Should the prop be feathered prior to shut-down or should I feather the propeller as I shut the engine down? Should I start the engine with the prop lever in fine pitch (forward) or in course pitch?Continue reading Prop Feather vs. Idle Times in the PT6A
The Douglas DC-3 Doesn’t Know the Meaning of the Word “Quit”
The same year the German airship Hindenburg crossed the Atlantic, the still-flying-today Douglas DC-3 was introduced to the world. The DC-3 is widely viewed as one of the most significant transport aircraft in history, due to its massive and long-lasting impact on the airline industry, and aerospace engineering. I got the chance to interview Ric Hallquist, the retired Chief DC-3 Pilot for Missionary Flights International who flew and worked on the beefy twin engine transport plane for over 30 years.Continue reading The Douglas DC-3: 85 Years and Going Strong
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The Pratt & Whitney PT6 engine has been in use since 1961 and has since logged more than 380 million flight hours, the equivalent of about 250,000 round-trips to the moon. The most advanced PT6A engine family includes three power levels, “Small”, “Medium”, and “Large” that all have the advantage of turbine cooling, aerodynamic design, and advanced technologies in materials. Keeping these engines running efficiently and safely wouldn’t be possible without a periodic Hot Section Inspection.
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The question of how a radial engine can be compared to a turbine engine is a question that has been asked many times over. Individuals in the Agricultural world are still asking themselves this question every year on a purely economic basis. However, the question can also be asked from a historic basis as well. In looking at the Pratt & Whitney family of Radial Engines and the PT6A family of engines, it is clear that the two are closely related.
A Bit of Background on Pratt & Whitney’s Engine Marvels: The PT6A, R-1340, & The R-985
A legendary engine deserves a story as extraordinary as it is, and such is the case with the early history of Pratt & Whitney’s PT6. This story begins decades before the turbulent history of the PT6 when radial engines were still the dominant engine for airplane use. The gas turbine engine of the PT6 revolutionized the industry, but not before the static, air-cooled radial engines had a few decades in the limelight.
Of all the radial engines, Pratt & Whitney’s R-985 was always a favorite since its inception in 1932. Simply sit back and watch a smile cross an aviation enthusiast’s face upon observing the sputter of the round radial engine as it starts up, and it is clear that these engines were something special.
However, the transition into the era of the PT6 was not an easy one. In fact, it was something of a miracle.
The Rise of the PT6
While the advancements of gas turbine engines were known to the aviation industry in the early 1950s, the expenses of the manufacturing, maintenance and repairing processes were problematic. However, that did not deter Pratt & Whitney Canada (PWC) while they forged ahead with their plans of designing a powerful gas turbine engine. They hired a team of specialists and proceeded with attempts to develop a 450 hp engine that had growth potential up to 500 hp. Their goal was to keep operating costs at a similar level as the previous radial engines, and their first foray into gas turbine engines was designed to fit small and lightweight airplane models.
However, they still needed to decide on a gas turbine technology, but eventually settled on a free turbine configuration that was more expensive, but had crucial advantages such as less starting power requirements, simplified controls for fuel and the ability for fixed-wing aircrafts to purchase off the shelf propellers rather than custom ones. Once the team decided to move in this direction, they still were not ready to get to work since they had to travel to Pratt & Whitney’s headquarters to convince the chief engineer that their plan was the right one. Upon securing his approval, the jubilant team started working on the ambitious project.
Unfortunately, their work was a blight on company balance sheets. The new design attempts led to a sort of development nightmare, but the chief engineer that approved the project still had faith in the vision. As a result, he sent a team of six experts spearheaded by a highly skilled engineer named Bruce Torell. The goal was to get the project back on track, and history reveals that this historic engine would have likely failed without his aid.
Progress was quickly made thanks to Torell’s engine expertise, but then the team faced obstacles from PWC itself. Despite aggressive attempts to terminate the project, work continued and was finally ready for flight testing in 1961. A search began for a suitable twin engine airplane to test with the PT6, and the team chose Beechcraft C-45 “Expeditor”. This Beechcraft Model 18 was equipped with two R-985s, meaning that the traditional radial engines played a huge role in the development and rise of the PT6. While further tweaks to the engine were made, the future of airplane engines was clear. Gas turbine technology was here to stay, it was just a matter of whether the PT6 was the engine that would dominate the airplane industry. It did, thanks to Beechcraft, the same company that used P&W’s radial R-985 engines of decades past. With that agreement, the PT6 finally saw mainstream success that produced its dominant run as one of the great engines of history and in fact was the first engine ever put on a King Air.
