|A PT6A-20 on display at the Canada Aviation and Space Museum|
|Type||Turboprop / turboshaft|
|Manufacturer||Pratt & Whitney Canada|
|Major applications||AgustaWestland AW139 |
Beech King Air and Super King Air
Cessna 208 Caravan
de Havilland Canada DHC-6 Twin Otter
|Number built||51,000 (as of November 2015)|
|Unit cost||750 hp -135A: $560,000|
1,050 hp PT6A-60A: $955,000
1,100 hp PT6A-68: $855,000
|Variants||Pratt & Whitney Canada PT6T|
The Pratt & Whitney Canada PT6 is a turboprop aircraft engine produced by Pratt & Whitney Canada. Its design was started in 1958, it first ran in February 1960, first flew on 30 May 1961, entered service in 1964 and has been continuously updated since. It consists of two basic sections: a gas generator with accessory gearbox and a free power turbine with reduction gearbox, and is often seemingly mounted backwards in an aircraft in so far as the intake is at the rear and the exhaust at the front. Many variants of the PT6 have been produced, not only as turboprops but also for helicopters, land vehicles, hovercraft, boats, as auxiliary power units and for industrial uses. By November 2015, 51,000 had been produced, 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. The PT6A covers the power range between 580 and 1,940 shp (430 and 1,450 kW) while the PT6B/C are turboshaft 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 the Wasp radial 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 of C$100,000. Guthrie recruited twelve engineers with experience gained at various places including the National Research Council in Ottawa, Orenda Engines in Ontario, Bristol Aero Engines and Blackburn Aircraft. They completed the detailed design of an engine for Canadair's small jet trainer, the CL-41. It was a 3,000lb thrust turbojet but the design was taken over by P&WA who developed it into the Pratt & Whitney JT12. The team had to wait for market assessments to define their next engine, a 450 shaft horsepower turboprop for twin-engined aircraft, the PT6. The early development of the PT-6, which first ran in December 1963, was beset with engineering problems, cost overruns and lack of sales. It was almost cancelled. The team lacked the ability to deal with the technical difficulties, i.e. how to develop the engine, because, as one of the team Elvie 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.
The PT-6 first flew on 30 May 1961, mounted as a third engine in the nose of a Beech 18 aircraft which had been converted by de Havilland at its Downsview, Ontario facility. 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 a Beech 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 the de Havilland Canada Dash 7 so a Vickers Viscount was modified as a PT6 test-bed with a Dash-7 installation in the nose.
The first production PT-6 model, the PT6A-6, was certificated in December 1963. The first application was the Beech Queen Air, enticing the U.S. Army to buy a fleet of the U-21 Ute variant. This helped launch the King Air with Beechcraft selling about 7,000 by 2012. From 1963 to 2016 power-to-weight ratio was improved by 50%, brake specific fuel consumption by 20% and overall pressure ratio reached 14:1. 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 and single-crystal turbine blades in the early 1990s. Its pressure ratio is 13:1 in the AgustaWestland AW609 tiltrotor, the highest that can be used without cooled turbine blades.
In response to the General Electric GE93, in 2017 Pratt & Whitney Canada started testing core technology and systems for a proposed 2,000hp engine to replace the most powerful versions of the PT6. It was considered likely to be a development of the PT6C core, and would fit between the 1,750 shp PT6C-67C/E and the 2,300 shp PW100 family. It was expected to be ready to launch by the end of 2017 for an initial helicopter platform with a 10-15% reduction in brake specific fuel consumption. 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.
When de Havilland Canada asked for a much larger engine for the DHC-8, roughly twice the power of the Large PT6, Pratt & Whitney Canada responded with a new design initially known as the PT7, later renamed Pratt & 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. A similar general arrangement with a free-turbine power take-off at the exhaust end ( the 1,000 shp P.181 engine) had been shown by Armstrong Siddeley Motors at the Farnborough Airshow in 1957.
