Safety Issue

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 aircraft operator:

• now requires all overhauled GCUs and BCUs to be internally inspected for corrosion, and the environmental protection inspected and reapplied as required in accordance with the manufacturer’s standard practices
• conducted a review of tech log reports on their 747-400 fleet to determine if specific AC power buses exhibit repeated intermittent failures or symptoms that may be attributable to internal corrosion or contamination
• is undergoing a scheduled removal of all BCUs and GCUs fitted to 747-400 aircraft to inspect internally for corrosion and environmental protection - this is being performed initially for aircraft that exhibit symptoms on AC bus 3.