What happened
On 12 March 1997, a Boeing 747-419, registration ZK-NBU, was operating as flight NZ18 from Auckland to Los Angeles when the Captain's electronic primary flight display (PFD) suddenly went blank. While the display remained powered, it ceased to show essential flight reference data such as attitude, heading, and vertical speed, instead displaying warning flags.
This loss of information was accompanied by several alerts on the engine indication and crew alerting system (EICAS), including traffic alerts and inertial reference system (IRS) messages. The crew attempted to resolve the issue by following the Quick Reference Handbook (QRH), switching the IRS source from the left unit to the center and then to the right. However, the center unit also triggered an advisory message, and the Captain's display continued to flicker for several minutes. To stabilize the flight instruments, the crew eventually turned the affected inertial reference units off. The aircraft returned to Auckland and landed safely without any injuries to the 19 crew members or 420 passengers.
The investigation
Investigators determined that the primary cause was a failure within the brake system control unit (BSCU) triggered by water ingress. The investigation found that a cracked plastic dripsield, located in the avionics equipment bay, failed to protect the BSCU from moisture accumulating from cargo containers above. This moisture caused an internal failure in the BSCU, which then sent a voltage spike through the data bus. This spike caused the inertial reference unit (IRU) to transmit error data across all its output buses, effectively invalidating the Captain's flight display information.
Further examination revealed that a Boeing Service Bulletin had previously recommended replacing these plastic dripsields with aluminum versions to prevent such cracking, but the airline had assigned this task a low priority. Additionally, the investigation looked into why the crew's attempts to switch to alternate IRUs inadvertently connected the new units to the same faulty electrical source.