PT6
A PT6A-20 on display at theCanada Aviation and Space Museum
Type	Turboprop /turboshaft
National origin	Canada
Manufacturer	Pratt & Whitney Canada
First run	1960[1]
Major applications	AgustaWestland AW139 Beech King Air andSuper King Air Cessna 208 Caravan de Havilland Canada DHC-6 Twin Otter Pilatus PC-12 Piper M700 Fury
Number built	64,000 (as of February 2023)[2]
Variants	Pratt & Whitney Canada PT6T

ThePratt & Whitney Canada PT6 is aturbopropaircraft engine produced byPratt & Whitney Canada.It was designed in 1958, first flew on 30 May 1961, entered service in 1964, and has been continuously updated since.The PT6 consists of two basic sections: a gas generator with accessory gearbox, and a free-power turbine with reduction gearbox. In aircraft, the engine is often mounted "backwards," with the intake at the rear and the exhaust at the front, so that the turbine is directly connected to the propeller.Many variants of the PT6 have been produced, not only as turboprops but also asturboshaft engines for helicopters, land vehicles, hovercraft, and boats; as auxiliary power units; and for industrial uses. By November 2015, 51,000 had been produced, which had logged 400 million flight hours from 1963 to 2016. It is known for its reliability, with an in-flight shutdown rate of 1 per 651,126 hours in 2016.ThePT6A turboprop engine covers the power range between 580 and 1,940 shp (430 and 1,450 kW), while thePT6B/C areturboshaft variants for helicopters.

In 1956, Pratt & Whitney Canada's (PWC) president, Ronald Riley, ordered engineering manager Dick Guthrie to hire a team of gas turbine specialists to design a small gas turbine engine. Demand for theWaspradial engine was still strong and its production was profitable but the aim was to become Canada's prime engine company by focusing on a small gas turbine engine. Riley gave Guthrie a modest budget ofC$100,000. Guthrie recruited twelve engineers with experience gained at various places including theNational Research Council inOttawa,Orenda Engines inOntario, Bristol Aero Engines andBlackburn Aircraft.^[3] They completed the detailed design of an engine for Canadair's small jet trainer, theCL-41. It was a 3,000-pound-force (13 kN) thrust turbojet but the design was taken over by P&WA who developed it into thePratt & Whitney JT12. The team had to wait for market assessments to define their next engine, a 450 shaft horsepower (340 kW) turboprop for twin-engined aircraft, the PT6.^[4] The early development of the PT6, which first flew in May-June 1961,^[1][5] was beset with engineering problems, cost overruns and lack of sales. It was almost cancelled.^[4] The team lacked the ability to deal with the technical difficulties, i.e. how to develop the engine, because, as one of the teamElvie Smith recalled, they came from research and design backgrounds. They learned how to run a development program, such as testing around the clock rather than on one shift, from a PWA team which directed the development for several months.^[6]

The PT6 first flew on 30 May 1961, mounted as a third engine in the nose of aBeech 18 aircraft which had been converted byde Havilland at its Downsview facility inNorth York, Ontario. Full-scale production started in 1963, with service entry the following year. The Beech 18 continued as a PT6 and propeller flying test-bed until it was replaced with aBeech King Air in 1980. The King Air test-engine or propeller replaced one of the standard ones. In 1974 the Beech 18 had been unable to fly fast enough and high enough to test the PT6A-50 for thede Havilland Canada Dash 7 so aVickers Viscount was modified as a PT6 test-bed with a Dash-7 installation in the nose.^[7]

The first production PT6 model, the PT6A-6, was certificated in December 1963. The first application was theBeech Queen Air, enticing the U.S. Army to buy a fleet of theU-21 Ute variant. This helped launch theKing Air with Beechcraft selling about 7,000 by 2012.^[8] From 1963 to 2016power-to-weight ratio was improved by 50%,brake specific fuel consumption by 20% andoverall pressure ratio reached 14:1.^[9]Its development continues and while today its basic configuration is the same as in 1964, updates have included a cooled first-stage turbine vane, additional compressor and turbine stages andsingle-crystal turbine blades in the early 1990s. Its pressure ratio is 13:1 in theAgustaWestland AW609 tiltrotor, the highest that can be used without cooled turbine blades.^[10]

