Diamond’s Twin For 2012

Diamond Aircraft of Austria reasoned there was room for one more multi. The first Diamond Twin Star was introduced in 2005, and it represented a paradigm shift in the whole philosophy of piston twins. The DA42 was the world's first certified, all-composite multi, constructed almost exclusively of carbon fiber. It also incorporated Garmin's innovative G1000 glass-panel avionics suite, another game-changing improvement.

Perhaps most significantly, however, the new Diamond employed engines that were cutting edge in technology, yet with roots in the 19th century. The diesel dates back to 1893 when Rudolph Diesel's compression-ignition engine was first patented. If your knowledge of diesels extends little farther than the old smoky Mercedes 190D sedan in the driveway across the street, don't feel alone. Diesel engines forego spark plugs altogether in favor of extreme compression ratios, on the order of 15-24 to one, that use the heat of compression to ignite the fuel/air mixture without benefit of a spark plug. (In contrast, avgas-powered piston engines rarely exceed 10-to-one compression.) It took nearly 100 years before someone figured a way to make diesel engines viable in an airplane.

Diamond's twin features FADEC diesels that are, strangely enough, a century ahead of Rudolph's first design. The initial Diamond Twin Star could burn jet fuel, a major advantage in those parts of the world where avgas is scarce or nonexistent. The DA42 offered a pair of German Thielert Centurion powerplants, a variation on Mercedes' automotive turbo-diesels. The Thielerts were rated for 135 hp each, burned a miserly 6.5 gallons/engine/hour and, at least theoretically, sported a 2,400-hour TBO.

None of the airplanes had the chance to reach that TBO. Thielert went bankrupt in April 2008, leaving Diamond with nearly 500 orphan twins and almost 400 diesel singles, with limited technical support and no warranty. As a result, Diamond had no choice but to halt production of the Twin Star until they could develop their own diesels to replace the Thielerts. Perhaps ironically, the company had captured something like 80% of the light-twin market when production was suspended.

In the interim, Diamond rushed development of the DA42-L360, essentially the same airplane fitted with conventional Lycoming IO-360 powerplants. These are fuel-injected engines, similar to the carbureted O360s mounted on the Piper Seminole, and rated for the same 180 hp. With 90 hp more enthusiasm at sea level, the resulting airplane was an impressive performer, but the trade-off was that fuel capacity remained the original 76 gallons. Burn jumped to 9.5 gallons/engine/hour, and endurance at 75% was reduced to three hours plus reserve. Fortunately, that wasn't a terrible disadvantage in training mode where flights rarely exceed 1½ hours.

In order to replicate the efficiency of the Thielerts, Diamond initiated its own diesel design in 2007. These were improvements derived from the same Mercedes turbo-diesel technology eployed by Thielert, but produced at Diamond's Wiener Neustadt factory in Austria. The resulting 2.0 liter Austro AE300 is a geared, FADEC-managed, turbo diesel with single-lever control, rated for 168 hp. R&D and EASA certification demanded two additional years, and at this writing, the engines have been certified in Europe, Russia, South Africa, Canada, the U.S., Brazil, Ukraine, South Korea and China. The new airplane was dubbed the DA42-NG (for Next Generation), and it was introduced in mid-2009.

Diamond had to drop the Twin Star name over the objection of Eurocopter, who apparently feared that some might confuse Diamond's twin with their EC 355 TwinStar Helicopter.

This meant that for the last two years, there have been two Diamond twins to choose from, and despite its better efficiency, the diesel model had (almost) all the aces. The Austro 300-powered Twin Star was certified at 4,189 pounds, while the Lycoming model was stuck with a gross of 3,935 pounds. This left a useful load on the diesel of 1,070 pounds versus 1,180 pounds on the avgas version. (Incidentally, pilots in the EEU pay takeoff, landing, parking and overnight fees based on weight in kilograms. The odd certification weights translate to more logical kilograms---1,650 kgs for the NG and 1,550 kgs for the L360.)

The reason for the disparity was simple. Diesel engines are far more efficient than avgas powerplants, offering a specific fuel consumption on the order of .37 pounds/hp/hour versus .42 pounds/hp/hour, but they achieve their efficiency at the cost of extra weight. In the DA42's case, the Austro diesels add over 170 pounds/engine to empty weight. Diesels generate extremely high compression ratios and temperatures, and the engines must be built to withstand those conditions.

This left first-generation DA42 owners with three choices, none of them especially attractive: 1) Live with what they had and hope for the best; 2) Sell their airplanes for whatever they could get and start over at a loss; or 3) Upgrade to the Lycoming or the new Austro AE300 engines at a cost.

Today, the resuscitated, diesel-powered DA42 brings new enthusiasm to the light-twin market, albeit at a somewhat higher price. Base tab for an entry-level Twin Star is 599,500 Euros, about $799,500 at the late-January 2012 exchange rate. This compares to about $624,500 for a DA42-L360.

We contacted a local California flight school that had one of the original Thielert airplanes totally upgraded to the new Austro engines, TKS, the bubble canopy, synthetic vision and other goodies to see what the new DA42-NG configuration offers. Angel City Flyers (www.angelcityflyers.com) on Long Beach Airport in Southern California is an all-Diamond school, and they had two freshly converted DA42-NGs on the line plus a fully operational Twin Star simulator. Owner Seosamh Somers agreed to let us fly one for a few hours to appreciate the new engines and all the other improvements.

