Continuing from his discussion of engine values last month, Jeremy Cox considers the world’s most-popular turboprop powerplant – the PT6A…
Turboprop engines trace their roots back to the Hungarian Jendrassik CS-1 which first ran in 1938 but never flew on an aircraft. It would be another seven years before the world’s first turboprop-powered aircraft – a Gloster Meteor F-1 – flew with a Rolls-Royce RB-50 Trent attached in September 1945.
The Vickers Viscount (1948) became the world’s first production turboprop aircraft, powered by the RR RB-53 Dart. Meanwhile, the world’s oldest ‘still flying’ turboprop aircraft is a 1953 Fairey Gannet powered by an Armstrong Siddeley Double Mamba ASMD 1 engine (two engines bolted to the same gearbox) driving two separate props concentrically mounted on the same thrust line.
It was in 1956 that the Canadian Pratt & Whitney Aircraft Company (later renamed Pratt & Whitney Canada (PWC)) proactively initiated the design and build of a new engine which would eventually become the world’s best-selling aircraft turboprop powerplant. The design mandate for the PT6A was to create a compact, lightweight, modular engine that could produce 500shp, and be serviced as much as possible ‘on-wing’ without removal.
The PWC team collaborated on a double-shaft, reverse flow, free-turbine design which produced more power than a fixed shaft engine, was quieter and virtually eliminated foreign object damage (FOD).
The initial target market for the PT6 engine was the piston-powered DHC-2 de Havilland Beaver. The power-to-weight ratio of a turbine engine versus a piston engine (the PT6 (325lbs) was less than half the dry weight of the current Wasp radial (653lbs), while producing 50hp more).
The first PT6 to take to the skies (1961) was mounted on an experimental Assault Support Helicopter. Later that same year, a Beech 18 was chosen as the first fixed-wing testbed aircraft. The nose section of its fuselage was modified to accommodate the new engine and Hamilton Standard Propeller and the rest, as they say, is history…
Today’s PT6A Series Engine
The modern PT6A Series Engine, starting from the output shaft (propeller mount) and working backwards to the rear of the engine, has the following modules and features:
- Drive Section Module: Epicyclic speed reduction gearbox enables a compact installation and output speed optimized for optimal power and low propeller noise;
- Power Section Module: Reverse-flow combustor optimized to provide low emissions, high stability and easy starting; single-stage compressor turbine which in many models has cooled vanes to maintain high durability; independent “free” power turbine with shrouded blades (forward-facing output for fast hot section refurbishment);
- Compressor Section Module: Multi-stage axial and single-stage centrifugal compressor that features reverse flow, radial inlet with screen for FOD protection;
- Accessory Section Module
Today, Pratt & Whitney Canada classifies the PT6A in three groups, as follows:
- Small engines: 500-999shp
- Medium engines: 1,000-1,400shp
- Large engines: 1,401-1,900shp
While we exclude specifics on military, helicopter or agricultural applications of the PT6A Series engine (our focus is exclusively fixed-wing Business Aviation applications), Business Aviation applications closely equal the other sectors mentioned approximately 50/50.
Specifically, in Business Aviation over 20,000 PT6A series engines have been installed in 54 different makes/models of business aircraft with the Beech King Air series the most prolific platform (10,774+ engines and counting), followed by the Cessna Caravan series (2,573+). The out-of-production Piper Cheyenne comes in third with 1,954 engines.
The ‘Top Three’ most prolific PT6A Models are as follows:
- PT6A-60A (as installed on the King Air 300 Series): 2,892+ engines
- PT6A-21 (as installed on the King Air C90 Series, excluding the GT models): 2,506+ engines
- PT6A-114A (as installed on the 208 Caravan Series, excluding the EX model): 2,149+ engines
PT6A-60A Powerplant Valuation
Noting the popularity of the PT6A-60A, we’ll use this model as the valuation example over the following paragraphs. When new, this specific model engine sells for an approximate list price of $1m.
The typical overhaul cost excluding Life Limited Component (LLC) replacements is approximately $500k, at a TBO interval of 3,600 hours. The Hot Section Inspection is accomplished at mid-time (1,800 hours) and typically costs in the region of $40k.