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Turbine Vs. Radial, Why the Comparison?
I’ve been privileged to know both the PT6A and the 9-cylinder Pratt engines. Both engines operate on a different technique for deriving horsepower from the combustion process, but at heart they are still both internal combustion engines that share the same engineering DNA.
One of the most complex parts of the R-1340/R-985 engine, which has remained relatively unchanged since December 24, 1925 when the very first R-1340 roared to life, is the supercharger or blower section. The blower section, which also serves as the anchor-point when installing the engine, is attached to the rear power case. The circular case receives the fuel/air mixture from the impeller assembly through diffuser channels then delivers the fuel/air mixture to the cylinders via the intake pipes. The blower is driven directly by the crankshaft through a spring loaded gear coupling located at the aft section of the crankshaft assembly. This ingenious design helps protect the blower gearing from sudden acceleration or deceleration. The spring loaded gear drives the floating gear. The impeller assembly, being indirectly driven by the crankshaft, turns ten or even twelve times crankshaft speed.
In like manner the PT6A Impeller is located in the gas generator housing which is the anchor point when installing the engine. The centrifugal impeller delivers air through diffuser tubes to the combustion chamber. The hot gases flow through a series of turbines which produce horsepower to the propeller shaft.
The impeller is only one area of similar design and function. The reduction gearing in both the PT6A & R-1340G engines are remarkably similar as well as many other features. It is not difficult to see a common engineering theory. Many pilots and mechanics love the history and engineering that goes along with engines and aircraft. Certainly looking and comparing two of the legacy engines from Pratt & Whitney is enjoyable information for many in the aviation community. I have always found it entertaining that as the PT6A engine took its first breath of life, there were R-985 engines on each side! The photo (left) is of the first flight of the PT6A, being test flown on a Beech 18 (May 1961).
In closing, I am a mechanic that holds to the history of aviation. Learning about the past can certainly give insight to the present while possibly holding a glimpse into the future. Drawing a comparison between these two engines certainly does that.
– Rob Seeman, Covington Aircraft Operations Manager
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THE PT6 ENGINE MILESTONES
It’s the remarkable story of a remarkable engine. With more than 51,000 engines delivered to power some 130 different applications, the PT6 engine can tell quite a story of creativity and transformation. While we had a lot to choose from, we’ve put together a list of milestones for the engine as we mark its golden anniversary.
1957 – P&WC assembled a team of 12 talented young engineers after studies showed a market opportunity for 500 shp (shaft horsepower) class turboprop engines in the aircraft market then powered by piston engines. P&WC saw an opportunity to channel some of the profits from its piston engine spare parts business towards the development of gas turbine engines smaller than those made by its U.S. parent.
1963 – It’s what our celebration is all about. In December 1963, P&WC shipped the first PT6 production engine, the PT6A-6, to Beech Aircraft Company for its Beech 87, which later became the King Air. The PT6A-6 was a highly innovative gas turbine that represented a significant advance in technology from the traditional piston-driven engines used to power small aircraft. Gas turbines have a higher power to weight ratio than piston engines.
1967 – Piper’s PA-31 Navajo took its first flight powered by PT6A-20s. Piper had enjoyed enormous success building light aircraft since the 1930s, but it took P&WC years of effort to get Piper to adopt turbine engines and move away from their traditional reliance on piston-driven engines.
1968 – P&WC’s ST6L73 engine (a derivative of the PT6A without the gearbox second stage) entered into service as an auxiliary power unit (APU) for the Lockheed L1011 airliner.
1968 – Bell Helicopter placed its initial order for P&WC’s first turboshaft, the PT6T Twin-Pac® engine
1970 – P&WC’s PT6T Twin Pac® entered into service. It is two engines coupled in a single package to power medium-sized, twin-engine helicopters.
