on avweb:
Diamond DA42 Engine Fix: Engine AD in the Works?
Diamond Aircraft said on Friday that it’s continuing its investigation into the dual engine failure of a diesel-powered DA42 Twin Star last month in Germany, and the fix might be a backup battery for the engine’s electronic control units (ECUs). In meantime, AVweb has learned that an Airworthiness Directive for the airplane’s two Thielert 1.7 Centurion engines is pending. Diamond North American president Peter Maurer told AVweb on Friday that Thielert -- the engine supplier -- and Diamond aren’t working at cross purposes, since the problem obviously needs to be corrected. Maurer said that both of the Twin Star’s engines quit immediately after the pilot retracted the landing gear. Activation of the gear retraction system caved the electrical system voltage and knocked both ECUs offline. When the engines quit, the props immediately feathered and the airplane’s dual alternators, which are supposed to provide failsafe power to the ECUs, also died. The airplane landed with the gear partially retracted and was significantly damaged. Fortunately, the crew survived.
According to Diamond, the pilot found the aircraft with a dead battery, then took off immediately after starting it with ground power, without completely charging the battery. Although the airplane has dual alternators and dual buses, it’s unclear how independent the two buses actually are, since they’re connected through a battery isolation relay. In any case, neither alternator was delivering power because the offline ECUs stopped both engines. The ECUs are designed to reset after a failure, but will do so only if provided with sufficient operating voltage. One fix -- although it hasn’t been decided yet -- is to provide each ECU with its own independent backup battery or to isolate the dual buses more effectively, as some all-electric aircraft do. Diamond’s single-engine diesel, the DA40tdi, has a backup battery and Diamond has also discrete batteries for improved starting and for instruments in its two-seat DA20 C1 model. Another approach, says Maurer, is to use capacitors to bridge momentary voltage transients. Diamond and Thielert have yet to decide whether the proposed fix will be an engine or an airframe mod. Either way, says Maurer, airplanes will be retrofitted in the field, once the fix is developed