History of the Flight
On 8 December 2020, a Diamond DA42 NG (registration G-HAKA) departed from Leeds Bradford Airport for a private flight to Southampton. Before departure, the engines would not start due to a low battery. Engineers from the operator's maintenance provider started the engines using a ground power unit and conducted engine ground runs and ECU downloads, finding no anomalies. The maintenance release was signed off assuming low battery charge. Approximately 1 hour 45 minutes behind schedule, the pilot and passenger reboarded.
After engine start, a low volts caution appeared but was disregarded as a likely result of earlier problems. During the takeoff, an audible alert and low voltage cautions for both main busbars appeared. The pilot decided to return but did not declare an emergency, believing 30 minutes of battery power remained. The abnormal checklist for low volts directed to checklists 4B.3.7 (voltage) and 4B.4.6 (alternator failure). The pilot completed 4B.3.7 but not 4B.4.6 as no alternator failure was displayed. The first action in 4B.3.7 is to ensure alternators are switched on. The pilot reduced electrical loads, but busbar voltages dropped further.
On downwind, the pilot sent a message via GPS tracker and selected the ILS approach on an electronic tablet. Further load reduction included switching off lights except strobes. During the base leg turn, voltages dropped rapidly, and the pilot informed ATC that communications might be lost. ATC provided vectors and landing clearance. After lowering gear and capturing the glideslope, all electrical power was lost, including electronic flight displays. The pilot activated the standby artificial horizon, leveled off, and turned right. Finding a gap in the cloud base at 700–800 ft, the pilot descended while maintaining visual contact. Using a mobile phone flight planning app, the pilot returned to the airport for an uneventful landing.
Aircraft Information
The DA42 NG is a four-seat twin-engine aircraft with two Austro diesel engines and a Garmin G1000 glass cockpit. G-HAKA was equipped with mission-specific equipment increasing electrical loads. Power is supplied by two alternators, one per engine, connected to main busbars via relays and circuit breakers. If an alternator switch is off, the busbar is powered by the battery. The status of alternator switches is not displayed on the G1000.
Operator's Investigation
After the incident, the main battery was found depleted. Engine ground runs and a check flight showed no anomalies. The operator concluded the most likely scenario was that the alternators were not switched on before the flight. During the earlier maintenance, the engineers had switched the alternators off to comply with a cautionary note for ECU download (Modification MAM 42-551 installed on G-HAKA). They did not switch them back on afterward.
Checklists and Pilot Actions
The pre-flight and before engine start checklists include checks that alternator switches are on. The pilot reported normally leaving them on. However, during the incident, the pilot twice checked the switches and thought they were on when they were off. The low volts abnormal checklist also prompts checking alternator switches. The pilot saw low volts on the ground but dismissed it; in flight, both low volts cautions were present, but no alternator failure indications, so checklist 4B.4.6 was not considered relevant.
Probable Cause
The alternators had not been switched on, resulting in loss of electrical power shortly after takeoff.