Diesel With A Jet Heart

Perhaps more than that, however, at least for aviators with most of their hours in avgas airplanes, there's the phenomenal smoothness of one or more turbine engines. Whether your turbine is driving a propeller or blasting you forward with pure jet thrust, the experience is akin to riding on velvet.

The new Cessna Skylane JT-A may not be quite that smooth, but it's close, and the airplane's power plant isn't even a turbine. As the designation implies, this 182 runs on jet fuel, but it's not driven by a jet engine. It's a turbo diesel, designed to operate on conventional Jet A, and the four-cylinder diesel is smoother than any six-cylinder avgas engine you've flown. (No, in perhaps the ultimate irony, the diesel Skylane isn't certified to burn diesel fuel, even if you were willing to drain some out of your Mercedes. You'd certainly never find any at local airports.)

For 2013, Cessna elected to retire the not-so-old avgas-powered Turbo Skylane T182T and replace it with a turbo diesel. Diesels have had a checkered career in aviation, though their heritage stretches way back to the 1920s. In the United States, Packard diesels were used on rigid airships starting in 1928, and German Jumo and Daimler-Benz diesels were employed on the famous Zeppelins (including the ill-fated Hindenburg) of the same period.

The major advantage of diesels in all applications was/is a comparatively low fuel consumption.

Piston aircraft engines experience an average specific fuel consumption of .43 lbs./hp/hr., whereas diesel engines record an sfc (specific fuel consumption) of more like .37 lbs./hp/hr. burning jet fuel. That's about a 12% improvement in efficiency. A second advantage is that avgas availability outside the U.S. is a sometime thing, whereas jet fuel is plentiful practically everywhere. Since American GA aircraft manufacturers sell roughly half their products overseas, the near-universal availability of jet fuel outside the U.S. and Canada makes a diesel engine more attractive.

Finally, jet fuel is almost universally cheaper abroad. To use an extreme example, when I flew through Narsarsuaq, Greenland, about a year ago, avgas cost $16/gallon while Jet A was priced at $5.89/gallon. Luckily for Americans, the delta between the two fuels on domestic soil is barely noticeable, but jet fuel is obviously far more attractive on the other side of either pond.

Thielert of Germany and SMA of France produced the first general aviation aircraft diesels in 2001. By now, practically everyone knows the story of Thielert. Despite developing what seemed a viable engine for the single-engine Diamond DA-40 Star and DA-42 Twin Star, Thielert went bankrupt in 2008, leaving Diamond holding the bag on hundreds of diesel-powered airplanes and an uncertain future for its two primary products. As a result, Diamond was forced to develop its own partner engine company. Diamond created Austro Engines and built the new manufacturing facility next door to the company's production facility in Weiner Neustadt, Austria.

Though Cessna's initial interest in diesels was in the Thielert/Mercedes product, the German Thielert company's bankruptcy caused Cessna to take a closer look at the French SMA engine. The 305 cubic inch/230 hp SMA mill had been fitted to older Skylanes under an STC in 2001, and since then, SMA has continued to develop the engine.

I flew one of those aftermarket Skylane diesels for this magazine in 2006 (see our April 2006 issue), when the predicted avgas shortage was still far in the future (we thought). Today, 100LL is becoming scarce in Europe, the Middle East, Indonesia, Africa, the Far East and other parts of the world. North America is one of the few places with adequate supplies of avgas at the moment, but 100LL isn't a high-volume fuel, and volume is everything in the fuel business.

These days, few refiners are willing to dedicate the resources necessary to produce avgas. In addition, the American Environmental Protection Agency has been trying to shut down avgas production for years because of its minuscule lead content. If the EPA ever gets its way, we'll have to turn to alternative fuels. Some of those show promise, and several are under development, but no one can guess when they might be ready for mass distribution and universal application.