In response to theGeneral Electric GE93, in 2017 Pratt & Whitney Canada started testing core technology and systems for a proposed 2,000 shp (1,500 kW) engine to replace the most powerful versions of the PT6.^[11]It was considered likely to be a development of the PT6C core, and would fit between the 1,750 shp (1,300 kW) PT6C-67C/E and the 2,300 shp (1,700 kW)PW100 family. It was expected to be ready to launch by the end of 2017 for an initialhelicopter platform with a 10-15% reduction inbrake specific fuel consumption.^[12]This 2,000 hp engine would target a possible new market such as a Super PC-12, a more powerful TBM, or a bigger King Air.^[13]

Whende Havilland Canada asked for a much larger engine for theDHC-8, roughly twice the power of the Large PT6, Pratt & Whitney Canada responded with a new design initially known as the PT7, later renamedPratt & Whitney Canada PW100.

The rate at which parts deteriorate in a gas turbine is unbalanced insofar as the hottest parts need replacing or repairing more often than the cooler-running parts. If the hotter parts can be removed without disturbing the rest of the engine, for example without removing the complete engine from the aircraft, maintenance costs are reduced. It was achieved with the PT6 by having the hottest parts, the gas generator turbine and combustor, at the propeller end. They are removed without disturbing the rest of the engine with its connections to the aircraft. This arrangement was patented by designer Newland, one of the original PT6 team.^[14] A similar general arrangement with a free-turbine power take-off at the exhaust end (the 1,000 shp (750 kW) P.181 engine) had been shown byArmstrong Siddeley Motors at theFarnborough Airshow in 1957.^[15]

An early design improvement, incorporated in the PT6A-20,^[16] was the pipe diffuser patented by Vrana, another of the original PT6 team.^[17] It replaced the vaned type diffuser used in centrifugal compressors. The pipe diffuser became standard design practice for P&WC.^[18] Another design change improved the part-speed functioning of the compressor. It is common to bleed air from a compressor to make it work properly at low engine speeds. The PT6 has a bleed arrangement which reuses the bleed air by returning it in a tangential direction at the entry to the compressor, an idea patented by Schaum et al. and titled "Turbine Engine With Induced Pre-Swirl at Compressor Inlet".^[19] It acts like a variable vane and is known as a "Jet-Flap".

All versions of the engine consist of two sections that can be easily separated for maintenance: a gas generator supplies hot pressurized gas to afree power turbine.^[20] The starter has to accelerate only the gas generator, making the engine easy to start, particularly in cold weather.^[20] Air enters the gas-generator through an inlet screen into the low-pressureaxial compressor. This has three stages on small and medium versions of the engine and four stages on large versions. The air then flows into a single-stagecentrifugal compressor, through a folded annularcombustion chamber, and finally through a single-stage turbine that powers the compressors at about 45,000 rpm. Hot gas from the gas generator flows into the power turbine, which turns at about 30,000 rpm. It has one stage on the small engines and two stages on the medium and large ones. For turboprop use, this powers a two-stage planetary output reduction gearbox, which turns the propeller at a speed of 1,900 to 2,200 rpm. The exhaust gas then escapes through two side-mounted ducts in the power turbine housing. The turbines are concentric with the combustion chamber, reducing overall length.

