Diamond Twins Reborn

I had the chance to fly both airplanes recently at Diamond’s North American headquarters in London, Ontario, Canada, and the airplanes are both clear improvements in numerous ways over the original Thielert-powered twin that Diamond launched in the United States in 2005.

How Diamond arrived at the point that it’s selling not one but two distinctly different versions of its all-composite twin is a complicated tale. But it’s one that speaks volumes about the agility of the Austrian airplane maker when faced with challenges, the risks and rewards associated with riding the cutting edge in airplane design, and the difficulties potential buyers face when assessing those risks.

Twin Star: History of Innovation
In developing its Twin Star light multiuse twin, Diamond created that rarity of rarities: a true, clean-sheet airplane. While the long, high-aspect-ratio wing is similar in spirit to other Diamond designs and in keeping with its sailplane ancestry, there's nothing derivative at all about the DA42. (The name Twin Star, by the way, is no longer used by Diamond.)

This is why, when it was developing the twin’s design a few years ago, the company seemed torn over the choice of powerplants. The conservative call was the 180 hp Lycoming IO-360 powerplant, a conventional avgas-burning four-banger that Diamond had been using for several years in its DA40 Diamond Star single. The innovative — and somewhat risky — call was to go with a new engine, an automotive turbodiesel powerplant repurposed for aviation use.

As we all know, Diamond went with the diesel, the Thielert Centurion 1.7 liter, a Mercedes conversion done up for aviation use by German company Thielert Aircraft Engines. While the Lycoming was well-known and trusted, the Thielert engine had some irresistible advantages. Despite the fact that it was only a 135 hp engine, compared with 180 hp for the Lycoming, the German diesel was, well, a diesel. And because it burned jet fuel, it could be fueled just about anywhere and, in many parts of the world, for far cheaper than airplanes requiring 100LL. And because it was an engine designed to be computer-controlled on the Mercedes sedans into which it was intended to go, computerized engine control, “fadec” in aviation parlance, was a natural, as well. So pilots would get a number of pluses: easy starting, no-brainer engine management and silky smooth power across the curve.

What sealed the deal, though, was fuel burn. In a day and age when fuel costs drive flight training decisions, the Thielert engines turned back the clock, giving fuel burns of less than 10 gph — combined — for typical training-scenario power settings. When I got my multi ticket in the DA42 at European American Aviation (EAA) in Naples, Florida, a couple of years ago, the flight school had just taken delivery of its second DA42 and was eagerly awaiting a third. The airplanes were flying nonstop, earning money for the school and requiring very little in return. And EAA’s experience wasn’t unique. For a time, Diamond couldn’t make the twins fast enough.

Unfortunately, we all know that that happy scenario quickly devolved into near chaos. The engines turned out to be too good to be true, and the ongoing engine maintenance costs, which few looked at carefully enough, turned nightmarish. In particular, the Thielert’s clutches were problematic. Not only did they require very regular overhaul — every 300 hours, in fact — but owners also had to send them back to Germany for the work, an expensive and time-consuming process.

The final straw came in early 2008 when Thielert, already under investigation from regulators for financial improprieties, declared insolvency, leaving the future of the Thielert engines, and that of the DA42, in jeopardy. The company parted ways with its founder and was taken over by a government insolvency administrator, who as part of the recovery plan declared warranties on existing engines null and void.

Shortly thereafter, facing an outcry from its DA42 owners, Diamond officially abandoned the Thielert engines, all the while putting pressure on the administrator to help DA42 owners keep their airplanes flying by lowering the prices of spare parts and overhauls.

Around that time, Diamond announced its intention to develop a Lycoming-powered DA42 model, on which it had already completed considerable work during the DA42’s initial development.

However, it was still crisis mode for DA42 owners. The high prices for Thielert’s engine parts, and their poor availability, left many DA42s grounded for lack of parts or parked for economic reasons.

Many DA42 operators called for Diamond to make them whole — either by honoring the engine manufacturer’s original warranty or by paying the cost of converting their Thielert airplanes to alternative power once those conversions were available.

While it was not obligated or able to do either of those things, Diamond has worked, despite the recession, to create new power options, as well as create what it says are attractive purchase options for many operators who want to trade in their Thielert-powered DA42s for newer models.

Some Solutions
What Diamond was able to accomplish in the past 18 months — successfully launching and certifying two new versions of its twin during the worst economic downturn in 80 years — seems nothing short of miraculous.

