What happened
On 4 June 2019, a Boeing 737-4Q8, registration G-JMCR, was performing a cargo flight from Oslo to Brussels. During the descent, the crew heard a loud electrical noise, followed by a partial electrical failure that deactivated the primary flight display screens on the left side of the cockpit. This failure also disconnected the autopilot and autothrottle, and caused the loss of several critical systems, including the flight management computer units and the yaw damper.
As the aircraft approached Brussels National Airport, the crew faced approaching thunderstorms. Although the pilots initially decided to continue the approach visually, they entered a heavy rain shower at approximately 300 feet above the ground. This loss of visual reference prompted the pilot flying to initiate a go-around. During this maneuver, the aircraft appeared to struggle with acceleration and a positive rate of climb. After orbiting the airfield to assess the situation and wait for the weather to clear, the crew successfully landed the aircraft on a second approach.
The investigation
The investigation focused on the cause of the electrical malfunction and the crew's response to the degraded cockpit environment. Investigators examined the aircraft's electrical system and identified that an internal short circuit in a transfer relay had caused a circuit breaker to trip. This trip resulted in the loss of power to several essential electrical buses.
Furthermore, the investigation looked into the crew's management of the approach. It was noted that the pilots were operating in a high-workload environment, dealing with simultaneous system failures, heavy radio static, and conflicting distance information from Air Traffic Control. The investigation also reviewed the aircraft's configuration, noting that the crew's understanding of the electrical backup systems was impacted by unclear fleet-wide documentation regarding how certain instruments are powered during a failure.
Findings
- The primary cause of the electrical failure was a fault in the transfer relay which triggered the opening of circuit breaker C819.
- The loss of power to the 115v AC Transfer Bus 1 and associated buses led to the loss of the left-side EFIS screens, the yaw damper, and the EGPWS.
- The crew's decision-making was complicated by the fact that the transfer bus off caption failed to illuminate, masking the true nature of the power loss.
- The aircraft was incorrectly configured for the approach because several non-normal checklist procedures were not completed during the high-workload period.
- The go-around maneuver was characterized by a period of slow acceleration, potentially due to insufficient thrust settings or windshear.
Safety action
- The operator has updated its aircraft documentation to clearly identify which specific aircraft in its fleet are configured to allow the left EFIS displays to be powered by the AC Standby Bus.