In most aircraft installations the PT6 is mounted so that the intake end of the engine is towards the rear of the aircraft, leading to it being known by many as the "back-to-front" engine.^[4] This places the power section at the front of the nacelle, where it can drive the propeller directly without the need for a long shaft. Intake air is usually fed to the engine via an underside mounted duct, and the two exhaust outlets are directed rearward. This arrangement aids maintenance by allowing the entire power section to be removed along with the propeller, exposing the gas-generator section. To facilitate rough-field operations, foreign objects are diverted from the compressor intake by inertial separators in the inlet.^[21] In some installation such as the PT6A-66B version in thePiaggio P.180 Avanti, the engine is reversed, with the propeller acting as a "pusher", the accessory gearbox facing the front of the aircraft.

By the 40th anniversary of its maiden flight in 2001, over 36,000 PT6As had been delivered, not including the other versions.^[22]Up to October 2003, 31,606 delivered engines have flown more than 252 million hours.^[23]Till November 2015, 51,000 have been produced.^[2]The family logged 400 million flight hours from 1963 to 2016.^[9]

The PT6 family is known for its reliability with an in-flight shutdown rate of 1 per 333,333 hours up to October 2003,^[23]1 per 127,560 hours in 2005 in Canada,^[24]1 per 333,000 hours from 1963 to 2016,^[9]1 per 651,126 hours over 12 months in 2016.^[25]Time between overhauls is between 3,600 and 9,000 hours and hot-section inspections between 1,800 and 2,000 hours.^[26]

Early PT6 versions lacked aFADEC,autothrottle could be installed as an aftermarket upgrade with anactuator, initially for single-engine aircraft like thePC-12 and potentially in twin-turboprop aircraft.^[27]In October 2019 the PT6 E-Series was launched on the PC-12 NGX, the first general aviation turboprop with an electronic propeller andengine control system with a single lever and better monitoring for longer maintenance intervals, increased from 300 to 600 hours, and aTBO increased by 43% to 5,000 hours, reducing engine operating costs by at least 15%.^[28]In April 2022,Daher announced that the updatedSOCATA TBM-960 would be powered by the PT6E-66XT.^[29]

The main variant, thePT6A, is available in a wide variety of models, covering the power range between 580 and 920shaft horsepower (430 and 690kilowatts) in the original series, and up to 1,940 shaft horsepower (1,450 kilowatts) in the 'large' lines. ThePT6B andPT6C areturboshaft variants for helicopters. In US military use, they are designated asT74 orT101.

Several other versions of the PT6 have appeared over time:

• theLarge PT6 added an extra axial compressor stage(4), and another power turbine stage(2) and a deeper output reduction, producing almost twice the power output, between 1,090 and 1,920 shaft horsepower (810 and 1,430 kilowatts).
• the MediumPT6 added another power turbine stage(2) -41 thru the -60 engines^[30]
• thePT6B is ahelicopter turboshaft model, featuring an offset reduction gearbox with afreewheeling clutch and power turbine governor, producing 1,000 horsepower (750 kilowatts) at 4,500 rpm.
• thePT6C is a helicopter model, with a single side-mounted exhaust, producing 2,000 horsepower (1,500 kilowatts) at 30,000 rpm, which is stepped down in a user-supplied gearbox.
• thePT6E is a large PT6A derivative equipped with digital engine control.
• thePT6T Twin-Pac consists of two PT6 engines driving a common-output reduction gearbox, producing almost 2,000 horsepower (1,500 kilowatts) at 6,000 rpm.
• theST6 is a version intended for stationary applications, originally developed for theUAC TurboTrain, and now widely used asauxiliary power units on large aircraft, as well as many other roles.^[31]

ThePT6A family is a series of free-turbine turboprop engines providing 500 to 1,940 shaft horsepower (370 to 1,450 kilowatts)

T74-CP-700

(PT6A-20) United States military designation for the PT6A-20/27, used in theBeechcraft U-21 Ute.

T74-CP-702

(PT6A-29)

T101

United States military designation for the T101-CP-100 / PT6A-45R, used in theShorts 330 andShorts C-23 Sherpa.

PT6B-9

ThePT6B-9 is a 550 horsepower (410.1 kilowatts) turboshaft engine for use in helicopters; a later mark of PT6B is rated at 981 horsepower (731.5 kilowatts).