An early design improvement, incorporated in the PT6A-20, was the pipe diffuser patented by Vrana, another of the original PT6 team. It replaced the vaned type diffuser used in centrifugal compressors. The pipe diffuser became standard design practice for P&WC. 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". 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 a free power turbine. The starter has to accelerate only the gas generator, making the engine easy to start, particularly in cold weather. Air enters the gas-generator through an inlet screen into the low-pressure axial 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-stage centrifugal compressor, through a folded annular combustion 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. 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. In some installation such as the PT6A-66B version in the Piaggio Avanti P180, 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. Up to October 2003, 31,606 delivered engines have flown more than 252 million hours. Till November 2015, 51,000 have been produced. The family logged 400 million flight hours from 1963 to 2016.
The PT6 family is known for its reliability with an in-flight shutdown rate of 1 per 333,333 hours up to October 2003, 1 per 127,560 hours in 2005 in Canada, 1 per 333000 hours from 1963 to 2016, 1 per 651,126 hours over 12 months in 2016.Time between overhauls is between 3600 and 9000 hours and hot-section inspections between 1800 and 2000 hrs.
The main variant, the PT6A, is available in a wide variety of models, covering the power range between 580 and 920 shaft horsepower in the original series, and up to 1,940 shp (1,450 kW) in the "large" lines. The PT6B and PT6C are turboshaft variants for helicopters. In US military use, they are designated as T74 or T101.
Several other versions of the PT6 have appeared over time :
- the Large PT6 added another power turbine stage and a deeper output reduction, producing almost twice the power output, between 1,090 and 1,920 shp (1,430 kW) ;
- the PT6B is a helicopter turboshaft model, featuring an offset reduction gearbox with a freewheeling clutch and power turbine governor, producing 1,000 hp (750 kW) at 4,500 rpm ;
- the PT6C is a helicopter model, with a single side-mounted exhaust, producing 2,000 hp (1,500 kW) at 30,000 rpm, which is stepped down in a user-supplied gearbox ;
- the PT6T Twin-Pac consists of two PT6 engines driving a common-output reduction gearbox, producing almost 2,000 hp (1,500 kW) at 6,000 rpm ;
- the ST6 is a version intended for stationary applications, originally developed for the UAC TurboTrain, and now widely used as auxiliary power units on large aircraft, as well as many other roles.
The PT6A family is a series of free-turbine turboprop engines providing 500 to 1,940 shp (433 to 1,447 kW)
|PT6A-6||525 eshp||500 shp|
|PT6A-11AG||528 eshp||500 shp||Air Tractor AT-400|
Schweizer Ag-Cat G-164B Turbine
|PT6A-15AG||715 eshp||680 shp||Air Tractor AT-400|
Air Tractor AT-501
Schweizer Ag-Cat G-164B Turbine
|PT6A-20||579 eshp||550 shp||de Havilland Canada DHC-6 Twin Otter Srs. 100-200|
|PT6A-21||580 eshp||550 shp||Beechcraft King Air C90A/B/SE|
Beechcraft Bonanza (turbine conversion)
Royal Turbine Duke
Evektor EV-55 Outback
|PT6A-25, -25A||580 eshp||550 shp|
|PT6A-25C||783 eshp||750 shp||Embraer EMB 312 Tucano|