There’s other good news. The insolvent Thielert company still won’t cover previous engines under warranty, but it has reduced the prices of parts for those engines and cut overhaul costs substantially.

It might be moot. Many owners of Thielert DA42s will surely switch to an alternate engine, as Diamond now has approval to retrofit the Austro or Lycoming engine models into Thielert airplanes.

Classic Gas Piston and New Diesel Engines
Given its disastrous experience with the Thielert diesels, some have asked why Diamond took the remarkable step of deciding to design and produce its own automotive derivative turbodiesel powerplant. The answer is easy. Its owners fell in love with fadec and low-fuel burns and turbo power and jet-A. Diamond always felt that the problem with the Theilerts was not so much in the conception as in the execution. If it could do the next diesel engine right, it would have a winner.

As Thielert did, when Diamond created its own engine — the Austro AE300 engine is technically a product of sister company Austro Engines — it started with a Mercedes automotive turbodiesel but went with a 2-liter model with an iron block and a higher compression ratio. The Austro engine, while heavier than the Thielert by a total of around 175 pounds for the pair, is more powerful, putting out 170 hp up into the flight levels. Austro/Diamond also went a different route with the gearbox, opting for a directly integrated gearbox with a torsional vibration damper, as opposed to Thielert’s clutch mechanism model. The TBO on the Austro engine is initially 1,000 hours; the goal is to increase that to 2,000 hours. Austro’s torsional dampeners have a 300-hour inspection interval, but that procedure takes less time and costs less money than on the Thielerts. Diamond expects the interval to increase over time, as there are no wearing parts in its dampener. The prop is the same three-blade MT prop used on the original.

The two diesels make use of the same fuel cooling system, as well as the same main and auxiliary fuel tanks and lines, though the Austro has fuel pumps for takeoff and landing. Both engines are fadec equipped, and the operation is similar, though the preflight safety checks are slightly different.

You can tell the difference between the three engines simply by looking at them. The Thielert and Austro cowlings have big oval inlets below the prop. The Austro engine has the turbocharger installed along the side, so there’s a noticeable hump on the starboard side. (Because the props on the Austros are conventionally rotating — that is, they both spin the same way — the hump is on the same side on both right and left engines.) The Lycomings, which are mounted farther forward for weight and balance considerations, are completely cowled and have their inlets just along the side of the cowls.

To mitigate the impact of the heavier Austro engines, Diamond increased the max takeoff weight of the NG model by more than 250 pounds, to 4,189 pounds, compared with 3,935 for the L360. The NG’s CG range is fairly narrow too, and flight schools will get used to adding ballast to the rear baggage compartment when flights involve just two front seat occupants. With the weight increase, the NG has a full-fuel payload with standard fuel of 865 pounds and with extended fuel of around 720 pounds — not bad at all for a twin powered by 170 hp engines.

DA42 L360: Surprising Lycomings
After flying the Lycoming-powered DA42 for the first time in September, I can say the Lycs are nothing short of a fantastic fit for the DA42. Flight schools, which have ordered dozens of the twins, apparently feel the same way. The higher power combined with the lighter weight gives you acceleration you can feel in a dramatic way: The L360 has better takeoff, climb and landing performance, and its slow-speed performance — both on one and two engines — is better across the board. And with its counterrotating engines, the L360 has another safety advantage as there's no critical engine, meaning that regardless of which engine fails, the right or the left, there's no discernable difference in performance or behavior.

In the pattern, where much of its training duties will be done, the Lycoming-powered DA42’s easy handling, lower weight, improved pitch feel and enhanced power output add up to happier students. You also get lower takeoff and landing distances and better climb, not to mention better landing manners, than the NG or the original.

The L360’s rate of climb is very strong for a light twin, both on two engines and on one. Best rate of climb by the book is better than 2,000 fpm, and cruise climbs are quick affairs, as well. At 5,000 feet with one engine shut down and the prop feathered, we were seeing a 300 fpm climb rate with excellent handling, which is almost twice as good as book figures, though we were admittedly flying on the light side. Still, the power advantage over the original DA42 is evident.

