Incident Description
On 30 November 2015, while taxiing at Warsaw Chopin Airport (EPWA), the flight crew of a Boeing 787-800 received a BRAKE SYSTEM INOP (WHEEL 4+8) warning. The crew decided to return to the parking stand. All 258 occupants on board were without injuries.
Maintenance Actions
Technical services performed maintenance procedures in accordance with CMM 32-45004 FIM DMC-787-A-32-40-F5-POA-421A-A. A system test was conducted and, after a positive result, the aircraft was released for flight. Later the same day, during taxi, the same fault reoccurred. This time, the decision was made to replace the Electric Brake Actuator Controller (EBAC) in accordance with the AMM. The aircraft was returned to service.
Follow-up and Resolution
The fault elimination proved ineffective, as the same fault occurred under identical circumstances on 2 December 2015. After consulting with the brake system manufacturer, the decision was made to replace the complete brake assemblies for wheels 8 and 4. According to information from the manufacturer, an internal fault of one Electric Brake Actuator (EBA) out of four on each brake assembly could have caused the EBAC fault. In subsequent flights, the fault did not reappear.
Probable Cause
The most probable cause of the fault occurrence was a malfunctioning one of the EBA blocks, which caused a fault of the entire EBAC (Electric Brake Actuator Controller) block.