Pilatus PC-7/PC-7 MKII Turbo Trainer
PZL-130 Orlik / TC-II Turbo-Orlik
|PT6A-27||715 eshp||680 shp||Beechcraft Model 99A, B99|
de Havilland Canada DHC-6 Twin Otter 300
Embraer EMB 110 Bandeirante
Let L-410 Turbolet
Pilatus PC-6/B Turbo-Porter
|PT6A-28||715 eshp||680 shp||Embraer EMB 121 Xingu|
|PT6A-29||778 eshp||750 shp|
|PT6A-34||783 eshp||750 shp||Embraer EMB 110 Bandeirante/111 |
Embraer EMB 821 Carajá
Grumman Mallard (turbine conversion)
PAC P-750 XSTOL
Vazar Dash 3 Turbine Otter
Viking DHC-6 Twin Otter 400
|PT6A-34AG||783 eshp||750 shp||Air Tractor AT-502B|
Frakes/Grumman Turbo-Cat Model A/B/C
Pacific Aerospace 750XL
PZL-Okecie PZL-106 Turbo Kruk
Schweizer Ag-Cat G-164B/D Turbine
Thrush Model 510P
|PT6A-35||787 eshp||750 shp||Blue 35|
|PT6A-36||783 eshp||750 shp|
|PT6A-38||801 eshp||750 shp|
|PT6A-110||502 eshp||475 shp||Schweizer AG-Cat Turbine |
Royal Turbine Duke
|PT6A-112||528 eshp||500 shp||Cessna Conquest I|
|PT6A-114||632 eshp||600 shp||Cessna 208 Caravan|
|PT6A-114A||725 eshp||675 shp||Cessna 208 Caravan|
|PT6A-116||736 eshp||700 shp|
|PT6A-121||647 eshp||615 shp|
|PT6A-135||787 eshp||750 shp||Beechcraft King Air F90-1/C90GT/C90GTi/C90GTx|
Blackhawk XP135A Cheyenne Series
Blackhawk XP135A Conquest I
Blackhawk XP135A King Air 90 Series
Cessna Conquest I
Silverhawk 135/StandardAero C90/E90
StandardAero Cheyenne Series
StandardAero King Air F90
T-G Aviation Super Cheyenne
Vazar Dash 3 Turbine Otter
|PT6A-40||749 eshp||700 shp|
|PT6A-41||903 eshp||850 shp||Beechcraft King Air 200/B200|
Piper Cheyenne III/IIIA
|PT6A-42||903 eshp||850 shp||Beechcraft C-12 HuronF|
Beechcraft King Air 200/B200
Blackhawk XP42 King Air 200
StandardAero King Air 200
Blackhawk XP42A C-208 Caravan Series (-42A)
|PT6A-45||1070 eshp||1020 shp|
|PT6A-50||1022 eshp||973 shp||de Havilland Canada DHC-7 Dash 7|
|PT6A-52||898 eshp||850 shp||Beechcraft King Air B200GT/250|
Blackhawk XP52 King Air 200/B200
Enhanced Aero B200GTO
StandardAero King Air 200/B200
|PT6A-60, -60A||1113 eshp||1050 shp||Beechcraft Super King Air 300/350|
|PT6A-60AG||1081 ehsp||1020 shp||Air Tractor AT-602|
Ayres Thrush 550P/660
|PT6A-61||902 eshp||850 shp|
Pilatus PC-9 Turbo Trainer
|PT6A-64||747 eshp||700 shp||EADS Socata TBM 700|
|PT6A-65B, -65R||1249 eshp||1173 shp||Beechcraft 1900/1900C|
Polish Aviation Factory M28 Skytruck
|PT6A-65AG, -65AR||1298 eshp||1220 shp||Air Tractor AT-602|
Air Tractor AT-802/802A/802AF/802F
Ayres Thrush 660/710P
|PT6A-66, -66A, -66D||905 eshp||850 shp||National Aerospace Laboratories SARAS|
Piaggio P.180 Avanti
Ibis Ae270 HP (-66A)
EADS Socata TBM 850
|PT6A-66B||1010 eshp||950 shp||Piaggio P180 Avanti II|
|PT6A-67, -67A, -67B, -67P||1272 eshp||1200 shp||Beechcraft Starship|
IAI Heron TP
Pilatus PC-12 (-67B)
Pilatus PC-12NG (-67P)
|PT6A-67D||1285 eshp||1214 shp||Beechcraft 1900D|
|PT6A-67AF, -67AG, -67R, -67T||1294 eshp||1220 shp||Air Tractor AT-802/802A/802AF/802F (-67AG)|
Ayres Thrush 710P (-67AG)
Basler Turbo BT-67 (-67R)
Shorts 360 / 360-300 (-67R)
|PT6A-67F||1796 eshp||1700 shp||Air Tractor AT-802/802A/802AF/802F|
|PT6A-68||1324 eshp||1250 shp||T-6A Texan II|
Pilatus PC-21 (-68B)
Embraer EMB-314 Super Tucano (-68C)
TAI Hürkuş (-68T)
- (PT6A-20) United States military designation for the PT6A-20/27, used in the Beechcraft U-21 Ute.
- United States military designation for the T101-CP-100 / PT6A-45R, used in the Shorts 330 and Shorts C-23 Sherpa.
