Tuesday, June 19, 2012
Turbines vs. Pistons
A turbine may be more logical than you imagine
PISTONS. While turbine-powered aircraft must fly high in order to justify their expense, pistons don't realize such major losses of efficiency by flying lower.
As everyone who hasn't been living under a rock for the last decade knows, avgas has been under assault for some time. Never mind that its impact on air quality is essentially negligible and that lead content has been reduced by 50% in the last 30 years. Ignore the fact that refiners are having a tougher time making a profit on such a low-volume fuel, and accordingly, the long-term future of avgas is in question. Avgas is becoming scarce in many of the planet's hinterlands and downright unavailable in much of the South Pacific, some parts of Southeast Asia and some destinations in the Middle East.
Fact is, single-engine turboprops are becoming a more reasonable alternative to top-of-the-line piston products. Yes, there's a significant disparity in price if you're comparing the top new pressurized piston model to the least expensive corporate turbine single.
These days, a 2012 Piper Mirage, unquestionably the most upscale piston single on the planet, has a list tab just over $1 million. In contrast, the 2012 Piper Meridian, a P&W PT6A-powered version of the same airplane, goes for almost exactly double that figure. (There are a few other production single-engine turboprops available: the TBM850, Pilatus PC-12 and Extra 500, plus several conversions—the PA-46-derived Jetprop DLX; the Cessna P210N-founded Silver Eagle; the Beech A-36 based Tradewinds; the Soloy Turbine 206 and others, but for the sake of simplicity, we'll confine our analysis to the two Pipers.)
Question is, which is the better buy, and how do you define "better?" Are there any factors that suggest you should spend the extra money for turbine over piston power?
Right up front, you have to consider an important intangible that's difficult to price—safety. If we all had unlimited money, we'd probably be flying in twin turbines, making the whole discussion moot. But let's say you could afford to purchase either of the two Pipers above. What factors should you consider in your evaluation?
First, it's important to remember that turbine-powered aircraft MUST fly high in order to justify their expense, while piston-powered machines don't realize such major losses of efficiency by flying lower. If you enjoy seeing the sights from below 15,000 feet, perhaps a turbine isn't for you. The Piper Meridian does its best work in the flight levels above 20,000 feet, where there's often nothing to see below but haze.
The Mirage's 350 hp Lycoming TIO-540 is a well-proven mill that has been around in various forms for nearly 40 years (on the Piper Chieftain and Mojave, Aerostar and others), and it's proven one of the piston world's most reliable and durable powerplants. When properly treated with standard periodic maintenance, proper leaning and a minimum of shock cooling, it can easily exceed Lycoming's recommended 2,000-hour TBO.
The Pratt & Whitney's PT6A gas turbine engine is a truly phenomenal machine, granted amazing reliability and equally impressive longevity. Specifically, the Meridian's 500 shp PT6A-42A sports a TBO of 3,600 hours, and it may even avoid the expensive hot-section overhaul if an electronic trend-monitoring system is installed. The PT6A is inherently smoother than large piston engines, simply because it has only about 10 moving parts, and they're all moving in the same direction.
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