The GCU/BCU manufacturer implemented procedures for a more thorough inspection of the external chassis of GCUs and BCUs received for repair for signs of liquid contamination, and a circuit card removal and inspection for any suspect units.
The GCU/BCU manufacturer drafted a service information letter to operators and maintenance providers advising that: Unexplained EICAS [engine indicating and crew alerting system] messages and aircraft electrical system anomalies may result from the presence of corrosion and contamination within the GCU or BCU circuitry. Operators suspecting water ingress of the GCU or BCU should remove the units to an avionics repair shop and request a physical inspection and a careful cleaning of the Printed Wiring Assemblies [PWA], the motherboard, and all interconnections to ensure contamination of the circuitry is not present. A test validation is not sufficient to detect corrosion and contamination of the internal circuitry. Avionic repair stations should examine the NVM [non-volatile memory] of each unit and perform a visual examination of PWAs for corrosion and contamination.
The ATSB is satisfied that the actions taken and proposed by the GCU/BCU manufacturer adequately address the safety issue.