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.
A friend stopped by my hangar a while back on a rainy Saturday with an interesting question. He was curious what I thought of the choice between high-end piston singles and turboprops. A few years ago, there would have been no question which one I would have recommended and why. Today, the choice isn't quite so simple.
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.
TURBINES. A turbine offers amazing reliability and impressive longevity. It's inherently smoother than a piston and is remarkably compact and lightweight.
The Lycoming has a complex system of at least 260 parts: rods, pistons, valves, lifters, camshafts, bearings, timing chains, belts, etc., many of them churning left, right, forward, back, up and down to produce power. The Lycoming is still remarkably smooth, but it stands no chance of running as evenly as the Meridian's P&W turbine.
Of course, you do pay for the turbine's added reliability. It operates at much higher temperatures, and the turbine spins as fast as 45,000 rpm, so a turboprop engine must be constructed of extremely heat-resistant metals and be very well balanced to handle rotational forces.
Turbine engines are remarkably compact and lightweight compared to piston mills. The Meridian's 500 shp PT6A is comparatively small and weighs a little over 300 pounds, whereas the big-bore, turbocharged, 350 hp Lycoming on the Mirage weighs more like 450 pounds. This imposes considerations of both design and weight and balance. The Meridian's fuselage is extended an extra foot over that of the Mirage, and that's all in the longer nose to control the airplane's balance point.
Turbines aren't as efficient as piston mills, but the difference isn't as much as you might think if you consider the improved performance. At optimum altitude, the Meridian burns about 31 gph compared to 20 gph on the Mirage, roughly 50% more. That's because piston engines are more efficient and offer a lower specific fuel consumption (.43 lbs./hp/hr) compared to turbines (.58 lbs./shp/hr).
As partial compensation, the Meridian climbs to its maximum altitude, 28,000 feet, in about half the time and cruises roughly 40 knots faster than the Mirage. (In fact, the Meridian is approved for flight at 30,000 feet, but few of the type are RVSM equipped for operation above 28,000 feet.)
As partial counterbalance to the efficiency of avgas, it's not efficient if you can't find any. Jet fuel is often less expensive than avgas, and it's more universally available around the world. (A few years ago, I delivered a Cessna 421 to a company in Subic Bay, Philippines, only to go back a year later and ferry the airplane back to the states. The company executives had grown tired of limiting their destinations to airports with avgas and decided to buy a King Air C90.)
The extra fuel burn of a turbine also has an effect on payload. Jet fuel is 13% heavier than avgas, so the Meridian must carry more fuel weight to manage reasonable range, specifically 150 gallons vs. 120 gallons on the Mirage. The Meridian's bigger power allows it to be certified at 5,092 pounds, almost 800 pounds heavier than the Mirage, but the top piston Piper still scores about 150 pounds more payload than the Meridian.
The logic of insurance is always difficult to define, and you'll pay a disproportionately higher premium to fly a turbine. Apparently, only pilots recognize the ease and simplicity of flying behind a jetprop. Both types demand high-performance/high-altitude checkouts, but for reasons that have nothing to do with accident history, insurance companies seem reluctant to acknowledge the benefits of a turbine.
Finally, there's resale value to consider. If you compare a seven-year-old Mirage to a similar-age Meridian, the Mirage has lost about 37% of its value, whereas the Meridian has declined about 39%, not much difference.
I've been lucky to fly both types quite a bit, including a dozen or so international deliveries. While that doesn't make me an expert, I'll take turbine every time. (Easy for me to say. I don't have to pay for it.)