Diamond Twin Star: 21st Century Multi

Using max cruise power, my flying Infiniti is tripping along at three times the freeway speed; meanwhile, it's sipping a mere 12 gph total, surprising economy for a twin (and not bad for a single, as well). While Diamond Aircraft didn't conceive the Twin Star design primarily as a cross-country machine, the airplane is capable of range well beyond the province of most standard singles and twins.

As if to prove the point, one Diamond pilot demonstrated exactly what the Twin Star was capable of during his return flight from Oshkosh AirVenture to Wiener Neustadt, Austria, in August 2004. The pilot supplemented the airplane's 76-gallon wing tanks with a 26-gallon ferry tank and flew 1,900 nm from St. John's, Newfoundland, Canada, to Porto, Portugal, in 12.5 hours, using 42% power. The Twin Star burned only 72 gallons of jet fuel, averaging a mere 2.85 gals./engine/hr. for the trip, meanwhile clocking a groundspeed of 152 knots.

Admittedly, the Diamond pilot had help from average 30-knot tailwinds, but the flight was an excellent example of what's possible with the Twin Star's remarkably efficient, Thielert, diesel engines. The eastbound ferry flight represented the first nonstop Atlantic crossing by a diesel-powered aircraft.

If economical cross-country travel is one of the Twin Star's major talents, it was only one of the airplane's original missions. The Austrian company foresaw a large market for a light/light multi, and that's not a surprise, considering the relative dearth of new minimum multis in the last quarter-century. New light twins temporarily disappeared from general aviation with the demise of the Duchess in 1982.

Prior to that time, there had been a half-dozen attempts to market entry-level twins, only one of which (the Piper Seminole) was modestly successful. Even the Seminole was discontinued in 1982, then revived from 1989 to 1991, and finally brought back into continuous production in 1995.

Now, Diamond has a whole new take on the light/light-multi formula, offering an innovative, turbo-diesel-powered, FADEC-controlled, multi-engine trainer with all the advantages of the mini twins, plus a larger cabin, better range and significantly simplified operating systems.

Considering the source, the choice of a diesel powerplant was only logical. While German engineer Rudolf Diesel's late-19th-century engines are among the world's oldest form of mechanical propulsion, and have been employed sparingly on military airplanes and dirigibles since the 1920s, they're a relatively new phenomenon in general aviation. SMA of France and Thielert of Germany have been the pioneers in aircraft diesel development for the little guy.

In fact, the Thielert 1.7 Centurion turbo-diesel is based on an automotive engine design by DaimlerChrysler. It's extremely similar to a mill used by Mercedes in one of its diesel automobiles, though geared down in the aviation application from 3,900 engine rpm to 2,300 prop rpm. Diamond Aircraft currently offers two Thielert, diesel-powered airplanes: the dedicated diesel Twin Star and the single-engine Star with your choice of a conventional avgas or diesel mill. The Twin Star was originally slated for Thielerts, with Lycomings as an option, but the diesel version has been so successful, Diamond dropped plans for the avgas model.

So why would anyone want a diesel-powered airplane in the first place? One reason is the aforementioned efficiency. Most avgas engines score a specific fuel consumption (SFC) of about 0.44 lbs./hp/hr. The Thielert 1.7 Centurion manages an SFC of more like 0.36 lbs./hp/hr., 20% better. Obviously, diesels can legally burn road diesel, but you're not liable to find that at most airports, so the alternative is Jet A1, still usually less expensive than avgas and, perhaps equally important, almost universally available at all but the smallest strips.

Avgas is rapidly disappearing at many international destinations. Back in the early '90s, I delivered a primo Cessna 421C to Subic Bay, Philippines, for a hospital management company. Two years later, the company called and said avgas was becoming so scarce in much of the Far East, they'd been forced to replace the Golden Eagle with a King Air C90. I picked up the 421 in Manila and returned it to the United States by way of Guam, Majuro and Honolulu. (Coincidentally, I had to have avgas shipped in to Majuro, as the airport no longer offers anything except Jet A1.)

The reduced hourly cost of operating a diesel is one reason the type has been so eagerly embraced overseas where avgas has long cost $5 per gallon or more. Jet fuel often sells for as much as $1 per gallon less. The bottom line is a dramatic savings for operators of airplanes that burn jet fuel.

If there's any bad news, it may be that the American FAA has dictated that the current Mercedes-built Thielert 1.7 Centurion engines must be replaced after only 1,000 hours. On the plus side, the all-Thielert-manufactured 2.0 engines that will supplant the 1.7 will be rated for 2,400 hours or 12 years, whichever comes first. Robert Stewart, of Diamond dealer U.S. Aero in Long Beach, Calif., says the new engines will likely sell for $24,500 in today's dollars, but that's for a firewall-forward new powerplant, and it's prorated to allow for the 1,000 hours already flown. In other words, the owner will pay more like $12,000 per engine. Multiply that by two, and it's about what you'd pay for a single factory overhaul on an IO-360 Lycoming.