- The PT6B-9 is a 550 hp (410.1 kW) turboshaft engine for use in helicopters. A later mark of PT6B is rated at 981 hp (731.5 kW).
- The PT6C is a 1600 to 2300 horsepower (1190 to 1720 kW) engine for helicopters and tiltrotors.
- 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.
- Soloy Dual Pac
- 2x PT6D-114A engines driving a single propeller through a combining gearbox, capable of independent operation.
- Twin PT6 power units combining outputs through a gearbox for use in helicopters.
- The ST6 is a variant of the PT6 that was originally developed as a powerplant for the UAC TurboTrain power cars, but later developed as a stationary power generator and auxiliary power unit.
- The ST6B-62 was a 550 bhp (410 kW) version of the PT6 developed for use in the STP-Paxton Turbocar, raced in the 1967 Indianapolis 500.
- STN 6/76
- The STN 6/76 was a 500 bhp (370 kW) version of the PT6 developed for use in the Lotus 56, raced in the 1968 Indianapolis 500 and later in Formula One races, in 1971.
The engine is used in over 100 different applications.
- AASI Jetcruzer
- Aero Commander 680T (PT6 conversion)
- Aero Ae 270 Ibis
- AHRLAC Holdings Ahrlac
- Air Tractor AT-400
- Air Tractor AT-501
- Air Tractor AT-602
- Air Tractor AT-802
- Antilles Super Goose
- Antonov An-28
- Ayres Turbo Thrush
- Basler BT-67
- Beechcraft 1900
- Beechcraft Model 99
- Beechcraft A36TC Bonanza (turbine conversion)
- Beechcraft C-12 Huron
- Beechcraft King Air
- Beechcraft Lightning
- Beech 18 series (turbine conversion)
- Beechcraft Model 87
- Beechcraft Model 99
- Beechcraft RC-12 Guardrail
- Beechcraft RU-21C Ute
- Beechcraft Starship
- Beechcraft Super King Air
- Beechcraft T-6 Texan II
- Beechcraft T-34C Turbo-Mentor
- Beechcraft T-44 Pegasus
- Beriev Be-30K
- CASA C-212 series 300P
- Cessna 208 Caravan
- Cessna P210N (turbine conversion)
- Cessna 404 Titan (turbine conversion)
- Cessna 421C Golden Eagle (turbine conversion)
- Cessna 425 Corsair/Conquest I
- Conair Turbo Firecat
- Conroy Tri-Turbo-Three
- de Havilland Canada DHC-2 Mk. III Turbo Beaver
- de Havilland Canada DHC-2T Turbo Beaver 
- de Havilland Canada DHC-3 Otter (turbine conversions)
- de Havilland Canada DHC-6 Twin Otter
- de Havilland Canada Dash 7
- Dominion UV-23 Scout
- Dornier Do 128 Turbo Skyservant
- Dornier Seawings Seastar
- Douglas DC-3 (turbine conversions)
- Epic LT Dynasty
- Embraer EMB 110 Bandeirante
- Embraer EMB 121 Xingu
- Embraer EMB 312 Tucano
- Embraer EMB 314 Super Tucano
- Frakes Mohawk 298
- Frakes Turbocat
- Gulfstream American Hustler 400
- Grumman Mallard (turbine conversion)
- Grumman Goose (turbine conversion)
- Harbin Y-12
- Helio AU-24 Stallion
- IAI Arava
- IAI Eitan
- Indonesian Aerospace N-219
- JetPROP DLX
- Kestrel K-350
- KAI KT-1
- Let L-410 Turbolet
- Lancair Evolution
- NAL Saras
- NDN Fieldmaster
- FTS Turbo Firecracker
- PAC 750XL
- PAC Cresco
- Piaggio P.180 Avanti
- Pilatus PC-6/B Turbo-Porter
- Pilatus PC-7
- Pilatus PC-9
- Pilatus PC-12
- Pilatus PC-21
