AO-1997-005

On Wednesday 12 March 1997, at 0032 hours, Boeing 747-419 ZK-NBU was enroute from Auckland to Los Angeles as Flight NZ18. The Captain's electronic primary flight display no longer displayed flight reference information and appeared blank. An internal failure within the brake system control unit, due to water ingress, in turn caused a fault with the inertial reference unit data bus supplying information to the Captain's display. An alternate inertial reference unit was selected and it failed to supply the required information. The selection of a third inertial reference unit resolved the problem and the Captain regained his display. The two affected systems were turned off and the aircraft returned to Auckland without further incident. Safety issues discussed include the adequacy of the Quick Reference Handbook and the vulnerability of an inertial reference unit to an external fault. Safety recommendations were made on both these safety issues.