The other huge upside to the Lycoming model, at least in terms of sales appeal, is that it is not a diesel. Burning extra fuel, they reason, is a small price to pay for the kind of operating-cost certainty that comes with the Lycoming engines. For some, however, the higher operating cost is a downside. The 20 to 30 percent higher fuel burn represents several gallons per hour increase over an Austro-equipped airplane. So max range is about 30 percent less than the NG model. And the L360 doesn’t match up in cruise with the NG at higher altitudes, though it is a decent performer. At 8,000 feet, we were looking at nearly 160 knots true on 17 gph total. At that altitude, there was already a dropoff in performance, as the normally aspirated engines can no longer produce full-rated power. At 10,000 feet, for example, we were getting only around 145 knots true on just over 20 gph total.

In terms of equipment, the L360 can be outfitted with just about everything the NG will come standard with, though its Garmin G1000 flat-panel avionics system has yet to integrate the excellent GFC700 autopilot, and it doesn’t yet have Garmin’s SVT synthetic vision displayed on the PFD. Instead, the basic-but-solid Bendix/King rate-based KFC 140 autopilot comes standard. For training, it’s an appropriate choice, though private owners, I’m guessing, will be begging for the Garmin box.

Of course, there are levers galore in the L360, two each for the throttles, props and mixtures. Depending on how you look at it, this is either a good thing or a bad thing. Flight schools will love it, I think, because it will give students the chance to learn about engine and fuel management. Individual owners, I’d wager, will prefer the single-lever fadec power of the Austro diesel.

DA42 NG: New Generation Diesel
The Austro engines have everything the Thielerts had: fadec power control, easy starting, smooth operation, turbocharged performance and excellent specific fuel consumption. And the additional 70 hp, compared with the Thielert model, gives the NG greatly improved takeoff, climb and cruise performance.

Plus, you get updated G1000 avionics, including the GFC700 autopilot. It’s a top-notch panel.

Starting the NG is a one-touch affair — well, it’s a twin, so make that two touches. Turn the key to the on position, and the engine simply spins to life. The electronic engine control units (EECUs) take charge of the prop and engine settings. The single levers are used simply to set the percentage of power.

On runup, you test the EECUs by setting the power levers to idle and depressing the two test buttons. The system automatically increases the rpm to 1900 and runs the engines through a series of tests, verifying the functioning of the EECUs and the prop governors.

On takeoff, the acceleration is clearly more brisk than with the Thielert model, and the initial climb is stronger too. With power set to takeoff (both levers full forward), we were seeing a cruise climb rate of around 1,200 fpm, which we could have increased by several hundred feet per minute by raising the nose a few more degrees.

At cruise, the NG steps away from the L360. We were getting close to 180 knots at 12,000 feet, and at 10,000 feet, we were seeing 175 knots true, all on around 17 gph total. At 8,000 feet, we were truing out at 170 knots at 16.6 gph, compared with 155 knots and 16 gph for the L360. Diamond test pilot Rob Johnson, who flew with me in both airplanes, says he regularly sees speeds in the NG of better than 180 knots at the same fuel flow. For a twin-engine airplane, those are remarkably frugal fuel flows.

The NG, like its Lycoming-powered stable mate, slows down easily, and its low-speed handling (with both engines operating and with one inop) is similar to the original DA42, thanks in part to the addition of VGs on the wing roots and gap seals on the elevator trim tabs. I think it would be a fine training airplane.

But instead, Diamond seems to be positioning the NG as the personal transportation choice. As such, it will come standard with a better avionics package than the L360, and many customers will opt for the platinum trim package, which includes custom paint, upgraded leather interior and premium carpet, electrically actuated adjustable rudder pedals, sun visors and lumbar adjustable front seats. Diamond hasn’t announced the U.S. price of the NG model yet, though it says it will be slightly more than the L360. But with typical options (meaning almost all of them), including Garmin XM satellite weather, TKS known-ice anti-icing system (the biggest ticket item) and Avidyne active traffic, among others, the price will probably be around $750,000, with a list price of around $600,000 for an L360 typically with standard equipment for a training mission.

Considering the nightmare experience of many Thielert owners, will NG buyers be taking another big risk by opting for the Austros? It’s a fair question, and I don’t think anyone has an answer yet. So far, the Austros are performing admirably, but the fleet leader engine (in a DA40) has only around 250 hours on it, so it’s too early to say for sure. But in its design of the AE300, Austro specifically tried to overcome the known issues with the Thielert engines. And it’s important to remember that this time the engine vendor is not a third party but Diamond’s own sister company.

For more information about the new DA42 models, visit diamondair.com.