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.
