Wings over the world: The PT6 turboprop and turboshaft engine, Part 2

An Air Tractor AT-502 agricultural aircraft operated by Southeastern Aerial Crop Service Incorporated, Belle Glade State Municipal Airport, Florida, June 2013. Wikipedia.

Greetings to you, my reading friend, and welcome to the second part of this article on one of the most successful aeroengine of the 20th and 21st centuries, the PT6.

Did you know that United Aircraft of Canada Limited (UACL) of Longueuil, Québec, a firm known today as Pratt & Whitney Canada Incorporated, played a significant role in the introduction of a new type of aeroengine on agricultural aircraft? Well, it sure did.

One could say it first became involved in agricultural aviation, somewhat indirectly however, in the first half of the 1960s. By then, aviation gasoline, or Av-gas, was becoming more difficult to find as well as less economical than aviation kerosene, or Av-tur, used in turboprop engines. As early as 1964, James Aviation Limited of Hamilton, New Zealand, one of that country’s largest and oldest top dressing firm, came to the conclusion that future top dressing aircraft could very well be powered by turboprop engines.

What is top dressing, you ask? A good question. If I may be permitted to quote myself, top dressing, the spreading of fertiliser over large areas to help feed cattle and sheep, may or may not fall within the purview of agricultural aviation. Many authors deem it closer to ranching than agriculture, but back to the mid-1960s.

At the time, James Aviation was one of the biggest, if not the biggest, user of the American-designed Fletcher FU-24 agricultural aircraft. Given this fact, it was hardly surprising that this aircraft was the one chosen to test the possible use of turboprop engines in top dressing. Thus, in 1966, a New Zealand aircraft manufacturing firm, Air Parts (N.Z.) Limited, began to redesign the FU-24, an aircraft it had been producing under license since 1964, for use with a UACL PT6 turboprop engine.

Construction of a prototype started in November 1966. Air New Zealand Proprietary Limited, the country’s main air carrier, may have been involved in the project. In any event, James Aviation appear to have paid most of the moolah needed to pay for the construction of the prototype. The lighter and more powerful turboprop engine made it possible to increase the payload while, at the same time, improving performance. The Air Parts Fletcher 1060, or FU-1060 Turbo-Fletcher, first flew in June or July 1967. This machine was the first turbine-powered agricultural airplane in the world.

Air Parts (N.Z.) seemingly hoped to launch the Fletcher 1060 while keeping the older, piston-powered FU-24 in production. As it had done with the FU-24, Air Parts (N.Z.) wanted to produce an agricultural as well as a utility / light transport version of the new machine.

Even though the Fletcher 1060 was granted a certification by the New Zealand Department of Civil Aviation in June 1969, no further aircraft of that type was built.

The one machine was used by James Aviation. Legally, it could carry a load comparable to that of an FU-24. In practice, however, ground crews were instructed to load with as much as they could. When conditions were especially good, the Fletcher 1060 could lift up to 1 500 kilograms (3 300 pounds) of fertiliser and still outperform a standard FU-24 carrying a one tonne (ton) load.

Unfortunately, the Fletcher 1060 was the very last New Zealand-built aircraft of this type not to have the reinforced wing introduced at the time. As a result, the continuous overloading significantly weakened its structure. Eventually, James Aviation contacted the FU-24’s manufacturer, the newly formed New Zealand Aerospace Industries Limited, to see how much it would cost to rebuild its unique aircraft.

The price quoted was very high, too high in fact. In addition, the aircraft’s PT6, lacking an effective air filter, had proved somewhat troublesome and expensive to maintain. Faced with these problems, James Aviation, by then New Zealand’s largest top dressing firm, retired the Fletcher 1060 in August 1979.

Some years before that, in May of 1972 to be more precise, James Aviation had contacted another New Zealand firm, Aero Engine Services Limited, and asked it to inquire into the possibility of creating an updated version of the Fletcher 1060. At the time, James Aviation reasoned that the path to the future laid in mounting second-hand PT6s in suitably-modified FU-24 airframes. Indeed, an engineer by the name of Patrick “Pat” Monk started work on such a design in October 1973. Unfortunately, he rapidly came to the conclusion that the only way he could come up with an aircraft more productive than a conventional FU-24 was to use new engines in an entirely redesigned airframe. James Aviation’s original idea went no further.

In spite of the difficulties with the Fletcher 1060, development of the basic FU-24 design did continue, under New Zealand Aerospace Industries, a firm reconstituted as Pacific Aerospace Corporation Limited in 1982. A turboprop version of the aircraft, the Cresco, meaning “I grow” in Latin, first flew in February 1979. This machine, designed by Monk, was powered by an American engine, however. In 1992, Pacific Aerospace launched a new version of its agricultural and utility aircraft which was powered by a PT6. This aircraft may, I repeat may, still be in production in 2020.