- Piper PA-31P (turbine conversion)
- Piper PA-31T Cheyenne
- Piper PA-42 Cheyenne III
- Piper PA-46-500TP Meridian
- Piper T1040
- PZL-130T Turbo Orlik and PZL-130TC-II Orlik
- PZL M-18 Dromader (turbine conversion)
- PZL M28 Skytruck
- Quest Kodiak
- Reims-Cessna F406 Caravan II
- Saunders ST-27/ST-28
- Scaled Composites ATTT
- Shorts 330
- Shorts 360
- Short C-23 Sherpa
- Socata TBM
- Spectrum SA-550
- Swearingen SA26-T Merlin IIA
- TAI Hürkuş
- US Aircraft A-67 Dragon
- 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
Engines on display
- McGill University, Montreal (cutaway)
- New England Air Museum, Connecticut (cutaway)
- Ontario Science Centre, Toronto, Ontario (cutaway)
Data from Jane's 62-63,
- 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 to MIL-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 (2 kg)/second
- Specific fuel consumption: 0.67 lb/hph (0.408 kg/kWh)
- Power-to-weight ratio: 2.14 hp/lb (3.52 kW/kg)
|PT6A-21||3, 1 / 1, 1||550||410||0.63||380||—||19 in
|327||148||Beech Bonanza, King Air C90A/B/SE|
|PT6A-25||3, 1 / 1, 1||550||410||0.63||380||—||353||160||Beech T-34C|
|PT6A-25C||3, 1 / 1, 1||750||560||0.595||362||—||338||153||Embraer Tucano, Pilatus PC-7, PZL-130 Orlik|
|PT6A-114/A||3, 1 / 1, 1||600–675||447–503||0.64||390||—||350||160||Cessna 208 Caravan|
|PT6A-135A||3, 1 / 1, 1||750||560||0.585||356||7||338||153||Cessna Conquest, Piper Cheyenne, Beech King Air F90|
|PT6A-42||3, 1 / 1, 2||850||630||0.601||366||8||66.9 in
|403||183||Beech King Air 200/B200, C-12 Huron|
|PT6A-60A||4, 1 / 1, 2||1,050||780||0.548||333||8.5||72.5 in
|475||215||Beech Super King Air 300/350|
|PT6A-64||4, 1 / 1, 2||700||520||0.703||428||8.5||70 in
|456||207||Socata TBM 700|
|PT6A-66||4, 1 / 1, 2||850||630||0.62||380||9.5||456||207||Piaggio P.180 Avanti|
|PT6A-65B||4, 1 / 1, 2||1,100||820||0.536||326||—||74 in
|481||218||Ayres Turbo-Thrush, PZL M28 Skytruck, Beech 1900/C|
|PT6A-67B||4, 1 / 1, 2||1,200||890||0.546||332||10.8||530||240||Pilatus PC-12|
|PT6A-67D||4, 1 / 1, 2||1,271||948||0.546||332||10.8||515||234||Beech 1900D|
|PT6A-68||4, 1 / 1, 2||1,250||930||0.54||330||—||72.2 in
|572||259||Beech T-6 Texan II|
|PT6A-68B||4, 1 / 1, 2||1,600||1,200||0.54||330||—||575||261||Pilatus PC-21|
|PT6B-37A||3, 1 / 1, 1||900||670||0.584||355||—||19.5 in
|385||175||Agusta A119 Koala|
|PT6C-67A||4, 1 / 1, 1||1,940||1,450||0.47||290||—||22.5 in
|PT6C-67C||4, 1 / 1, 2||1,100||820||0.49||300||—||—||—||Agusta A 139|
|PT6T-3B/BF||2 × 3, 1 / 1, 1||1,800||1,300||0.6||360||—||43.5 in
|PT6T-3D/DF||2 × 3, 1 / 1, 1||1,800||1,300||0.595||362||—||692–681||314–309||Bell 412/SP/HP/EP|
|PT6T-6||2 × 3, 1 / 1, 1||1,875||1,398||0.591||359||—||660||300||Bell 212, 412/SP/HP/EP, Sikorsky S-58T|
|PT6T-68||2 × 3, 1 / 1, 1||1,970||1,470||0.591||359||—||665||302||Bell 412HP|
- compressor / HP, LP turbine
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|Wikimedia Commons has media related to Pratt & 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. Lay summary.