What happened
On December 20, 2018, a 777-32WER, registration PT-MUG, operated by TAM Airlines, was performing a scheduled international flight from São Paulo (SBGR) to London (EGLL) with 339 passengers and 16 crew members on board. During the leveling phase of the flight, the crew identified an "ELEC BACKUP SYS" warning message. Although the crew followed the Quick Reference Handbook (QRH) procedures to restart the system, the message persisted.
Approximately eleven minutes later, the aircraft suffered a significant electrical failure. The pilot flying and pilot monitoring both lost access to the Primary Flight Displays (PFD), Navigational Displays (ND), and the Engine Indicating and Crew Alerting Systems (EICAS). Simultaneously, the primary lighting in both the passenger and crew cabins failed. While some displays were eventually restored, the electrical synoptic indicated that the main and backup systems were unable to supply power to the aircraft buses, despite the availability of the APU generator and left Backup Generator. This led to the deployment of the Ram Air Turbine (RAT) to provide standby power.
Due to the electrical instability, the crew declared an emergency and diverted to Tancredo Neves Aerodrome (SBCF) in Confins, MG. The aircraft landed above its maximum certified landing weight. The high-energy braking required for the overweight landing caused the main landing gear brakes to overheat, triggering the thermal fuses and resulting in the deflation of the tires. The aircraft sustained minor damage, but all 355 occupants escaped without injury.
The investigation
CENIPA's investigation focused on the electrical system's failure to distribute power despite active generation sources. Investigators examined the aircraft's electrical configuration, including the status of the Integrated Drive Generators (IDG) and the Backup Generator (BUG). The investigation also reviewed the maintenance history of the backup converter, which had been replaced shortly before the incident. Additionally, the investigation looked into the physical damage found on the landing gear and specific electrical connectors.