This being said (typed?), it was really in 1974 that UACL became seriously interested in agricultural aviation. That year, an American aerial applicator came to Longueuil for the overhaul of the crankshaft of the Pratt & Whitney R-1340 Wasp engine, one of the mainstays of the aerial application industry in the United States, on one of its aircraft. During a conversation with a market development representative at UACL, Derek Emmerson, this operator mentioned that the annual convention of the National Agricultural Aviation Association was about to begin.

Emmerson quickly prepared a display on the PT6 and left for Florida. He was truly stunned to discover that virtually all the operators he met wanted to talk about the possibility of installing a PT6 on an agricultural aircraft. Indeed, Leland Snow, a well-known operator and designer of agricultural aircraft, suggested to Emmerson that it would be in UACL’s interest to go after this market. Upon his return, Emmerson discussed the idea with his superiors. It was agreed to have a go at it.

The Canadian team’s first task, and a difficult one it proved to be, was to convince agricultural aircraft makers that a turbine-powered version of their machines would find customers. The main hurdle was cost. An overhauled Wasp piston engine cost approximately $11 000. A PT6, on the other hand, would increase the cost of an agricultural aircraft by $ 100 000 or so.

Grumman American Aviation Corporation, a subsidiary of Grumman Aerospace Corporation responsible for marketing the famous Grumman G-164 Ag-Cat aircraft, showed no interest. Still, one or more of its representatives suggested that UACL contact a number of operators in the southern United States. In that region, aerial application played an essential role in large-scale agriculture. As a result of this, the Canadian firm could get a pretty good idea of what the potential market for PT6-powered aircraft might be.

In May 1975, Emmerson visited a number of operators in a 160-kilometre (100 miles) long strip along the coast of the Gulf of Mexico, in both Texas and Louisiana, where rice was cultivated in huge areas. The producers in that region were thoroughly dependent on aerial seeding, fertilising and spraying of both insecticides and herbicides. Indeed, 85 % of all this work was being done by air. The 400 or so aircraft required for the 7 or 8 applications each crop needed were piston-powered aircraft like the aforementioned Ag-Cat, the Cessna Model 188 Agwagon, and the Piper PA-25 Pawnee.

One of the operators contacted by Emmerson was Lyon Air Service Incorporated in Louisiana. The Canadian representative spent several days with people of this sizeable firm, observing spraying procedures and talking to employees of all sorts. He even bought a stopwatch to time the many phases of a spraying operation.

As well, Emmerson spent some time at Air Rice Incorporated in Texas, another important operator of the southern states. Like a number of his competitors, the firm’s boss, William Priester “Bill” Cardiff, expressed some concern regarding the long term reliability of the Wasp. Engines were starting to fail which meant that operators would find it increasingly difficult to keep up with the growers’ demands. Thus, Cardiff’s reaction to UACL’s project was quite enthusiastic. He was confident that a PT6-powered aircraft would bring some added benefits thanks to its greater payload, faster turnarounds, reduced pilot fatigue and smaller maintenance costs.

Indeed, Cardiff suggested that UACL loan a PT6 to help with the conversion of an aircraft. The work would be done by a Texas firm, Frakes Aviation Incorporated. The aircraft chosen, an Ag-Cat, became the Frakes / Grumman Turbo-Cat. It first flew in the spring of 1976. Trials showed that its productivity was 60 % greater than that of piston-powered agricultural aircraft. A number of aerial applicators anxious to use the new engine approached Frakes Aviation to have their machines converted. In fact, the firm eventually offered to convert some versions of the Ag-Cat.

Frakes Aviation’s boss, Joseph Frederick “Joe / Fred” Frakes, had already worked with UACL. In 1967, he had begun work on a PT6-powered conversion of the Grumman G-73 Mallard twin-engined amphibian. This machine, known as the Frakes Turbo-Mallard, had flown for the first time in 1969 and proved successful. Many conversions were produced in the 1970s. Several / many of them were still flying in the United States and elsewhere in 2020. But back to our story.

What happened afterward remains somewhat obscure. We do know that a second PT6-powered prototype flew in July 1978. This aircraft may have been an Ag-Cat built by Schweizer Aircraft Corporation, the sole producer of the piston-powered Ag-Cats since day one, in 1959. Like its predecessor, this machine had been converted by Frakes Aviation. Air Rice evaluated the aircraft for some time. These trials showed that a production model of the turbine-powered Ag-Cat could be fitted with a less powerful version of the PT6 engine without any undue loss of performance or economy. This resulted in the conversion of yet another aircraft, apparently for use as a pre-production machine, in September 1978.

By then, American Jet Industries Incorporated had bought the 80 % holding of Grumman Aerospace in Grumman Aviation and made a cash offer to the holders of the remaining 20 %. The firm’s new owner renamed it Gulfstream Aviation Corporation. Starting in 1979, Schweizer Aircraft gradually transferred production of the piston-powered Ag-Cat to Gulfstream Aviation. Shortly thereafter, it started to market its new PT6-powered Turbo Ag-Cat. This state of affair, however, did not last very long.

