The market for Diamond DA 42 multiengine diesel-powered aircraft has matured now that the aircraft has been on the market since 2006. The aircraft and its powerplants have, like any groundbreaking and innovative product, experienced many iterations as the new diesel engine technology proved itself over time. Usually an airframer introduces a new airframe with a conventional, proven powerplant. With the introduction of the DA 42 Diamond broke with that tradition and introduced an entirely new airframe design along with dual FADEC controlled diesel engine power plants made by the German company Theilert. The goal was and is now to build aircraft free of Avgas limitations. Today there are hundreds of DA 42 aircraft in service flying with either Austro engines or Continental TDI engines. They sell for prices close to what they originally sold for over twelve years ago. So, the Diamonds have maintained their resale values better than many competitors. DA 42s under Piston Power programs sell for more than when they were new. The engines have always had a twelve-calendar year factory recommended replacement time regardless of hours. For this reason, in 2020 the topic of repowering these aircraft is front and center for many owners or potential buyers of these early airframes. There remains to this day misconceptions and confusion about the diesel engine choices for repowering these airframes. So, let’s take a trip down memory lane to best understand what options are available now.

The aircraft was introduced with two G1000 panels, dual FADEC controlled Theilert 1.7 liter 135 hp diesel engines based on a Mercedes Benz OM668 car engine. 1500 were built by the end of 2006. At the end of 2006, the engine was upgraded to the Mercedes Benz OM640 engine cylinder block and became the Centurion 2.0-liter 135 HP Theilert engine. The engine burns JET A burning only 12 gallons an hour (both engines) at cruise. The original Mercedes automotive block was cast iron. Theilert preferred to use an aluminum block to save weight. The engine has a reduction gear and is liquid cooled and is not designed to be overhauled. The original time between replacement (TBR) was 1000 hours. These 1.7 liter equipped early versions had expensive gear box inspections every 300 hours. Thus, the operating costs negated the low fuel costs. These early airframes were equipped with KAP 140 auto pilots.  Diamond was working through the early teething problems in 2006 and 2007 when lightning struck in April of 2008. Theilert went bankrupt leaving the early adopter owners with no engine warranties and had to find parts. This was the perfect storm as it unfolded at the start of the Great Recession.

To Diamond Aircrafts credit even in the face of the recession it stood up its own diesel engine manufacturing facility in Austria called Austro Engines to save the airframe and its superior technology. The new Austro AE 300 168 HP 2.0-liter engine is also based on the same Mercedes engine but uses the actual Mercedes engine block. This makes the AE300 an overhaulable engine. The AE300 currently has an 1800-hour TBO. Diamond upgraded to GFC 700 autopilots to pair with the AE300 diesel engines. Because the AE300 uses a cast iron block the engine is heavier than its aluminum block-based predecessor. The airframe was beefed up to carry the heavier higher-powered engine.  Over time the factory upgraded assembly line aircraft to the G1000Nxi panels and did an aerodynamic cleanup of the airframe. The latest version is the DA 42- VI. It has the same AE300 168 HP FADEC operated engines as was first introduced. In order to assist the early adopter owners, the factory converted several of the original 1.7 liter Theilert equipped KAP 140 autopilot aircraft to Austro AE300 GFC 700 equipped aircraft. These are marketed as DA 42NGs.

Theilert was eventually bought by Technify (Continental Motors) in Mobile Alabama. Continental improved the engine design by introducing a mass flywheel to replace the problematic clutch assembly. This reduced the inspections to 600 hours from 300 and the inspection itself is much less expensive. The latest version of the 135 HP aluminum block TDI engine has a 2100-hour Time Between Replacement.

While it I possible to convert a DA42TDI airframe to a DA 42 Austro equipped airframe few owners follow that expensive and labor-intensive path. The improvements Continental has made makes replacing TDI diesels with newer longer life TDI engines the most economically sensible thing to do with little difference in performance. Similarly, Austro engine equipped DA 42s universally have their engines replaced with new or overhauled Austro engines.

The mission to break free from dependence on Avgas for multi engine aircraft has only been successful for two manufacturers. Diamond Aircraft with their DA 42 and DA 62 aircraft and Tecnam Aircraft with their P2006 four seat twin and also their eleven seat P2012 twin, both running on inexpensive and available conventional car gas. So it is easy to see why these multi engine aircraft have become preferred by flight training organizations and owners seeking to keep their operating costs low.

Written By Errol Bader

President Gulf States Aircraft Sales, LLC & PistonPower Board of Directors, Member.