What happened
On December 20, 2023, an Airbus A321-253NX, registration SE-DMS, was conducting a ferry flight from Copenhagen (EKCH) to Oslo (ENG and ENGM) under Instrument Flight Rules. During the initial climb, the flight crew noticed an error on the Electronic Centralized Aircraft Monitor (ECAM) regarding fuel distribution, specifically an "ACT XFR FAULT" message indicating that fuel remained in the Additional Center Tanks (ACT) when it was expected to be empty. The crew attempted a manual fuel transfer to resolve the issue.
As the aircraft climbed through Flight Level 250, the first officer reported the smell of an electrical fire. The commander immediately ordered oxygen masks to be donned and declared a MAYDAY, instructing air traffic control for an immediate return and descent to Copenhagen. The aircraft landed safely on runway 22L. Upon taxiing, the crew noted that while the smell was less noticeable after removing the oxygen masks, a strong odor of burning was present in the cockpit area.
The investigation
Following the landing, maintenance crews inspected the avionics compartment located beneath the cockpit floor. They discovered a Ground Fault Interrupter (GFI) relay (124QH) that had sustained a burnt hole in its plastic housing and was covered in black soot. An adjacent relay also showed signs of soot deposits.
Technical analysis, including a Computed Tomography (CT) scan by the French BEA and inspections by the component manufacturer, confirmed that significant overheating had occurred inside the GFI relay. The investigation revealed that various internal components, including the capacitor, resistor, and Pulse Width Modulator (PWM) chip, had been damaged by voltage levels exceeding their design limits.