1970 – The United States Military ordered 294 Bell 212s under the designation UH-1N equipped with PT6T Twin-Pac® turboshaft engines. Delivery also began in 1970.
1973 – The second-stage power turbine was introduced on the PT6A-41. This was a step change in engine power and efficiency.
1979 – An Air Tractor agricultural aircraft is powered by a PT6A engine and flies to the National Agricultural Aviation Association convention in Las Vegas – the first time such a combination was displayed in public.
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As most of you know, there are life-limited components in a PT6. These components include the CT and PT Disks and the Compressor Disks. After a certain number of cycles they must be replaced.
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Driving on an early May morning to Sandpoint, Idaho, to see the Quest Aircraft factory and then fly a new Kodiak 100 Series II to California, it was clear that icing conditions were not only forecast but likely in the wet gray clouds that shrouded the local mountains. For the flight-into-known-icing-certified Kodiak, however, icing is not a problem, and in the 11 years since it entered service, the capable utility single-engine turboprop has proven its mettle in challenging flying all over the world.
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This article first appeared over at AOPA here.
Concurrent with a max takeoff weight increase to 8,600 pounds, Cessna dropped the Corsair moniker and renamed the 425 Conquest I while reassigning the 441 the name Conquest II. Confused yet? The 425 is best described as a 421 Golden Eagle with turbines in place of pistons. Aside from sharing the same basic dimensions, the similarities between the 421 and Conquest I fade quickly. The 425 is substantially beefed up structurally and has more robust systems.
Since it’s based on a piston design, the 425 is easy to fly and an easy step up. In fact, with the easy-to-operate turbines, many would argue that the 425 is less complex than the 421. Cockpit visibility is excellent, as is the instrument panel layout. Cabin seats are comfortable once seated. Cessna’s “wide oval” cabin biases more toward elbow room than headroom, so there will be nothing close to stand-up comfort.
Performance-wise, the Conquest is good for 250 KTAS at typical cruise altitudes in the mid teens to low 20s. As is usual with turbines, the fuel burn drops off the higher you fly. Also typical of turbines, the winds will dictate choice of cruise altitude vs. fuel burn. Owners often figure 500 pounds of Jet-A the first hour and 400 pounds/hour after that. Blackhawk Modifications Inc. offers 425 owners PT6A-135 engines in place of the original -112s. The Blackhawk holds its max power to much higher altitudes than the original engines, resulting in faster time to climb and a 20-knot increase in true airspeed.
Range with tanks full is about 1,200 nm, which leaves about 700 pounds of payload. With six adults on board, range is about 700 nm. The 425 is confident at all weights on 4,000-foot runways at sea level. At lighter weights, 3,000-foot runways are doable.
Vref values a 1981 Conquest I at $625,000 while a 1986 model fetches an average of $875,000. Once an owner swallows the reality of six-figure engine overhauls, higher fuel burn, and other substantial cost increases of owning a turbine, he or she will be impressed with the Conquest’s performance and reliability.
Pete Bedell is a pilot for a major airline and co-owner of a Cessna 172 and Beechcraft Baron D55.
Cessna 425 Conquest I
Powerplants | (2) 450-shp Pratt & Whitney PT6
Length | 35 ft 10 in
Height | 12 ft 7 in
Wingspan | 44 ft 1 in
Seats | 2+6
Max takeoff weight | 8,600 lb
Takeoff distance over 50-ft obstacle | 2,800 ft
Max cruise speed | 264 kt
Landing distance over 50-ft obstacle | 2,482 ft
Range | 1,200 nm
Peter A. Bedell
Pete Bedell is a pilot for a major airline and co-owner of a Cessna 172M and Beechcraft Baron D55.
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As we head into the winter months. the question is often asked about Preservation and Depreservation of engines. To help answer some of these questions, the following information was gathered from the PT6A-34AG Maintenance manual (Manual Part Number 3021242).Continue reading Preservation and Depreservation of PT6A Engines
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The success of the PC-6 Porter is built around its STOL capabilities. This feature makes it very sought-after for missions such as international relief, firefighting, medevac, parachuting, search and rescue, and even sightseeing.