Tech – A Flying Fashion Victim: The PC-6 Engine Saga

By me

In a dazzling display of consistency, my research for a magazine article about S5-CAM – the Pilatus PC-6 that had visited Lučko some months ago – took only moments to veer completely off track, invariably as soon as I began to delve deeper into the type’s rich history :). The culprit for my deviation was the fantastic database at www.pc-6.com, documenting in amazing detail the life and times of this amazing aircraft.

Such huge collection of sometimes obscure information was right up my runway, so with my initial research goal completely forgotten, I began to read through the type’s version list. Pretty soon I began to notice that the PC-6 had a tendency to change engines as often as I change clothes, prompting me to dig even deeper and attempt to make a list of all the powerplants (and their evolutions) that had ever been fitted to the Porter… 🙂

From A to D

While the PC-6 is today universally – and pretty much exclusively – associated with the venerable PT6A turboprop, the design actually had much more humble beginnings, starting out in life as “just another piston”. First flying back in 1959, the original PC-6 had been equipped with a Lycoming GSO-480 engine, whose six supercharged (S prefix) cylinders, linked to a reduction gearbox (G prefix), produced around 340 HP (the reason why this series is also known as the PC-6/340). Despite the comparatively low power – and the piston engine’s well known anemia at altitude – the Porter prototype had nevertheless managed to capture the period record for the highest (successful 🙂 ) landing, touching down at an impressive 18,856 feet somewhere in the Himalayas.

However, the GSO-480 was quite a complicated and temperamental thing to run and maintain, leading pretty soon to the development of a simpler model called the PC-6/275, powered by a normally aspirated – but still geared – 250 HP GO-480. But the loss of 90 HP – a whopping 26% – over the equally heavy standard model had meant the performance suffered dramatically, spurring the introduction of the follow-on PC-6/350, equipped with a fuel injected (I prefix) IGO-480 developing a much more reasonable 350 HP :).

The apex of the piston PC-6 however did not come until 1970 – rather late by large piston standards – with the brutal PC-6/D, powered by the monstrous eight cylinder Lycoming TIO-720 producing a sizable 400 HP. One of the more ludicrous ideas to come out of the Lycoming works, the 11.8 liter TIO-720 was created by joining together two IO-360s and then – for that little extra something – screwing on a massive turbocharger (T prefix) :). Despite the power and potency of the engine – which had also been used to great effect on the Piper PA-24-400 Comanche and the PA-36-375 Pawnee Brave – it was very heavy and its rear cylinders were notoriously prone to overheating. The superiority of turboprops – which had been introduced to the PC-6 line nine years earlier – had slammed the final nail into the coffin of the D model, the program being quietly dropped after just one prototype had been completed.

The French Connection

This “first contact” with the turbine came in 1961 with the 523 HP Turbomeca Astazou IIE, creating the PC-6/A, the first of the Turbo Porters :). A very light, compact and durable engine, the Astazou would go on to become one of the world’s great small turboprops – but would sadly have a short and largely unremarkable career on the PC-6. The only major revamp in the period was the one-of PC-6/Ax, powered by the new Astazou X which had – through the addition of another compressor stage – been boosted to 573 HP. This model was followed by the very similar PC-6/A1 and PC-6/A2, which had only differed in engine versions (Astazou XII and XIVE respectively) with no change in power. All in all, only 43 Astazou-powered examples were ever built, all of which were eventually re-engined with the PT6A – thus confining the A models to the pages of history…

What would eventually become the PC-6 we know today had started emerging in the mid-60s, when the Astazou of one example was swapped for a 550 HP PT6A-6A, creating the enduring legend – the PC-6/B :). This first foray into PT6 World was however short lived, with only 12 examples produced before the introduction of the definitive early B model, the PC-6/B1-H2, sporting the PT6A-20 of equal power output, but higher torque.

Like fine wine (or cheese 😀 ), the PT6 PC-6 would then take some time to mature – 17 years in fact until the arrival of the penultimate Turbo Porter, the PC-6/B2-H2 of 1984. Representing 80% of the way to today’s standard, the B2-H2 was fitted with a 680 HP PT6A-27, flat rated down to the “original” 550 HP. While this may seem like a questionable move, it does have a raft of benefits for a “hauler” designed for operations at high weights and in all weather conditions. The first advantage is the engine’s larger core, which gives a measurable increase in torque throughout its operating range without a (significant) increase in fuel consumption. Additionally, running slower and cooler than it was designed for means engine wear is noticeably reduced, boosting overall reliability and noticeably prolonging the engine’s service life.

However, the biggest advantage is a stable power output regardless of outside air temperature. In a conventional non-flat-rated system, the maximum power the engine can produce with the throttle wide open – the so called thermodynamic power – varies greatly with air density, itself a function of air temperature. The higher the temp, the lower the density and vice-versa. When the density is low, the mass flow through the engine is reduced, the combustion efficiency is reduced and the engine’s thermal limits are more constricting – all of which results in a reduced power output. Conversely, when the density is high – such as on a cold day – the mass flow is high, combustion efficiency is high and the engine runs cooler, allowing more fuel to be injected and thus produce more power. The upshot is that an engine producing, say, 1000 HP in standard conditions (15 degrees Centigrade, used for all performance specs) may produce upward of 1100 HP at 0 Centigrade and as low as 900 HP at 30 Centigrade – which complicates performance calculations and adversely affects the aircraft’s overall performance. Obviously enough, the more critical condition is at lower densities – since very few pilots will object to having additional pep at takeoff :D.