Unlike the early SMA effort, the installation on the Skylane JT-A is a dramatic improvement over the old engine, with an estimated 40% new parts. The C1 version features a huge turbocharger and an equally impressive intercooler, the latter designed to keep the diesel cool while pulling as much as 90 inches of manifold pressure. Yes, you read that correctly, 90 inches. No one expects a Skylane JT-A to run against the Mustangs and Bearcats at Reno, but that's the kind of power they pull from their Merlins and Pratts, at least numerically.

As we went to press, Cessna was anticipating an FAA type certificate for the Skylane JT-A by April. I flew a pre-production model with flight-test equipment in the aft fuselage, but the design had been frozen for production, so there should be no differences in performance or specifications between the prototype and the completed airplanes.

The Skylane JT-A is virtually identical to the Turbo Skylane from the firewall aft. Weights and CG are the same. All the changes (except for diesel instrumentation on the G1000) are confined to the power section and prop of the aircraft. Even the cowling is recognizable. It fits in the original footprint, though its configuration does boast two large intakes out front, one for intercooler and the second for oil cooler. Other than that, you could easily mistake a JT-A for an older Lycoming-powered Skylane.

Previous diesel cycle engines have suffered from significant weight increases over avgas mills. That's a function of the fact that diesels need to be stronger to endure the extreme compression ratios and temperatures of ignition without spark plugs or magnetos. Most aircraft avgas piston engines employ compression ratios of 7:1 or 8:1, diesels compress their fuel/air load to 17:1 or more.

The JT-A avoids part of the weight problem by mounting a lightweight three-blade Hartzell carbon-fiber propeller behind the spinner. As a result of that and other economies, the total weight increase forward of the firewall is only 15 pounds.

As mentioned above, the JT-A uses the same fuel tanks as the avgas model--- 87 gallons. That means if you fill the tanks, you'll be carrying an extra 70 pounds of fuel, since Jet A weighs an extra .8 pound per gallon more than avgas.

Theoretically, that increases the delta to 85 pounds, but the good news is that the airplane's range is so exemplary with the lower-Jet A sfc, you may not need to fill the tanks on some flights, a characteristic common to turbines. Cessna doesn't mount sight gauges on the JT-A's high wings, so you may not be able to trust the line boy to hit your exact fuel requirement, but the company claims the fuel gauges on the Garmin G1000 are extremely accurate. Still, payload is only 400 pounds, so you may need to mind the weight and balance if you need to carry a full fuel.

(Cessna recognized the distinct possibility of line personnel misfueling the JT-A, and accordingly, the fuel tanks are designed to accept the duck-billed "jet only" filler nozzles. This makes it extremely difficult---nothing is impossible---for anyone to pump avgas into the tanks.)

Since horsepower hasn't changed, aerodynamics are identical and airframe weight is essentially the same, it's no big surprise that the Skylane JT-A flies basically the same as last year's avgas-powered T182T, only better. I flew with Cessna demo pilot Charles Wilcox out of the company's Wichita Mid-Continent facility. Wilcox did much of the test work on the JT-A and probably knows the airplane better than anyone. He had worked out a standard demo routine that showed the JT-A to maximum advantage.

Predictably, the new Skylane demonstrated the manners we've all come to expect from a 182. Climb is simply a matter of pushing the single power lever (you're almost tempted to call it a thrust lever) full forward, and letting the ECU computer set power. There's no waste gate on the engine, so all intake air goes through the turbo. As a result, you'll see something you've probably never seen before on a Cessna: 90 inches of manifold pressure during climb. This turbo is indeed stout.

Despite that, climb fuel flow will be almost exactly half of equivalent power on the turbo Lycoming airplane, about 12 gph. Ascending at 85 knots on a near-ISA day, the JT-A showed me a consistent 1,000 fpm all the way to 10,500 feet.

As we cleared the Wichita class C and pointed west toward Kingman, Kansas, for some landings, I leveled, set 90% power and watched speed walk around to cover 155 knots. That's the spec, and it's no surprise that it's very comparable to that of the T182T. Manifold pressure ran about 70 inches for cruise. If I owned a JT-A, I doubt if I'd ever get used to seeing that.