If you learned to fly twins in a comparative antique, such as a Travel Air or Apache, as I did, the Twin Star will come as a revelation. These days, Diamond's glass is more than half full, and those who allege there's been no innovation in general aviation need only take a ride in a Twin Star to understand the error of their position.

Take FADEC, for instance. Full Authority Digital Engine Control regulates every parameter of engine operation except manifold pressure, expressed as percent of horsepower on the Twin Star. From startup to shutdown, FADEC samples air temperature, atmospheric pressure, humidity and throttle position to manage electronic fuel injection, rpm, mixture and timing, and deliver optimum performance and fuel efficiency for any conditions through a single lever for each engine.

Whether you're flying a max gross takeoff out of Leadville, Colo. (elevation 9,927 feet MSL), or merely cruising at 6,500 feet over Cape Cod, the Engine Control Unit reads the engine environment, optimizes power and fuel burn, turns the electric fuel pump on and off as necessary, regulates ignition timing and minimizes the possibility of out-of-limit cylinder and exhaust gas temperatures. In short, FADEC diagnoses and automates all engine functions.

This simplifies the pilot's job and allows him or her to concentrate on navigation, communication and simply enjoying the trip. The standard avionics suite for the Twin Star is the Garmin G1000--integrated, two-screen, flat-panel display. Once you learn the operating principles, the G1000 automates communication and navigation functions nearly as much as FADEC does engine operation.

The bottom line is simplicity that initially threatened one of the Twin Star's primary missions. The FAA originally questioned whether FADEC's automation and the lack of prop and mixture controls compromised the airplane's ability to train pilots for the multi-engine rating. The question was whether the airplane was truly a "complex" design, since the props weren't traditionally controllable. Eventually, the FAA concluded that the props were controllable, even if controlling them required only moving the throttle. In the event of a failure, the pilot still needed to identify the sick engine and shut it down, feathering the prop, even if that process was as simple as flipping a single switch.

Indeed, single-engine operation is about as uncomplicated as it can be without incorporating an auto-feather system. During the air-to-air formation session that produced Jessica Ambats' photos, I shut down the left engine and chased the Cessna Skylane photo ship around the clouds above Catalina Island with the left prop caged. The shutdown and restart process consisted of merely turning off the appropriate ECU, then switching it back on when it was time to restart. Uncommanded yaw was mild, and the Twin Star remained docile while in single-engine formation.

Whatever the mission, Diamond configured the DA42 to offer super-simple operation, a large, comfortable cabin and something no other manufacturer has---a back door. Both the single-engine Star and Twin Star feature a fold-up door at aft left, allowing independent access to the rear two seats. Pilot and copilot board through an overhead hatch that hinges at the front and rotates up and forward. The cabin measures 46 inches across, nearly as wide as a cabin-class Piper Navajo.

Standard fuel is 50 gallons, but practically everyone opts for the long-range, 76-gallon tanks. Climb is excellent; the turbos' critical altitude is 8,000 feet, so as long as your body can take it, there's no reason not to fly at 10,000 to 12,500 feet on practically every flight. Incidentally, single-engine service ceiling is 10,000 feet.

When level at 12,500 feet with 80% power dialed in, you can expect about a 160-knot cruise speed on 12.5 gph. Extrapolate that over five hours, and you could reasonably expect to transit 800 nm at one sitting. If you have the inclination and the time, you can pull back to 50%, endure for more like 9.5 hours and range out nearly 1,200 nautical miles.

At this writing, Twin Stars are fully assembled and certified in Austria, then disassembled and shipped to Diamond's London, Ontario, Canada, facility where they're reassembled and ferried to the various North American dealers. Within a year, the Canadian plant will begin producing its own Twin Stars.

Current base price of the Twin Star is $527,313, including FADEC and the G1000. TKS known icing and a number of other options can elevate that figure well above $550,000. Air-conditioning, the ultimate luxury, is expected to be available sometime in 2008.

Initial sales of the Twin Star have been encouraging. Lufthansa has ordered 40 for its European training facility and Embry-Riddle Aeronautical University in Daytona, Fla., has contracted for 10 more, and current deliveries and orders total more than 700.

Since designers began mounting a second engine on airplanes in search of redundancy, the industry has struggled with the problem of asymmetric thrust and twin-engine safety. The Diamond Twin Star doesn't totally solve the problem, but its unusual combination of automatic systems and easy handling may make it one of the simplest---and safest---twins in the sky.

SPECS: 2007 Diamond DA42 Twin Star