Flat rating systems get around this issue (up to a point of course) specifically by limiting the engine’s maximum power so they always have a reserve to compensate for any drop in output due to reduced density. In the case of the PC-6, the capacity to produce that additional 130 HP is used to compensate for the reduced efficiency at higher temperatures, allowing the engine to produce its stated 550 HP regardless of outside conditions :). Additionally, since the same principle applies to the reduction in density with altitude, flat-rated engines have a lower power decay while climbing, and can produce their stated power to a higher altitude, helping out greatly in hot-and-high operations.

Having finally sorted the engine out (after decades of trying 😀 ), Pilatus then turned to the other remaining propulsion item – the propeller – replacing the usual three-blade unit with a new four-blade model, creating today’s production standard PC-6/B2-H4.

However, while Pilatus themselves had stopped fiddling with the powerplant, the Porter’s users had other ideas and decided to carry on the tradition themselves :D. An immensely popular skydive platform, the PC-6 had at time been found wanting for power in the climb, leading to the logical idea of refitting it with a more powerful engine. This was eventually achieved in 2001, when an old B2-H2 was upgraded with the 750 HP PT6A-34 (flat rated to 620 HP), becoming the progenitor of a series of 30 such conversions (both H2s and H4s), all done under a new Supplementary Type Certificate :).

The Alpine Yank

But the “fulfillment” of the PT6A installation is not the whole of the PC-6 engine saga – not by a long shot :). The success of the first turboprop models had created a lot of interest on the other side of the Atlantic, where operators were keen on a home-grown version using locally-available components. Not oblivious to the huge potential of the aircraft on the American – and especially SOUTH American – market, Pilatus quickly complied with demand, and in 1964 struck a deal with Fairchild-Hiller to produce the aircraft under license in the States. Initially, the aircraft rolling off the line were stock B models – but it took the locals only a year to come up with their own version, the PC-6/C, powered now by the 575 HP Garrett (AiResearch) TPE-331-25D :).

Generally “confined” to the US market, the C models would eventually rise to worldwide fame, thanks most of all to the PC-6/C2, known in military service as the AU-23A Peacemaker. A type still happily flying with the Royal Thai Air Force, the AU-23A is/was powered by the 665 HP TPE-331-1-101F, and had flown into the spotlight during its exploits in the skies of Vietnam, Laos and Cambodia :). Another C2 had also landed in the record books, having performed an amazing 424 take offs and landings in a single day, a feat achieved over 21 consecutive hours without breaks (except for oil and fuel top ups)…

Chronologically out of tune, the last of the C models was the the PC-6/C1 of 1969, powered by a 576 HP TPE-331-1-100. Apparently a TPE conversion intended for the European market, the C1 eventually ended up being just a one-of model, with the PC-6 already having a suitable engine in the form of the PT6A :).

Overview – piston:

  • PC-6 (PC-6/340) – Lycoming GSO-480-B1A6 (340 HP) (1959)
  • PC-6/275 – Lycoming GO-480-D1A (250 HP) (1960)
  • PC-6/350 – Lycoming IGO-480-A1A (350 HP) (1961)
  • PC-6/D – Lycoming TIO-720-C1A2 (400 HP) (1970)

Overview – turboprop:

  • PC-6/A – Turbomeca Astazou IIE (523 HP) (1961)
  • PC-6/Ax – Turbomeca Astazou X (573 HP) (1964)
  • PC-6/A1 – Turbomeca Astazou XII (573 HP) (1967)
  • PC-6/A2 – Turbomeca Astazou XIVE (573 HP) (1967)
  • PC-6/B – Pratt & Whitney Canada PT6A-6A (550 HP) (1964)
  • PC-6/B1-H2 – Pratt & Whitney Canada PT6A-20 (550 HP) (1967)
  • PC-6/B2-H2 – Pratt & Whitney Canada PT6A-27 (680 / 550 HP flat rated) (1984)
  • PC-6/B2-H4 – Pratt & Whitney Canada PT6A-27 (680 / 550 HP flat rated) (1996)
  • PC-6/B2 (mod) – Pratt & Whitney Canada PT6A-34 (750 / 620 HP flat rated) (2001)
  • PC-6/C – Garrett (AiResearch) TPE-331-25D (575 HP) (1965)
  • PC-6/C2 – Garrett (AiResearch) TPE-331-1-101F (665 HP) (1967)
  • PC-6/C1 – Garrett (AiResearch) TPE-331-1-100 (576 HP) (1969)

Sources:

One thought on “Tech – A Flying Fashion Victim: The PC-6 Engine Saga

  1. Pingback: History – Turbomess: The L-410 Turbolet Family Tree | Achtung, Skyhawk!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s