What happened
On 19 September 2022, an ATR 72-212A, registration EI-GPN, was performing a commercial flight arriving at Belfast City Airport. The aircraft was operating under a specific engineering bulletin that required the Transformer Rectifier Unit (TRU) to remain powered throughout the flight. During the descent and approach phase, the aircraft experienced a sudden and widespread failure of several electrical systems.
As the aircraft descended through 4,000 ft, the autopilot and yaw damper disengaged, and flight director command bars disappeared. The crew noted that various electrical busbars turned amber on the system displays. The situation intensified as the aircraft continued its approach; the crew reported that engine warning displays appeared to freeze, and the landing gear and flaps failed to deploy upon selection. Additionally, the aircraft experienced a loss of primary flight data on the co-pilot's display, including airspeed, altitude, and vertical speed, and the engine propeller speeds increased unexpectedly.
While approaching the runway, the crew realized they had no ILS indications and transitioned to a visual approach. To resolve the cascading failures, the commander instructed the co-pilot to turn the TRU off. This action de-energized the faulty contactor, restoring power to the affected DC buses and allowing the landing gear and flaps to deploy. The aircraft landed safely at 0859 hrs.
The investigation
AAIB investigators examined the aircraft at Belfast City Airport and identified a malfunction in the SM150D19 contactor (designated 95PA). This component is responsible for supplying 28 V DC output from the TRU to the standby and emergency bus bars. Testing revealed that the contactor's internal pin had slipped within the plunger, leading to poor electrical connection and localized arcing.
Further investigation of similar contactors found that the pin-and-plunger assembly was prone to slippage in several units manufactured around 2015. The investigation also explored the human factors involved, noting that the crew experienced significant startle and surprise due to the sudden, multi-system nature of the failure and the lack of a specific checklist for this exact scenario.
Findings
- The primary cause of the electrical failure was the malfunction of the 95PA contactor, specifically a loss of continuity caused by the internal pin slipping out of position.
- The failure occurred while the aircraft was operating under an engineering bulletin that required the TRU to be continuously powered, a configuration that placed the faulty contactor under increased operational use.
- The crew experienced significant startle and stress, which impacted their information processing and decision-making during the approach.
- At the time of the incident, there was no specific procedure in the Quick Reference Handbook (QRH) to troubleshoot this particular combination of failures.