprop and turboshaft experience to power the innovative Leonardo AW609 tiltrotor aircraft.
Earlier this month, P&WC delivered the 100th PT6 E-Series turboprop to Switzerland to power Pilatus PC-12 NGX aircraft, marking yet another chapter in what is almost certainly one of Canada’s greatest contributions to the aerospace industry of the past 60 years.