Diamond DA 62 N84LT

While descending in preparation to fly a visual approach, the pilot lowered the landing gear and contacted the tower controller for landing clearance. During the controller’s response, the pilot reported hearing a “pop” and observed the avionics display screens lose power. A few seconds later, he observed that both engines had lost power. The pilot executed a forced landing on to a road, during which the airplane struck a power line in the descent, and after touchdown, two road signs, which resulted in substantial damage to the right wing. A postaccident examination of the airplane revealed that the electronic control unit (ECU) backup batteries were wired incorrectly (parallel versus series) and the in-line fuses for the backup ECU power system for both engines were blown. The incorrect wiring would have resulted in only 12 volts instead of 24 volts being available which would have resulted in a higher current draw and subsequent blown fuses. According to the airplane’s maintenance records, 4 new ECU battery backup batteries were installed during the annual maintenance inspection about 4 months before the accident. The 1,000 hour/12-month Concorde battery capacitance check was also completed, and the battery had 92% capacitance. It is likely that during this maintenance the batteries were incorrectly rewired in parallel. A review of the aircraft maintenance manual showed that it did not provide a procedure to verify the ECU backup batteries were functioning correctly after replacement. The last step in the ECU back-up battery installation was to run engines and verify that the electrical system operated correctly. However, this step does not verify that the ECU backup batteries were installed correctly and were ready to provide power to the ECUs. In addition, there was no ECU backup battery operational test. If there had been such a test, the improperly connected batteries would have been discovered immediately after replacement. Completing this check on a routine basis would determine if the in-line fuse has blown. Furthermore, the pilot did not have the ability to check for the proper functioning of the ECU batteries before every flight during the before-takeoff checklist. The airplane manufacturer does complete a test of the ECU back-up batteries as part of the production process. Per the airplane’s maintenance records, the main battery was a serviceable one that was installed while maintenance waited for a new battery that was on order. The battery had been installed 23 days before the accident. The airplane had flown about 15 hours from the time the serviceable battery was installed to the accident. The main battery capacitance was tested and found to be 11.04 Amperes-hours (Ah) or 81.2% of rated capacity. The battery capacitance requirement was 85% or better. The loss of main battery power most likely occurred during the landing gear extension when the hydraulic pump turned on, and simultaneous with the radio transmissions. However, during the aircraft and component testing a definitive root cause of the initial power failure could not be determined. Testing could not duplicate the conditions of the flight when the electrical system anomaly occurred or the anomaly itself. With the ECU backup batteries and the alternator relays wired incorrectly, there was no back-up power to the ECU, which resulted in the complete loss of power to both engines and the inability for the engines to restart. While multiple anomalies with the wiring and battery system were documented, a definitive root cause of the initial power failure could not be determined. An electrical system anomaly caused the aircraft electrical system to shutdown and subsequent engine shutdown due to the mis-connected ECU back-up batteries. The testing done could not duplicate the actual conditions of the flight when the electrical system anomaly occurred or the anomaly itself.

Quoted verbatim from the NTSB record.