What happened
On 8 December 2020, a DA 42 NG, registration G-HAKA, was scheduled for a private flight from Leeds Bradford Airport to Southampton. Prior to departure, the aircraft had experienced engine starting difficulties, which required ground engineers to use a ground power unit. During the maintenance process, the engine Electronic Control Units (ECUs) were downloaded, a procedure that involved switching the alternators off.
Shortly after the engines were started for the intended flight, a low voltage alert appeared in the cockpit. The pilot, attributing this to the earlier starting issues, did not perform the necessary abnormal checklists. Following takeoff, an audible alert sounded, and both main electrical busbars showed low voltage. The pilot elected to return to Leeds Bradford.
As the aircraft progressed through its approach, the electrical load was reduced by switching off various lights. However, during the base leg of the approach, the busbar voltages dropped rapidly. The pilot notified air traffic control that communications might be lost. Shortly after, the aircraft suffered a total loss of all electronic flight displays as the battery voltage fell below the required threshold. The pilot managed to maintain control using the standby artificial horizon and a mobile phone-based flight planning application to navigate back to the airfield, where a safe landing was completed.
The investigation
The AAIB investigation focused on why the electrical system failed despite the engines remaining operational. It was established that the alternators had been left in the 'off' position following the maintenance work performed earlier that day. The investigation examined the cockpit's Garmin G1000 interface, noting that the system does not visually indicate the status of the alternator switches.
Furthermore, the investigators looked into the maintenance procedures, finding that while the specific ECU download procedure did not mandate switching the alternators off, other related maintenance notes in the aircraft maintenance manual did. This discrepancy likely led the engineers to leave the switches in the off position. The investigation also assessed the pilot's use of checklists and the impact of confirmation bias during the pre-flight phase.
Findings
- The primary cause of the electrical failure was that the alternators were not switched on prior to takeoff.
- The pilot failed to identify the incorrect switch setting during pre-flight and engine start checks, likely due to confirmation bias, as the switches were checked twice and appeared to be in the correct position.
- The G1000 display system does not provide a visual indication of whether the alternator switches are on or off.
- The ammeters continued to show electrical current even with the alternators off, because the alternators were still supplying power to the ECUs and fuel pumps, which may have misled the pilot.
- The pilot dismissed the initial low voltage warning on the ground, assuming it was a residual effect of the earlier battery depletion.
Safety action
Following the incident, the operator implemented an Operational Crew Instruction requiring pilots to use a 'read-and-do' method for the before-engine-start checklist rather than relying on memory. The instruction also emphasized the lack of alternator status indication on the G1000 and the necessity of studying the low voltage abnormal checklists.