The ‘Life Limited’ components found inside a PT6A-60A engine include:
- First Stage Compressor Hub/Rotor (15,000 cycle-life): Full-Life Value (FLV) = ~US$60k
- Second Stage Compressor Disc (20,000 cycle-life): FLV = ~$18k
- Third Stage Compressor Disc (20,000 cycle-life): FLV = ~$18k
- Centrifugal Impeller (20,000 cycle-life): FLV = ~$105k
- Compressor Turbine Disc (15,000 cycle-life): FLV = ~$90k
- First Stage Power Turbine Disc (30,000 cycle-life): FLV = ~$80k
- Second Stage Power Turbine Disc (30,000 cycle-life): FLV = ~$75k
- Combined FLV of all LLCs = ~$446k.
The replacement value of a full set of Turbine Blades, when worn below limits or damaged, is approximately $100k.
Blackhawk XP67A Conversion
Waco, Texas based Blackhawk has been providing PT6A Conversions in accordance with a plethora of FAA Supplemental Type Certificates since 1999. Blackhawk offers an upgrade installation of the PT6A-67A engines as direct replacements of the original engines and props (exchange) for $1.735m.
This conversion provides the stock King Air 350 (for example) with two 1,200shp engines which, after being flat-rated by Blackhawk, provides a 25-30% increase in available power delivering an increased True Airspeed (35+ kts dependent on aircraft weight, altitude level selected and the outside air temperature (OAT) at that flight level) and a higher rate of climb (1,950fpm at ISA +20C versus 780fpm for a stock aircraft). In addition, the single-engine service ceiling increases from 21,500ft to 35,000ft dependent on OAT.
The Blackhawk modified King Air 350 is commercially rebranded as a 350XP67A, and includes the following items:
- Two factory new PT6A-67A engines
- Two factory new composite MT Propellers with Spinners
- Installation kit, hardware, STC documentation and AFM supplement
- PWC warranty (five-years or 2,500 hours)
In addition to the King Air 350 upgrade, Blackhawk also sells upgrades for the following PT6A powered aircraft:
- King Air 200 Series (XP61, XP52 or XP42)
- King Air 90 Series (XP135A)
- Conquest I (XP135A)
- Cheyenne Series (XP-135A)
- Caravan Series (Vx or XP-140)
JetPROP DXL/DL Conversions
Since 1998, another popular PT6A conversion for business aircraft (the Piper PA46 Malibu and Mirage) has been provided by JetPROP LLC, a Spokane, Washington based subsidiary of Rocket Engineering Corp. JetPROP LLC to date has performed 316 conversions of PA46 aircraft, which is broken down into three engine models:
- 88 aircraft as the original JetPROP utilizing the PT6A-34 (no longer offered)
- 38 aircraft as the JetPROP DL utilizing the PT6A-21
- 190 aircraft as the JetPROP DLX utilizing the PT6A-35
The DL modification provides 550shp in a package that’s approximately 100lbs lighter than the original Continental TSIO-520. Best of all the conversion provides an additional 240shp at a cost of around $550k.
The DLX modification provides 560shp in a package that is again about 100lbs lighter than the original Lycoming TIO-540 as fitted on the Piper Malibu Mirage (PA46-350P). Here, an additional 250shp is provided at a cost of around $630k.
Included in the conversions are the following:
- Factory new PT6A-35 or -21 engine (DLX or DL);
- Factory new propellers:
- Option 1, Hartzell four-blade metal Scimitar;
- Option 2, MT four-blade composite;
- Option 3, MT five-blade composite;
- Complete installation, STC documentation and AFM supplement;
- PWC warranty (five-years or 2,500 hours).
So why take an existing Piper pressurized piston-single and convert it into a turbine, when the PA46-500TP Meridian, M500 and M600 were/are available from the OEM with the PT6A-42A factory installed?
The answer is found under three separate headings:
- The initial purchase price. As an example, a 2015 model’s Malibu Mirage factory-new price was $1.1m. Add the $630k JetPROP DLX price, and the total outlay would be $1.73m. The price of a 2015-model Meridian was $2.22m factory new. That’s a difference of almost $490k.
- The JetPROP DLX offers better performance (as much as a 500fpm better climb rate; lower fuel burn; and allowance for extra baggage).
- Higher Residual Value. A 2007 JetPROP DLX has retained 68.2% of its value versus a 2007 model Piper Meridian which has retained 52.7% of its value.
With over 20,000 PT6A powerplants produced for Business Aviation to date, there are no signs of this popular model diminishing any time soon. Not only does it continue to be used on some of today’s popular production turboprops, but as demonstrated, it provides significant power through conversion programs, including Blackhawk and JetPROP – and for very good reason.