Settled into the left seat at our final cruise altitude of 26,000 feet, we were showing a true airspeed of 304 knots and burning about 700 pounds of jet-A per hour. As the lush rolling landscape of central Pennsylvania slid by far below, a nagging question had entered my mind. What is it about the Beechcraft King Air family of twin turboprops, I asked myself, that keeps these airplanes rolling out of the factory in Wichita, Kansas, more than 53 years after the first one emerged? I always thought I knew the answer to that question, but there in the confines of the King Air 250’s cockpit a quiet crisis of confidence was beginning to bubble up in my mind. Who, precisely, should be buying this airplane anyway? I wondered.
Piper’s sleek Malibu/Mirage pressurized singles have always been good performers once they get up to altitude. It’s the takeoff and climb phases that leave a little to be desired. JetProp LLC’s DL and DLX conversions solve that with an infusion of an extra few hundred horsepower.
The SOCATA TBM (now Daher TBM) is a family of high-performance single-engine turboprop light business and utility aircraft manufactured by Daher. It was originally collaboratively developed between the American Mooney Airplane Company and French light aircraft manufacturer SOCATA.
The design of the TBM family originates from the Mooney 301, a comparatively low-powered and smaller prototype Mooney developed in the early 1980s. Following Mooney’s acquisition by French owners, Mooney and SOCATA held a series of in-depth discussions on the potential for co-developing a new enlarged turboprop design derived from the earlier 301; these resulted in the formation of a joint venture for the purpose of developing and manufacturing the envisioned aircraft, which was designated as the TBM 700. From the onset, the emphasis was placed upon the design’s speed, altitude, and reliability. Upon its entry into the market in 1990, it held the distinction of being the first high-performance single-engine passenger/cargo aircraft to enter production.[
Shortly after launch, the TBM 700 was a market success, which quickly led to the production of multiple variants and improved models, often incorporating more powerful engines and new avionics, amongst other features.
The prefix of the designation, TBM, originated from the initials “TB”, which stands for Tarbes, the French city in which SOCATA is located, while the “M” stands for Mooney. At the time of its conception, while several aviation companies had studied or been otherwise considering the development of such an aircraft, the envisioned TBM 700 was the first high-performance single-engine passenger/cargo aircraft to enter production. From the onset, key performance criteria were established for the design, demanding a high level of reliability while also being capable of an unequaled speed/altitude combination amongst the TBM 700 other single-engined peers.
The Pratt & Whitney CanadaPT6A-64 engine, providing up to 700 shp (522 kW) powers the TBM 700. According to Flying Magazine, the PT6A-64 engine is “the secret to the TBM 700’s performance. At sea level, the engine is capable of generating a maximum 1,583 shp (1,180 kW), which is intentionally limited to 700 shp (522 kW) on early TBM models; the limit allows the aircraft to maintain 700 shp (522 kW) up to 25,000 ft (7,620 m) on a typical day. Engine reliability and expected lifespan are also enhanced by the limitation. While the typical engine overhaul life is set as 3,000 flight hours between overhauls, on-condition servicing can also be performed due to various engine parameters being automatically recorded by the engine trend monitoring (ETM) system. Data from the ETM can be reviewed by the engine manufacturer to determine the level of wear and therefore the need for inspection or overhaul. The ETM, which is connected to the aircraft’s air data computer, also provides information to enable easy power management by the pilot.
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
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.
A clean-sheet wing, more power, touchscreen avionics and stylish interior enhancements lift Piper’s latest incarnation of the classic PA-46 to new heights.
The New Generation model builds on the PC-12’s established reputation as the bizjet rival that takes unprepared strips in its stride
This article originally appeared on the P&WC Airtime Blog.
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For PT6 engines, general MRO practices should be performed regularly, while a total overhaul of the turbine will vary on timing, depending upon usage and other factors that can affect wear and tear on the engine. However, while a total overhaul should be handled by a DDOF for optimal aircraft support, pilots and owners should engage in basic maintenance routines for safety and performance. Continue reading The Basics Of PT6 Engine Maintenance
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