As of January 1981, Schweizer Aircraft acquired all production rights for the Ag-Cat family of aircraft from Gulfstream Aviation. While the latter kept a small number of Turbo Ag-Cats for sale, Schweizer Aircraft embarked on an evaluation of the potential market for these aircraft. It eventually came to the conclusion that said market was not quite large enough for aircraft with the very large hopper then in production. Indeed, only 22 of these were eventually produced.

Instead, Schweizer Aircraft proposed its own version, the Ag-Cat Turbine or Ag-Cat Super-B Turbine. Even so, most aircraft it produced were still fitted with piston engines. One only had to read the prices of each version to understand why: less than US $ 150 000 for an aircraft powered by a Wasp piston engine and almost US $ 375 000 for one powered by a PT6.

Another person who was seriously interested in turbine-powered agricultural aircraft was Frederick Ayres, the boss of Ayres Corporation. On the same site as this firm, the General Aviation Division of Rockwell International Corporation was producing its Thrush Commander agricultural aircraft. Faced with an increasingly scarce supply of good Wasp engines, the firm eventually decided to use the Wright R-1300 Cyclone 7. The operators, loyal to the Wasp, were not happy with this idea. Ayres proposed a solution: fitting a Thrush Commander with a PT6. Rockwell International showed but little interest.

Seeing this, Ayres contacted UACL. A deal was signed and Ayres hired an American firm, Serv-Aero Engineering Incorporated, to convert a Thrush Commander. This machine flew in September 1975. Shortly thereafter, 5 conversion kits were sold to Sterner Aero Aktiebolaget, a Swedish agricultural aviation firm which already flew a number of Thrush Commander. The first 30 Turbo-Thrushes were aircraft originally built with the Cyclone 7 that their manufacturer had been unable to sell.

Strangely enough, Rockwell International would not allow any aircraft to leave the factory without being fitted and certified with a piston engine. In a further intriguing twist, a single Wasp engine was moved back and forth between the production line and the conversion site. It would be mounted on each new Turbo-Thrush so that it could be certified in accordance with Rockwell International’s wishes. Once this was done, the aircraft would be fitted with a PT6 and made ready for delivery. The Wasp engine would then go back to the factory to start the process again.

Overall, it seems that the first 40 or so conversion kits were made by Serv-Aero Engineering. Ayres took over production of the kits at that point. Then, in late 1977, this firm acquired both the manufacturing rights for the Thrush Commander and the factory. The success of the turbine-powered conversions was one of the main reasons behind this decision.

Twelve conversion kits and 30 aircraft were produced in 1978. By the end of the decade, 50 to 60 aircraft were sold each year. As of January 1980, no less than 125 Turbo-Thrushes were in service and 4 aircraft were being produced each month. The recession of the early 1980s, not to mention farm surpluses, high interest rates and the rising American dollar, had quite an effect on the firm’s business.

In 1982, Ayres, arguably the world’s leading manufacturer of agricultural aircraft, won a United States State Department contract to develop a special 2-seat version known as the NEDS. This acronym described the aircraft’s mission with great accuracy; it stood for Narcotics Eradication Delivery System.

Nineteen aircraft were delivered between 1983 and 1988, to the department’s International Narcotics Matters Bureau. They were used on Operation Roundup, a series of drug eradication flights, in half a dozen countries:

- in Burma, today’s Myanmar, Thailand and Colombia, against poppy crops,

- in Mexico, against marihuana and poppy crops, and

- in Belize and Guatemala, against marihuana crops.

And yes, the liquid chemical herbicide used was Roundup, designed to greatly accelerate the plants’ growth, thus ensuring that they would wilt and die very fast.

To ensure that these small fields, all but invisible in the jungle, could be both found and treated properly, the NEDS were equipped with advanced navigational aids. A quiet propeller made it easier for them to fall upon a field without being detected. Given the fact that the aircraft performed their mission at pretty low speed (195 to 210 kilometres/hour / 120 to 130 miles/hour), special armour plate had to be fitted to the crew and engine compartments.

Fully aware that a military version of its drug-fighting aircraft could be of interest to a number of air forces thanks to its simplicity and ruggedness, its load-lifting capability, and its ability to take off and land on short, unprepared landing strips, Ayres developed the V-1-A Vigilante. This low cost surveillance and close support aircraft was designed in part with the cooperation of the United States State Department and the United States Army. The prototype flew in May 1989. Production aircraft were fitted with an American engine.

Over the years, other agricultural aircraft have been fitted with PT6 engines. The only manufacturer which remained in operation in 2020 seemed to be the American firm Air Tractor Incorporated. And yes, it was with one of its machines that today’s article began.

And that is it for today. Be careful, my reading friend.

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Rénald Fortier