The diesel Skylane feels comfortable at max cruise---fuel burn is down to about 11 gph, the noise level is very civilized at the engine's constant 2,200 rpm and vibration is virtually non-existent. Skylanes have always offered a smooth ride; stable and not skittish in turbulence, and the JT-A only makes a good thing better. Critical altitude is 10,000 feet, and the turbo diesel will still deliver 75% power at the airplane's maximum altitude of 20,000 feet.

At max cruise, the 87-gallon capacity allows for seven hours' endurance plus reserve, enough for easy 1,000 nm legs. If you're one of those rare folks who likes to practice ferry flying, the JT-A will accommodate. Unlike avgas engines, diesels don't care what power setting you use. If the mission demands it and you're into masochism, you can retard the power on the SMA engine to 35% and endure for 14 hours, enough to cover 1,500 nm.

Since the JT-A has no aerodynamic changes, I wouldn't have expected any revisions to stall and slow flight manners, and sure enough, there weren't any. Directly over Kingman, I tried a few 60-degree steep turns, and the airplane had nothing new to tell me. It maintains all the habits of a Skylane.

Wilcox was eager to show me how the big diesel reacted to an in-flight shutdown, so we cooled the engine for a minute or two, then brought the power lever to fuel cut-off. At first, you have no hard indication that you've just lost power. The giveaway comes when you raise the nose and see the rpm begin to decay. The prop will continue to windmill even in a dirty stall, however. To restart, you simply bring the power lever out of the cut-off position to reintroduce fuel and let the computer do the rest. The point is that the electronic control unit simplifies shutdown and startup so much that there's almost nothing you can do wrong, it says here. I, Robot.

Stalls in a variety of configurations were next. The airplane hangs in there until airspeed is practically off the dial. The spec is 41 knots with flaps fully deflected. That means, in theory, you could maintain an approach speed of 50 knots and still preserve 1.2 times Vso.

Landings at Kingman were classic Skylane. Plenty of control pitch right into the flare and lots of elevator trim to stay ahead of the heavy elevator. No one has ever claimed Skylanes are short-field airplanes, but the new JT-A could probably use unobstructed runways of 1,500 feet or less without problems.

One anomaly I noticed during landings was that the JT-A isn't as amenable to no-flap approaches as was the avgas version. Residual thrust without the benefit of flaps means a reluctance to bleed off speed in the flare. Cessna is still working that problem and hopes to have a solution by the time the first airplanes are delivered this summer.

Cessna recognized early on that engine maintenance would be a major concern for prospective buyers, and to that end, they enlisted the aid of Lycoming. Cessna and Lycoming are both Textron companies, and Cessna arranged for the Lycoming Service Centers around the country to handle all engine service/warranty problems. Lycoming is in process of bringing its shops up to speed, so buyers should be able to rest easier when first deliveries begin late this summer.

TBO has been set at a preliminary 2,400 hours, and that may not be as optimistic as you think. Automobiles with ECU-directed turbo-diesels under the hood benefit from almost ridiculous reliability, though it's expressed in miles rather than hours, and there's no reason aircraft diesels shouldn't offer the same durability. The engine will require an oil change every 100 hours, a turbo inspection and possible overhaul at half TBO and an inspection of the fuel injection system every 600 hours.

Of course, the big question, conveniently saved for those who aren't punch- drunk from plowing through all the numbers above, is price. As with any other Cessna product, there will be some options, but the base price has been set at a preliminary $515,000. At the press briefing prior to the flight, a British reporter suggested that price would be "a significant challenge" in Europe, but "the shrinking supply of avgas may be a compelling argument for diesels."

Recent developments in turbo-diesel technology have been exponential, and the day of the general aviation diesel- cycle engine may very well have arrived. Diesels are no longer dirty contributors to pollution and noise, and offer some significant advantages over avgas-burning engines. Even in a down market, price is always subjective, and Cessna's Skylane JT-A is certainly a worthwhile subject.