If you learned to fly in conventional, all-metal, avgas-powered airplanes sometime back in the last century as I did, the Diamond models look like something conceived by Lucas and Spielberg. All the Diamonds feature composite construction that provides otter-sleek surfaces and a configuration more reminiscent of the Millennium Falcon than an air-breathing teaching machine.

Make no mistake, this is one plush trainer, beautifully constructed, exquisitely appointed and meticulously engineered. There's more leather in sight than in a rock star's closet, space in front and back is generous, and knobs and switches have a distinctly professional feel. Also, like the DA40 Diamond Star, the DA42 has an aft left back door, a feature not shared by any other four-seat single or twin.

The DA42 offers a double dose of aerodynamic and ergonomic sophistication with swept, raked cowlings; a fuselage as slick as Murano glass; long, glider-like, high-aspect-ratio wings with swept winglets at the tips; a 50-inch-wide front cockpit; single-lever power controls and conventional joysticks springing from the forward, center seat cushions for pitch and roll control.

That last feature might seem an anachronism in this age of more modern side sticks, but while joysticks date back to the Wrights, they continue to be arguably the most effective and comfortable method of directing a general aviation aircraft.

My mentor on all things aviation was the late Roy LoPresti, father of the Mooney 201 and 231, Grumman American Cheetah, Tiger and Cougar, and VP of engineering on the Beech team that got the Starship certified. When it came to control system, LoPresti was a stickler on sticks, and it's revealing that LoPresti's choice for the upcoming Fury homebuilt was a center stick. I'm no expert on flight control design, but I'm allowed to fly center sticks, yokes and side sticks on a regular basis, and unless I'm granted the front seat of an F-16, I'll take a center stick every time. (The F-15 Eagle, almost universally regarded as the world's best jet fighter, uses a center stick.)

One peripheral benefit of the FADEC system is that starts are pretty much automatic, whether it's hot or cold. The electronics take care of most of the usual chores except for closing the hatch and fastening your seat belt. The airplane comes off the blocks with minimum breakaway power, and you can taxi on even less. Pretakeoff checks are few and simple, again with the help of FADEC. The airplane even does its own runup.

After the final lights/camera/action check, the DA42-NG turns out to be an enthusiastic performer. With two up-front and full mains (50 gallons), the test machine scored an easy 1,300 fpm climb. That was at 90 knots Vy.

I tried a 120-knot cruise climb, and the VSI only dropped to about 1,000 fpm. It's apparent the airplane's low-drag signature provides an efficient trip uphill.

At 10,000 feet, the NG was still scoring over 1,000 fpm up at Vy. The L360 Lycoming model with normally aspirated engines would probably be down to 800 fpm or less at that height. Both models are approved for a max altitude of 18,000 feet, but it's guaranteed the force-fed NG will reach that height sooner and with far more reserve power. The NG also offers over double the single-engine service ceiling (14,000 feet) of the L360 (6,400 feet).

In training mode, the NG again has the edge because of the turbo. Vmc and stall are virtually the same on both airplanes, so there's little chance of getting into real trouble. The NG's turbos provide consistent power to 14,000 feet, the critical altitude. Vmc demonstrations will be consistent all the way up, whereas the standard Lycomings lose power with altitude, so a Vmc demo in the L360 above, say, 5,000 feet, should be fairly easy. The airplane's reduced power up high won't induce much of a roll, no matter how badly you abuse power management.

When it's time to cruise, the NG scores an impressive 180 knots-plus at 14,000 feet, provided you're willing to wear the mask. Operationally, 10,500 or 12,500 are more realistic altitudes, and our cruise checks suggested an easy 163 knots at 10,500 feet, 175 knots at 12,500 feet. Come back to 60% power, and you can see 150 knots on about 10 gph total. That means endurance with the 76-gallon tanks is an easy six hours, worth probably a 900 nm range. Diamond suggests a max range of 1,180 nm, presumably at higher altitude. FADEC manages mixture control automatically, adjusting the flow to any power setting. It's reassuring that the NG's turbo diesels allow a single-engine service ceiling of 14,000 feet.

The DA42's high wing loading imparts a ride that's serene even when the sky isn't. The airplane has almost a locomotive's sense of straight ahead, and the Garmin GFC 700 autopilot is nearly psychic on climbs and descents once you've psyched out which button to push.

Descents can be as you like them, since the engines are water cooled. You can practically treat the diesels like turbines, reducing power to 30-35% for letdowns without fear of shock cooling. Similarly, landings are the next thing to automatic. Ninety-five knots works well around the pattern, and 85 knots is plenty on short final. Dirty stall is only 61 knots, so if the runway is short, you can come back to 75 knots without violating the 1.2 Vso rule.

The DA42 NG continues in production,and Diamond plans to keep the London, Ontario, Canada line running. For the nonce, the company is emphasizing the engine-replacement program. When general aviation recovers from the financial doldrums of the 2008 recession, the Diamonds' unique combination of 21st/19th century technology should make them leaders of the pack. The NG is already out front.