PT6B-16

PT6C

The PT6C is a 1,600 to 2,300 shaft horsepower (1,200 to 1,700 kilowatts) engine for helicopters and tiltrotors.

PT6D-114A

The PT6D-114A is based on the PT6A-114A; the main difference is the deletion of the second-stage reduction gearing and output shaft, because the engine is intended for integration with a combining gearbox incorporating power turbine governors and a propeller output shaft.^[37]

Soloy Dual Pac

2x PT6D-114A engines driving a single propeller through a combining gearbox, capable of independent operation.

PT6T

Twin PT6 power units combining outputs through a gearbox for use in helicopters.

ST6

TheST6 is a variant of the PT6 that was originally developed as a powerplant for theUAC TurboTrain power cars, but later developed as a stationary power generator andauxiliary power unit.

ST6B

The ST6B-62 was a 550 brake horsepower (410 kilowatts) version of the PT6 developed for use in theSTP-Paxton Turbocar, raced in the 1967Indianapolis 500.^[38]

STN 6/76

The STN 6/76 was a 500 brake horsepower (370 kilowatts) version of the PT6 developed for use in theLotus 56, raced in the 1968Indianapolis 500 and later inFormula One races, in 1971.^[39][40]

The engine is used in over 100 different applications.

• AgustaWestland AW119 Koala
• Avicopter AC313
• Changhe Z-8F
• Lockheed XH-51
• Sikorsky S-76B
• Westland Lynx 606

• Airbus Helicopters H175/Avicopter Z-15
• AgustaWestland AW139
• AgustaWestland AW609
• Bell UH-1 Global Eagle upgrade

• Soloy Pathfinder 21

• Pilatus PC-12NGX

• Lockheed L1011 TriStar (as an APU)^[42]
• STP-Paxton Turbocar Indy racer
• UAC TurboTrain

• Lotus 56USAC andFormula 1 racing car

• New England Air Museum, Connecticut (cutaway)^[43]

Data from Jane's 62-63,^[44]

• Type: Turboprop
• Length: 62 in (1,575 mm)
• Diameter: 19 in (483 mm)
• Dry weight: 270 lb (122.47 kg)

• Compressor: 3-stage axial + 1-stage centrifugal flow compressor
• Combustors: annular reverse-flow with 14 Simplex burners
• Turbine: 1-stage gas generator power turbine + 1-stage free power turbine
• Fuel type:Aviation kerosene toMIL-F-5624E /JP-4 /JP-5
• Oil system: Split system with gear type pressure and scavenge pumps, with pressure to gearbox boosted by a second pump.

• Maximum power output: 578 hp (431 kW) equivalent power at 2,200 output rpm for take-off
• Overall pressure ratio: 6.3:1
• Air mass flow: 5.3 lb/s (2 kg/s)
• Specific fuel consumption: 0.67 lb/hph (0.408 kg/kWh)
• Power-to-weight ratio: 2.14 hp/lb (3.52 kW/kg)

Related development

• Pratt & Whitney Canada PT6T

Comparable engines

• Garrett TPE331
• General Electric Catalyst
• HAL HTSE-1200
• Klimov VK-800
• Walter M601

Related lists

• List of aircraft engines

Wikimedia Commons has media related toPratt & Whitney Canada PT6.

• "PT6A".Pratt & Whitney Canada.
• "PT6B".Pratt & Whitney Canada.
• "PT6C".Pratt & Whitney Canada.
• "Embraer EMB-110 type certificate"(PDF).National Civil Aviation Agency of Brazil. 20 December 1972.
• How PT-6 Turbines Are Overhauled. AVweb. 14 August 2015 – via YouTube.
  • Paul Bertorelli (17 August 2015)."Video: How PT-6 Turbines Are Overhauled".AVweb.

• 1960s turboprop engines
• Pratt & Whitney Canada aircraft engines
• Mixed-compressor gas turbines

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