What happened
On 17 January 2023, an ATR 72-202, registration G-NPTF, was conducting a commercial cargo flight from Belfast International to East Midlands Airport. During the final stages of a CAT II automatic approach in freezing fog, the aircraft experienced a major electrical malfunction.
As the aircraft descended through 500 ft, the crew observed a DC generator fault and noted that the co-pilot's flight instrument displays began flashing. The autopilot disconnected, and the crew subsequently initiated a go-around. During the climb, the aircraft was beset by numerous audio and visual warnings, including terrain alerts from the EGPWS, and the co-pilot's instruments continued to malfunction. The crew also experienced a temporary loss of radio communications with air traffic control due to severe electrical interference.
Following the procedure, the crew diverted to Birmingham International Airport. To stabilize the electrical system, the crew manually switched off the faulty generator. This action stopped the warnings but left the aircraft with significant equipment loss, including the loss of several flight instruments and the inability to use autopilot functions. The commander flew a manual approach to Birmingham, landing the aircraft without further incident.
The investigation
The AAIB investigation focused on the electrical system failures and the root cause of the generator fault. Examination of the No. 2 DC starter-generator revealed that the speed sensor wiring was damaged. Specifically, the white wire was found to be broken at the soldered pin connection, and the black wire showed a damaged core.
Testing confirmed that these defects caused an intermittent and rapid signal fluctuation. The generator control unit (GCU) interpreted these erratic signals as a fault, causing the generator contactor to open and close rapidly. This rapid cycling triggered a self-protection mode in the Bus Tie Contactor (BTC), which locked the contactor open, preventing the electrical networks from being reconnected and leaving the co-pilot's side of the cockpit unpowered.
Findings
- The electrical malfunction was caused by a wiring defect in the speed sensor connector of the No. 2 DC starter-generator.
- The damage to the wire cores was likely caused by the incorrect use of mechanical wire stripping tools during a previous overhaul at a third-party maintenance organisation.
- The rapid power fluctuations caused the Bus Tie Contactor to enter a protective lock-out state, preventing the restoration of power to the co-pilot's instruments.
- The crew managed a high-workload situation effectively, successfully navigating the diversion despite degraded instrumentation and temporary communication loss.